Solid-State Relays
Technical Manual
Contents
Crouzet manufactures and sells a complete range of high-performance solidstate relays complying with the new electro-magnetic compatibility standards.
■
Introduction to Solid-State Relays
The aim of this manual is to facilitate access to these components by
presenting the technological bases for the product and specifying the basic
rules for their use.
■
Comparison between the Solid-State Relay
and the Electro-Mechanical Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . ■ 06
■
Typical Applications for Solid-State Relays
■
The Input Circuit
■
The Trigger Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ■ 20
■
The Output Circuit
■
Thermal Characteristics of Solid-State Relays
■
Mechanical Design of Solid-State Relays
■
Load Switching on Solid-State Relays . . . . . . . . . . . . . . . . . . . . . . . . ■ 43
■
Protecting SSRs against Transient Phenomena
■
Special Precautions and Help with
Circuit Diagnostics for SSRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ■ 66
■
Characteristics and Terms relating to Solid-State Relays
■
Applications
For further information, refer to the
Crouzet “Control” catalogue:
Control Catalogue
or contact our International Customer Service Centre (ICSC)
on 0 823 333 350
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1
Introduction to Solid-State Relays
Definition_
A solid-state relay is an electronic component which performs an interface
function with electrical isolation between a control circuit, usually at low level,
and a power circuit connected to loads which may have high power ratings
(motors, pumps, solenoid valves, heaters, etc).
Moreover, this function is performed in an entirely "static" fashion, with no
moving parts, thus endowing the component with an almost unlimited lifetime.
SOLID STATE
RELAY
Based on electronic
components
electrically isolated
switching system
Although the technology was developed many years ago, the first solid-state
relays became available as standard components towards the end of the
1960's, although their structure was much simpler then. In the meantime,
miniaturisation of electronic components has both improved the performance
of these relays, and also made it possible to add complementary functions,
so that today the solid-state relay is particularly well-suited to certain
applications: soft start, reversing the direction of rotation, power
proportioning.
Structure of a Solid-State Relay_
The solid-state relay, also termed SSR (Solid-State Relay), consists basically
of 5 functions as described below (functions in bold typeface)..
6
1
2
3
4
5
7
8
1 input circuit
5 protection
2 optical coupling
6 load
3 trigger circuit
7 main AC supply
4 switching circuit
8 solid-state relay
This structure is technically equivalent to and comparable with that of an
electro-mechanical relay (EMR).
2
3
The Input Circuit
In an electro-mechanical relay, the input characteristics (voltage, current, …
level) are determined by the coil. Similarly, the solid-state relay has a more or
less complex input circuit, which may, at the lower end of the range, consist
of a simple serial resistor with its polarisation diode, or, for more complex
relays, a circuit generating a constant current for extended input voltage
ranges, or an analog-to-digital converter for analog relays.
Figure 1.2
EMR
The Isolation
In an EMR, galvanic isolation is ensured naturally by the electro-magnetic
coupling between the moving armature and the coil. In the case of a solidstate relay of the semiconductor type, this isolation is provided by optical
coupling (photo-transistor, photo-triac…). On some older versions, isolation
may be by magnetic coupling, or even a REED relay.
The Trigger Circuit
This circuit processes the input signal received and switches the output
circuit. Where the switching is complex (zero voltage switching, pulses, phase
control …), this circuit guarantees the desired switching mode: in the case, for
example, of zero voltage switching, the circuit will ensure that the output will
only switch when the voltage next passes zero after application of the control
input.
Complete Solid-State Relay
The Switching Circuit
This circuit consists of an element providing for the electrical power to be
switched to the load. This component may be either a bipolar transistor or a
MOS transistor to switch a DC voltage to the load, or a triac or back-to-back
SCRs to switch an AC supply.
Figure 1.3
As opposed to electro-mechanical relays where the switching element is a
simple contact capable of operating either in AC or DC mode, in a solid-state
relay, the type of main supply switched is pre-determined by the output.
Inside a Solid-State Relay
The Protection Circuit
Electro-mechanical relays usually do not have a protection subassembly.
Because of their totally electronic structure, solid-state relays are particularly
sensitive to the interference present in the main AC supply; consequently the
switching circuit needs to be protected from the surges and interference in low
voltage supplies. Although such protection could be installed outside the
solid-state relay, it is more and more often integrated in the relay itself.
4
5
Comparison between the Solid-State Relay and
the Electro-mechanical Relay
The all-electronic structure of solid-state relays endows them with
considerable advantages in relation to electro-mechanical relays, but also
entails certain limitations. The Tables below summarise the advantages and
disadvantages of solid-state relays and electro-mechanical relays.
Electro-Mechanical Relay_
Advantages
These Tables show that there is no major disadvantage to solid-state relays
as against electro-mechanical relays in normal switching applications and, in
this comparison, we have preferred to specify certain restrictions applicable
to solid-state relays which need to be known and which may have an impact
on the final choice of the type of relay.
Firstly, it has to be accepted that no one type of relay is suitable for use
regardless of the application. Relay applications vary enormously and depend
on the physical and electrical environment, so that it is impossible to define a
precise set of parameters capable of providing the user with a complete guide
to the best choice. So the final decision can only be made bearing in mind the
parameters specific to each application.
Disadvantages
• Low residual output voltage
• No heat sink
• Low purchase price
• Multiple contacts and reversing contacts
• No leakage current
• AC or DC output
• Compact in size
• Output power
• Limited max. switching frequency
(5 to 10 Hz)
• Acoustic noise
• Electro-magnetic interference
• Contact life : ~ 250 hrs at a switching
frequency of 1 switching operation / sec
• Switching is not synchronous
• Contact bounce
• Low performance with high currents (arcing)
• Interfacing with digital circuits
(control power >>)
• Control power ≥ 200mW
Main Reasons for Using a Solid-State Relay_
Relay Life
Used properly, the most important features of the solid-state relay are its
reliability and its lifetime. In practice, the outputs of an SSR last practically for
ever, while the switching contacts of an EMR are subject to wear, corrosion,
sticking, etc. depending on the use. An electro-mechanical relay may
malfunction as a result of mechanical fatigue in the moving parts (spring,
moving armature). It is commonly accepted that a solid-state relay lasts
between 50 to 100 times longer than an EMR; i.e. 25 000 hours operating at
a frequency of 1 switching operation / sec (compared with 250 hrs for a
traditional relay).
Cheaper in the Long Run
Solid-State Relay_
Advantages
Disadvantages
• Low control power
(typically: 10 to 50 mW)
• Synchronous switching
• Asynchronous switching
• Low electromagnetic interference in
synchronous mode
• Lifetime / reliability 25000 hrs
• Fast response time
• No moving mechanical parts
• No mechanical wear
• Compatible with digital circuits
• Resistance to shock
• Silent operation
• Residual output voltage (1 to 1.6 V)
• Output is either AC only or DC only
• Heat sink often mandatory
• Used with low output signals
• Resistance to transient voltages
• Some leakage current
• Single contact
6
The cost factor is important when choosing a relay. The initial outlay for an
EMR is usually lower than that for an SSR with similar technical
characteristics. However, in this sort of calculation, the subsequent costs of
monitoring, maintenance and possible replacement of the EMRs are usually
forgotten.
Similarly, the cost - which is often high - of adapting the control circuits to the
EMR inputs should not be overlooked. Additional costs may also be incurred
by the need for efficient filtering to counter interference caused by contact
bounce.
Control Power
The sensitivity of an electro-mechanical relay is around 10 to 20 times less
than that of an SSR, i.e. to obtain equivalent output power, an EMR would
need 10 to 20 times more power at the control input (200 to 500 mW). This
characteristic is vital for compatibility with other electronic equipment,
especially digital systems.
7
Immunity to the Environment
Basic Technical Differences between an SSR and an EMR
Immunity to the application environment is a more complex criterion, but
SSRs are invariably superior where this parameter is concerned.
The mechanical resistance of an SSR, which has no moving parts, is better
than that of an EMR. The resin coating on the SSRs offers complete
protection against vibration and shock in addition to providing very good
protecting in particular against corrosion.
Moreover, humidity has very little effect on SSRs, only reducing slightly the
insulation resistance. An EMR is often more sensitive to humidity, which is
likely to cause corrosion in the long term.
Operating Mode
Whereas an electro-mechanical relay can only switch a load in asynchronous
mode, i.e. the output contact switchover is controlled solely by the control
signal without any particular relationship in time with the power signal, in a
solid-state relay switching can be synchronised with the output signal. This
particularity makes the following switching modes possible in addition to
asynchronous switching:
• Asynchronous Mode,
• Synchronous Voltage Zero or Peak Mode,
• Phase Angle Mode,
• Pulse Mode.
Switching Speed
The switching parameters are often the prevailing factors when it comes to
choosing an SSR or an EMR. Speed may be extremely important, even
critical in certain process control or automatic machine applications.
In certain exceptional cases where very low power factors are involved, the
electro-mechanical relay cannot be used, and the same applies if switching
without bounce is to be guaranteed.
Electro-magnetic Emission
The possibility offered by the solid-state relay of switching loads when the line
voltage passes through zero will limit transient phenomena considerably, as
well as current peaks and also electro-magnetic emission as a result.
In certain exceptional cases where very low power factors are involved, the
electro-mechanical relay cannot be used. The same applies if switching
without bounce is to be guaranteed. So here the AC or DC solid-state relay is
inevitable.
The most important of these and their specific uses will be covered in detail
later.
Special Precautions and Output Circuit Protection
The all-electronic structure of the solid-state relay usually requires that some
special implementation precautions be taken in addition to using protection
"systems" on the power output circuit. These measures include in particular:
a) Measures which can be implemented independently of the type of load
connected to the relay, and as a result, will often be incorporated directly in
the relay housing by the manufacturer. These involve mainly protection
against the phenomena which may occur on the main AC supply.
• RC filters to protect against sudden voltage variations including those
generated by the switching itself.
• Protection using a varistor or transil diode against high instantaneous or
high-energy surges.
Other, more technical factors are obviously also involved in the choice of a
relay, all of which will be addressed in detail in the subsequent chapters.
b) Measures to be determined depending on the characteristics of the load
and of the external circuit, the latter of which can only be fully defined or
optimised by the user himself.
• Protection against short circuits on the load using a fuse or in certain cases
a circuit breaker.
• Protection against excessive temperature rise by mounting the relay on an
appropriate heat sink with heat transfer compound or thermal interface. This
protection shall be provided systematically. Special attention shall be paid to
the flatness of the contact surface and to the maximum tightening torque of
the screws to avoid distorting the base plate. To facilitate the user's task, the
8
9
manufacturers are constantly increasing the offer for solid-state relays with
integrated heat sinks defined by the acceptable limit characteristics for
current, voltage and temperature.
Special attention shall also be paid to complying with standard industry
practices when wiring the power circuit; in particular by adapting the crosssection of the wires to the rated current for the load and to the characteristics
of the device protecting against overcurrent.
Typical Applications for Solid-State Relays
Solid state relays have been used successfully for 20 years in a wide range
of applications, and current experience shows that, although universal, the
solid state relay is particularly well-suited to process applications, where
machine tools are controlled by PLCs or other microcontroller-based circuits.
Due to their very high input sensitivity (less than 15 mA for controlling up to
120 A) over a wide voltage range, solid state relays are directly compatible
with most standards for electronic components such as CMOS, TTL,
microprocessors, etc…
Potential uses include (non-exhaustive list):
Production equipment:
• Printing
• Special machines
• Textile machines
• Machines for the food and
beverage industry
• Pumps
• Conveyors
• Machines for the plastic and
rubber industry
• Packing machines
• Ovens and heat chambers
• Electronics industry
Industrial monitoring equipment:
• Temperature monitoring
• Test machines
Service equipment:
• Lighting
• Photocopiers
• Lifts
• Vending machines
• Automatic doors
• Air-conditioning
• Computer peripherals
• Electric water-heaters for showers,
water heaters, hot-air hand dryers
• Printers
Alarm systems
Highway equipment
Medical equipment
10
11
Examples of Uses for Solid-State Relays
As mentioned earlier, the applications in which solid-state relays are mainly
used can be found in all spheres, both industrial and commercial. The few
real-life examples given below only provide a very limited glimpse of the
potential applications for the SSR.
Traffic Light Control
Traffic lights are managed by a logic system controlling solid-state relays
mounted in a metal cabinet at the roadside. The SSR's resistance to vibration
ensure total operating reliability.
Automatic Vending Machines
Solid-state relays are mounted directly onto the printed circuits controlling
these vending machines and command various electro-magnets.
Temperature Control
The temperature of injection pipes and nozzles for plastic moulding machines
is measured by sensors, and the device heating elements are controlled via
solid-state relays by a digital system based on the data acquired.
Controlling Motors
Solid-state relays are used to control the motors on water pumps used in
decorative fountains. These ornamental fountains are installed in both public
and private gardens; those with variable, programmable operating cycles are
used in entertainment shows.
Controlling Re-fusion Ovens
Heating elements in re-fusion ovens are powered via solid-state relays,
piloted by a micro-controlled logic system. Various output modules (GA8) are
used to control electro-magnets which position the PCBs prior to soldering.
Lighting Control
Automatic lighting control is often used in commercial buildings outside office
hours or outdoors in very harsh operating conditions. The use of synchronous
relays increases the bulb life.
Controlling Electric Hoists
The solid-state relay controls the direction in which the 3-phase motor in the
hoist operates. This type of hoist is often used extensively in industry,
especially in mechanical applications.
Regulating a Corrosion Tester
The air in the climatic test chamber is heated, cooled and humidified
cyclically. Both the heating elements and the motors are controlled by SSRs,
which are particularly suitable for corrosive atmospheres.
12
13
The Input Circuit
Complementary Applications using I/O Modules
I/O modules (Input / Output Modules) such as low output power SSRs which
are specially adapted to industrial control applications using digital systems,
expand the range of potential applications for solid-state relays considerably.
The input circuit, together with the output circuit, is a fundamental element of
the solid-state relay. You need to be familiar with its parameters to ensure
optimum use and interfacing with the circuit in which the relay will be
incorporated.
By combining an output module (whose function is identical to that of an SSR
with a higher output) and an input module (whose function is the opposite of
an SSR since it transforms any AC or DC signal into a logic level compatible
with CMOS, TTL …) with the inputs of a digital control system, we can satisfy
such demands as controlling pushbuttons and indicators in lifts as shown in
the Figure below.
6
1
2
3
4
5
7
8
1 input circuit
5 protection
2 optical coupling
6
3 trigger circuit
7 main AC supply
4 switching circuit
8
load
solid-state relay
To satisfy the numerous demands in the industrial field, the solid-state relay input
circuit may be designed for direct voltages which will usually be limited to 35
VDC, or for alternating voltages ranging up to 280 VAC. However, some SSRs
can be used with either voltage.
I/O Module
DC-Voltage Input Circuit_
Input Circuit Configuration
•1
•2
•3
•4
•5
•6
1 output module
2 input module
3
3 lift
The diagrams below show two different DC input configurations.
2
1
•7
•8
•9
•10
•11
•12
2
1
+
3
Fig. A
Controlling lift pushbuttons and indicators. The I/O modules
interface with the control PC.
Fig. B
1 protection diode (parallel)
Main Input Circuit Structures
2 protection diode (series)
3 constant current generator
14
15
In the version in Figure (A), the control voltage is applied to the terminals of
a LED-type diode integrated in an optocoupler which is activated when the
voltage applied to its terminals exceeds the threshold of 2 or 3 volts (direct
threshold voltage of the LED diode) with a minimum current of the order of 2
mA (i.e. a turn-off power of about 5 mW). The optocoupler will turn off again
when the control level falls below approximately 1 volt again.
Simply connecting a resistor in series will enable voltages of up to 35 volts to
be applied to the control input, while complying with the optimum operating
conditions for the optocoupler without adversely affecting its lifetime (well
above 200 000 hours).
Protection against accidental polarity inversion is achieved by adding a diode
in parallel to the optocoupler; this will limit the reverse voltage level to 0.6 –
0.7 volts, i.e. less than the reverse flashover voltage of the optocoupler diode
(around 2 to 3 volts).
The major drawback of this type of circuit is that it really does not allow
optimal use of a status LED indicating the status of the control – the
brightness of a LED depends on the current passing through it, and in the
case shown in Figure (A), the current varies with the control voltage; so,
depending on the circuit calibration, a 5V control voltage may well not provide
optimal LED illumination. This is why solid-state relays with status LEDs
usually include a circuit of the type shown in Figure (B). This circuit includes
a current generator which, whenever the control voltage exceeds a threshold
of around 0.6 to 0.8 volts DC, maintains§ the current in the coupling diodes
and the status LED at a predetermined value of around 10 to 15 mA
independently of the control voltage level.
A circuit with a current generator has the drawback of a low flashover voltage,
which may render the circuit very sensitive to surges. On the other hand,
protection against inversions in the control voltage is ensured by installing a
diode in series; this significantly increases the resistance to reverse polarity
(up to 500 Volts).
DC Input Circuit Wiring
The solid-state relay with a DC input can be controlled either by its " + " or by
its " - " input, as long as the polarities are complied with, and the output
current capacities of the control organ are verified and compatible. The main
wiring modes for an SSR with DC control are shown in Figure 2.
TTL Circuits
Most solid-state relays can be switched by the 16 mA current available with
low residual voltage (0.4 to 0.5 V) on TTL circuits. Since TTL outputs are
mounted in current sinks (which can be assimilated to an NPN output), it must
be controlled by the " - " input only.
CMOS Circuits
Standard CMOS components are not capable of controlling a DC solid-state
relay directly because of the low output current available (< 1 mA). CMOS
components with an integrated buffer and an output current of around 10 mA
should be preferred. They must be wired in the same way as the TTL
components in a current sink.
+
DC 5V
Connecting CMOS Circuits to a
Solid-State Relay Input
R
3
1 CMOS circuit with Buffer
2
2 SSR
10 mA
max
3 load
1
-
Depending on the voltage and the power supply, which for a CMOS may be
above 5 Volts, and on the SSR to be controlled, especially where a relay with
an input without a current generator is used, care should be taken additionally
to limit the current and consequently the power dissipated in the CMOS
component by adding a serial resistor R. The value of the resistor is defined
by the various circuit parameters; between 1 000 and 2 000 W is a commonly
accepted value for a voltage of 10 to 15 VDC.
Special Precaution for 2-Wire Sensors
+
DC
1 open NPN collector
1
2 relay
2
3
3 switch
+
4 open PNP collector
-
+
-
4
2
3
When a solid-state relay is
controlled by a 2-wire
device or any other device
with high leakage current,
such as a sensor or other
solid-state
relay
for
example, it is advisable to
+
VDC
2
1
3
TTL
V
SSR
-
1 sensor or SSR
-
2 leakage current when open (1 to 8 mA max.)
Fig. 2 : : DC Input Connection for a Solid-State Relay
16
3 shunt resistance is determined such that V < 1 volt
17
2-Wire Sensor
Connection
check that the leakage current of the device alone does not switch the solidstate relay. In this case, a resistor must be mounted in parallel on the SSR
input to reduce the voltage level generated by this current to a level below the
turn-on threshold (1 Volt).
Additional Precautions
It should be noted that an AC/AC solid-state relay can be connected at both
the input and the output to two different voltage sources within the limits of the
voltage and frequency specifications.
Furthermore, a solid-state relay with a DC input can become an SSR with an
AC input by adding an external filter-rectification circuit (Figure 3). This can
prove useful if low level AC control is required (24 VAC for example).
To limit the effects of voltage surges (DC or AC) on the input causing
inadvertent switch-off, or even destroying the input circuit, it may be useful to
mount a Zener diode in parallel with the input for DC relays.
Controlling an SSR with a DC Input
using an AC Voltage
R1
1
VAC
1
C
DC-Input SSR
(R2 = SSR internal resistor)
AC-Voltage Input Circuit _
Input Circuit Configuration
Solid-state relays with an alternating control input are generally designed for
an input voltage range of 90 VAC to 280 VAC. The alternating control signal is
rectified in half or full wave, filtered and sometimes regulated before being
applied to the optocoupler diode; and to the status LED.
As a result, a solid-state relay with an AC input can be controlled by a DC
voltage. As the input impedance of this type of filter rectification circuit is high,
the maximum acceptable input voltage is usually high.
The value of the DC control voltage may be equivalent to the acceptable RMS
voltage for alternating control.
The value of resistor R1 can be determined, when R2 is known, given that the
input voltage on the relay is defined approximately across the bridge R1 – R2
by the equation:
VDC . (R2 + R1) = VAC . R2
R1 = (VAC – VDC) . R2
VDC
AC Input Circuit Wiring
A relay with an AC input circuit can be controlled like a DC-input relay using
a dry contact (switch or electro-mechanical relay). A solid-state control can be
obtained by inserting the SSR in the anode or the cathode of an SCR or a
triac.
If VDC << VAC
R1 = VAC . R2
VDC
AC Input Connection for an SSR
1
2
1
switch
2
relay
3
AC-input SSR
and C = 4,7 µF (typical value)
As for DC input circuits, it may be helpful to mount MOV (Metal Oxide Varistor)
components in the AC control circuit to limit possible effects of line surges.
These precautions will be detailed in the following pages.
3
18
19
The Trigger Circuit
The trigger circuit provides the interface between the input circuit and the
output circuit to best adapt the switching parameters to the power element.
This circuit also determines the switching mode of the solid-state relay. These
modes will be detailed later. At this point, it is enough to say that the main,
most frequently used modes are:
4
1
2
3
t
3
5
4
2
5
• Instantaneous Switching
• Zero Voltage Switching
Principle of Instantaneous Switching
1
2
3
4
5
1
0
1 control signal
1 input circuit
6
1
2 trigger circuit
2 current
3 thyristor
3 main supply voltage
4 load
4 I load
5 transmission delay = < 0.1 ms
5 the current is only blocked here
7
8
General Structure of a Solid-State Relay. The Trigger Circuit
1 input circuit
5 protection
2 optical coupling
6
3 trigger circuit
7 main AC supply
4 switching circuit
8 solid-state relay
load
In this instance, the trigger function simply updates the switching signals.
Instantaneous switching relays are generally used in applications requiring
very short response times or for switching resistive or inductive loads as long
as the power factor is less than 0.7 (significant current-voltage de-phasing).
The major drawback of asynchronous switching is its extreme sensitivity to
stray pulses at the input which may cause the relay to switch as soon as the
nominal voltage thresholds are reached. Special attention should therefore be
paid to protecting the input on this type of relay, especially in an environment
with a high level of electro-magnetic interference.
Instantaneous Switching (Asynchronous Switching)
In a solid-state relay with instantaneous switching, the time interval between
the application of the control signal and the output closing (turn-on response
time) is very short (around 0.1 to 0.2 ms) and is only limited by the response
time of the SSR components.
As a result, closing can occur at any point on the sine wave, since the output
element (SCR or triac) will always be opened or blocked again the first time
the load current next passes through zero once the control signal is no longer
applied, i.e. a response time on opening which will always be less than a half
period (10 ms max. for 50 Hz).
20
Zero Voltage Switching (Synchronous Switching)_
Synchronous switching is characterised by switching the power to the load
only when the main supply voltage first passes through zero following the
application of the control signal on the input. As in asynchronous mode, the
SSR will open the first time the load current passes through zero following the
removal of the control signal.
However, the relay is not turned on when the supply voltage reaches 0 volt
precisely, but at a voltage which is close enough to activate all the SSR's
internal circuitry. This voltage is known as synchronisation voltage or
synchronism, and its value is close to +/- 15 volts, depending on the SSR
internal circuits, corresponding to a negligible phase delay of around 2 to 3°
for a main supply voltage of 240 VAC.
21
4
3
Principle of the Trigger Circuit in
Synchronous SSRs
1 voltage at the SCR terminals
2 load current
1 input circuit
1
2
4
3 control voltage
4 main supply
2 updating circuit
5 possible switching window
3 detecting passing through zero
4 load
3
6 trigger voltage (minimum)
7 max voltage drop (when closed)
2
As a result, any control signal applied to the SSR when the output voltage is
above the synchronisation voltage, will only be taken into account at the start
of the following half-period, i.e. a response time of up to 10 ms for a 50 Hz
supply (8 ms for 60 Hz). This property endows synchronous solid-state relays
with very good immunity to electromagnetic interference at the input.
8
In this switching mode, the voltages switched at turn-on are low, so the power
ratings involved during this phase are also low with the result that
electromagnetic radiation generated is kept to the minimum.
The voltage at the relay output terminals is shown below.
This type of relay is therefore strongly recommended for switching heating
elements, even at high frequencies, while limiting electromagnetic radiation.
Similarly, using a synchronous relay will limit the inrush current on igniting
tungsten lamps with low resistance when cold, which considerably extends
the life of the filaments.
22
conduction zone
1
6
7
5
9
8
Electrical Signals at the Relay Terminals
23
The Output Circuit
DC SSR with MOS FET Output
Transistor for Currents Generally
Below 10 A
+
2
3
1
2 load
6
1
2
3
4
1 control electronics
5
3 power supply voltage
7
8
General Structure of a Solid-State Relay ; Output Circuit
1 input circuit
5 protection
2 optical coupling
6
3 trigger circuit
7 main supply
4 switching circuit
8
load
solid-state relay
The output circuit of a solid-state relay is nothing more than an element which
switches the power to the load; and we will continue to call the solid-state
relay by the type of signal it switches - either AC or DC. For economic
reasons, the technologies employed do not allow AC or DC switching with the
same type of relay as is the case with an EMR. Similarly, for reasons of
thermal dissipation and also cost, the electronic structure does not allow
multiple outputs either, as is possible with EMRs.
DC Output (DC SSR)
In an SSR with a DC output, the power element usually comprises either a
bipolar transistor or an FET transistor (Field Effect Transistor) depending on
the specifications required.
To obtain rapid response times with limited currents (< = 10 A) in an SSR with
a DC output, a relay with a bipolar transistor as the output should ideally be
used. The FET transistor should be chosen for applications requiring very low
leakage current (< 10 µA) with limited temperature rise despite significant
load currents (30 to 40 A).
The output element in a DC SSR may be wired as a 2-wire or a 3-wire output.
The 2-wire output is mainly used for SSRs in "Hockey Puck" housings with
output currents below 10A, while the 3-wire solution is often reserved for I/O
modules.
In the 2-wire configuration, the load can be mounted in series with either of
the other outputs as long as the power supply voltage polarities are complied
with.
Where the output element is a bipolar transistor, the 2-wire connection
prevents the transistor from being totally saturated and the voltage drop at its
terminals when closed continues to be around 1.2 to 1.5 Volts DC; which is
acceptable for most applications, especially where the load power supply
voltage is fairly high (> 24 V DC).
Where the operating voltage at the output is lower, as for example in digital
interfacing systems, a residual voltage of 1.5 V DC may be too high; in this
case a 3-wire connection should be used, allowing total saturation of the
output transistor with a residual voltage in this case of 0.2 to 0.3 Volts DC. I/O
modules are often used as an isolated interface between 2 low-level digital
systems or subassemblies; so they will generally be wired in the 3-wire PNP
or NPN mode.
2
R>>
3
1
4
DC SSR with Bipolar Output Transistor
+
I = Ic + i
2
I
c
NPN-Output Connection
4
SSR in Hockey Puck Housing
1 control electronics
3
1
-
2 load
1 electronic trigger
3 power supply voltage
2 power supply
4 SSR with DC Output
3 output
4 logic ground
24
25
When closed, the MOSFET transistor becomes the equivalent of a low value
resistor (50 to 80 mW). The residual voltage to be taken into account depends
on the current in the load according to ohmic law. A current of 20A, for
example, will result in a residual voltage of 1.2 to 1.5 Volts.
SCR Voltage-Current
Characteristics
I
2
1 peak reverse voltage
3
1
AC Output (AC SSR)
VRM
2 minimum holding voltage
V
VB0
In an AC SSR, the switching element may consist of either SCRs or a triac,
selected according to requirements.
3 turnaround voltage
4 blocked state
4
It is preferable to use two SCRs mounted back-to-back rather than a triac in
AC SSRs with a high nominal output current (> 50 A) or high peak voltage
(1200 to 1500 V), while a triac is preferable for versions which are cheaper
but do not perform as well in terms of current or dv/dt characteristic.
The SCR
The SCR is most often used as the switching element in an AC SSR, because
of its ability to switch currents as high as several thousand amps while limiting
the voltage drop at its terminals to low levels (~ 2 volts max) and to withstand
non-repetitive current peaks of 10 or 12 times the nominal current for which it
is designed. The SCR also withstands peak reverse voltages as high as
several Kv.
3
3
3
5 gate active characteristic
5
The SCR can be characterised as an assembly of two transistors mounted in
the same package and on the same silicon chip so as to create a positive
inverse feedback between them. When V1 starts to conduct following a pulse
on its base or gate, it creates a current in the base of transistor V2 causing
the current in the V2 collector (= base of V1) to increase, resulting in further
saturating V1… (positive inverse feedback). The SCR can then no longer be
blocked by its gate.
The SCR is equivalent to a switched diode, and yet it has the serious
disadvantage of being a one-way component. To overcome this disadvantage
and enable operation on positive and negative half waves, two SCRs are
usually mounted in parallel and in opposition (back-to-back mounting) in
solid-state relays.
Principle of the SCR
2
P
N
P
Solid-State Relay with
Back-to-back Output SCRs
V2
2
N
P
N
2
1 stray coupling
V1
24
4
1
1
2 gate
1 control circuit
3 anode
2 AC load
4 cathode
4
3 output element
3
The SCR is equivalent to a one-way diode which blocks the current in both
directions when its control input (gate) is not active. The SCR is turned on
(anode to cathode) either by a short pulse on the gate, or by exceeding the
turnaround voltage VB0 at its terminals, and can then only be stopped by
reversing the voltage or reducing the current between anode and cathode
below the minimum holding value IH for a minimum period (~ 15 µs).
26
The Triac
Semiconductor technology has made it economically feasible to integrate the
equivalent of two opposing SCRs on the same chip, controlled by a single
gate: thus the triac was born. However, the triac is much more limited in
current (max. = 40/50 A), in voltage (max. = 800 V) and in maximum dv/dt
characteristic. This last parameter, which is considered to be of prime
importance, can be improved by using filters which may be integrated in the
triac; such triacs are commonly known as "snubberless" triacs.
27
dv/dt Limitation on Switching
I
Triac Voltage-Current
Characteristics
2
1
VB0
V
VB0
1
2
1 gate
2 gate active characteristic
3 blocked state characteristic
3
The same phenomenon of uncontrolled SCR turn-on can occur during
breaking at current zero on an inductive load. In this instance, when the SCR
opens on passing through current zero, the instantaneous value of the main
supply voltage will appear at the SCR terminals instantly. The greater the dephasing between the current and voltage, the more abrupt the voltage
variation, and a capacitive coupling effect between anode and gate may turn
the SCR back on, causing an opening fault. 20V/µs is a typical value for the
dv/dt parameter on switching a triac.
However, this type of fault is only encountered very rarely and when switching
highly inductive loads with currents close to the nominal current values. Using
"snubberless" components also reduces this phenomenon.
There are other power switching solutions, such as the alternistor (2 SCRs
mounted back-to-back and controlled by a triac); their operation is similar to
that of back-to-back SCRs.
Main Limitations of Power Components _
The characteristics of SCRs and triacs, and of any switched component in
general, are particularly well-suited to switching high currents at high speeds.
In addition to the usual restrictions such as maximum current, voltage, …,
these components have other specific limitations which must be taken into
account in optimising their implementation and to avoid certain malfunctions.
IT
5
6
4
VT
3
SSR
7
2
Static dv/dt Limitation
1
The positive inverse feedback of SCRs and triacs is the source of the
particularly attractive characteristics of these components (max. current and
switching speed), but also of the component's dv/dt limitation. Applying too
sudden a variation in the voltage between the anode and cathode can result
in uncontrolled SCR closure, owing to a capacitive effect between the anode
and gate. If C is the stray capacity, the current across this capacity, and hence
the base of V1 (see “Principle of an SCR” diagram) will be defined by:
5 mains supply
1 ON state
2 load current
6 (dv/dt)c
I = C. dv/dt
3 voltage at the SCR (triac) terminals
7 dI/dt
Depending on the value of C and of dv/dt, the current may rise high enough
to cause the SCR to turn on. The maximum voltage variation which can be
applied to the SSR terminals without causing uncontrolled closure is
generally stipulated in the specifications (static dv/dt) and is given in volts per
micro-second. 500 V /µs is a typical value for an SCR, while the dv/dt for a
triac would be about 200 V/µs. By way of comparison, the maximum dv/dt of
the 230 V 50 Hz main supply on passing through zero is 0.1 V/µs.
4 de-phasing
8
These dv/dt values may be increased significantly by adding RC-type filters
internally or externally. However, these filters known as "Snubbers" have the
disadvantage of increasing leakage current.
28
8
dv/dt Phenomenon on
Switching to a Highly
Inductive Load
blocked state
di/dt Limitation
Because of their internal semiconductor structure, triacs and SCRs have a
limited ability to absorb a di/dt current variation, at the risk of being destroyed
by overheating in the semiconductor junction. The maximum di/dt value
acceptable to an SSR depends on the output element and ranges from 10 to
200 A/µs.
29
In real-life applications, these values are rarely reached because of stray
inductive (although weak) and resistive impedance present in all wiring and
which limits the rate at which the current increases.
Thermal Characteristics of Solid-State Relays
For a semiconductor to provide nominal performance, it is essential that the
operating temperature does not exceed a maximum value specified by the
manufacturer.
When the circuit opens (when the current reaches zero), the RMS value of the
(di/dt)c parameter can be calculated as a function of the current in the load
and of the frequency (50 Hz in the Table below).
IT (RMS)
(dl/dt)c theoretical (A/µs)
5A
10 A
20 A
50 A
2.2
4.5
9
22.5
In most semiconductors, this maximum temperature is never reached in
nominal utilisation conditions. Beyond a given utilisation power, it is often
necessary to cool the semiconductor using a dissipator.
The SSR is a power element which is prone to temperature rise, and it has to
be cooled sufficiently to avoid the junction temperature of the power
semiconductor element mounted in the relay exceeding a temperature of
around 120, 125°C.
Leakage Current
AC solid-state relays always have 2-wire output elements and have the
disadvantage - as with al electronic components of this type - of leakage
current which is usually between 2 and 10 mA depending on the components
used, and which must be taken into account when integrating them into a
control system (see special precautions for 2-wire sensors). Using "snubbers"
or external RC filters increases leakage current still further.
General Theory of Thermal Conduction_
The electric power applied to the control input of a solid-state relay is usually
very low, so the power dissipation of the relay is basically related to the
current in the power element, and consequently in the load, per the formula:
Pd = V0Ic
where:
V0 is the voltage drop at the SSR terminals
(1.25 to 1.4 V for an SCR - 1.65 to 1.85 V for a triac,
on average for Imax)
and
Ic is the current across the relay
• Thermal relationships are governed by a law similar to ohmic law and
thermal resistance is defined by the equation:
R=
∆T/Pd where Pd is the power dissipated and T the difference in
temperature between the two measurement points.
5
2
3
4
1 temperature
6
2 semi conductor
3 substrate
TJ
4 base plate
5 dissipator
1
TC
RθJA =
TS
Tj - TA
Pd
6 ambient air
TA
TJ
RθJC
RθCS
RθSA
7
30
31
TA
Thermal Dissipation
• The equation for the thermal schematic above would be:
GN 25 AMP (1200 & 600 V)
25,0
RθJA =
=
RθJC + RθCS + RθSA
TJ – TA
(°C/W)
Pd
20,0
A
18,0
1 Load current (A)
15,0
2 Ambient temperature (°C) T
1
RθSA =
TJ – TA
Pd
3 Dissipators
10,0
– (RθJC + RθCS)
5,0
0,0
where
RθJC =
20 25 30 35 40 45 50 55 60 65 70 75 80
thermal resistance between the semiconductor junction and the
SSR housing.
This value is given in the SSR technical specifications.
RθCS =
thermal resistance between the relay housing and the heat sink. If
the relay has been mounted correctly using heat transfer
compound, this value is close to 0.1 °C/W. Without heat transfer
compound, the resistance can be multiplied by a factor of 10 or 20,
i.e. 1 to 2°C/W.
RθSA =
thermal resistance of the heat sink. Given by the manufacturer, the
resistance values are generally between 0.5 and 3°C/W depending
on the model.
Simplified Calculation / Use of Graphs_
While the above calculation is very accurate, in practice it entails a tedious
process involving measuring temperatures using thermocouples or other
such devices. To make this calculation easier and assist in selecting a heat
sink, CROUZET has devised graphs which are easy to use to select the
optimum heat sink for its entire range of solid-state relays.
Knowing the load current (A) and the ambient temperature (T), the minimum
thermal characteristic of the heat sink to be used can be determined from the
graph below. To guarantee that the operating margin is sufficient, always
select a heat sink with a thermal coefficient above that passing through point
(T – A).
2
T
3
1,0 C/W
1,5 C/W
1,7 C/W
2,0 C/W
3,0 C/W
Draw a horizontal line through the current used, a vertical line through the
ambient temperature, and select the curve above the intersection of the two
lines: this defines the type of heat sink to be used.
E.g.: T = 50°, A = 18 A
➪ Draw the A and T lines
➪ Select the 2°C/W heat sink
To define the optimal dissipator, now simply consult the dissipator
manufacturer catalogues, and according to the dissipator profile selected,
define the length necessary to obtain the desired thermal coefficient (profile
as a function of length).
Remember: As a general rule, these coefficients are given for use in vertical
position, which facilitates thermal exchange. Reduction coefficients are
applicable for utilisation in other positions; these coefficients are explained
below.
Special Utilisation Conditions
Forced-Air Cooling
Forced-air cooling around the solid-state relay and the heat sink ensures
better thermal dissipation for a given size.
The thermal resistance of any profile in forced-air cooling conditions is
described by the equation:
Rth forced air = α. Rth
32
natural convection
33
α Coef.
Air Speed in m/s
and h = 1.42 k
a 1
in vertical position
0.5
0.8
0,8
h = 1.21 k
in horizontal position, facing upwards
1
0.64
0,6
h = 0.63 k
in horizontal position, facing downwards
1.5
0.53
0,4
2
0.42
0,2
3
0.3
4
0.25
6
0.2
0
0
1
2
3
4
5
6
v (m/s)
values in which K is a coefficient dependant on the temperature and
the geometry of the heat sink
The full calculation shows that there is an increase of the order of 7 to 10%
in thermal resistance between the vertical and horizontal/upwards positions.
Effect of Forced-Air Cooling on Heat Sink Performance
Dissipator Orientation
In general, the thermal resistance of dissipators are always given for optimal
natural convection and in vertical position.
The thermal performance of a heat sink is seriously downgraded when its
orientation is changed from vertical to horizontal, facing down, or horizontal
facing upwards.
A similar calculation leads to an increase of the order of 100 % in thermal
resistance between the horizontal/upwards and horizontal/downwards
positions.
The thermal resistance values of heat sinks are generally expressed by the
manufacturers based on the position providing the best thermal dissipation
(vertical position). To simplify, the following correction coefficients can be
applied depending on the mounting position :
Mounting
Correction Coefficient
Vertical
1
Horizontal, upwards
1.1
Horizontal, downwards
2.1
Effect of the Temperature Difference T
a) Vertical Position
b) Horizontal, Facing
Upwards
c) Horizontal, Facing
Downwards
Mounting Type
Globally, thermal resistance can be described by the simplified equation
below :
I
Rθ =
m2
hc . (total surface of the heat sink)
where
The thermal resistance of a heat sink is defined by the formula Rθ = T
P
where T is the difference in temperature between the ambient air and that of
the heat sink.
The values given by the heat sink manufacturers are defined for the given ¢T
conditions (usually 50°C). Consequently, it has to be corrected to take
account of the real ¢T value and to define precisely the optimum heat sink for
the application under consideration. The following rule applies without
exception :
"The higher the temperature difference T, the higher the thermal dissipation
capacity and conversely".
Rθ is expressed in °C/W
hc = thermal transfer coefficient
expressed in W/m2 °C
34
35
Effect of the Dissipator's Surface Condition
Utilisation with a Baseplate_
The total thermal emissivity of a material is a complex function of the
dissipator profile (fin shape, position,…), of the material used, but also, and
for a large part, of the surface condition.
Solid-state relays are often mounted on metal plates in cabinets for reasons
of simplicity as much as cost, the mounting plate serving in this case as a
dissipator. The thermal resistance of the baseplate can be calculated from the
graphs below, provided for aluminium and steel.
Today, most standard dissipators for solid-state relays are either made of
extruded aluminium either untreated, chromium-plated (alodine) or anodised ;
and each of these surface treatments produces a difference appearance and
emissivity.
It is important firstly to note that most thermal radiation in a material occurs
mainly outside the spectrum of visible frequencies; It can easily be deduced
therefore, that the colour of the material will be of little significance in its total
thermal emissivity. So a heat sink made of anodised aluminium will perform
in the same way whatever the colour of the anodising (black, green, blue).
10
8
10
8
6
6
5
5
0,5
1
1
4
0,5
3
1
2
1
4
2
3
3
3
5
2
2
5
The variations in performance related to the heat sink's surface treatment
also depend on the shape of the fins. However, the following averages can be
considered:
Relative Performance
Anodised Chromium-plated
(Alodine)
Untreated
Aluminium
100 %
87 %
92 %
It should be noted that with forced-air cooling, the thermal transfer between
heat sinks and the environment take place mainly by convection rather than
by radiation; so the effect of the surface condition is negligible, and anodising
will not provide any additional advantage in terms of performance.
1
10
12
14
16
18
20
22
24
26
28
1
30
2
thermal resistance
2
dimensions / cmxcm
3
thickness / mm
12
14
16
18
2
Thermal Resistance of an Untreated
Aluminium Baseplate (Vertical Position)
1
10
20
22
24
26
28
30
e
x
x
Thermal Resistance of an Untreated Steel
Baseplate (Vertical Position)
As an example, an aluminium plate 5 mm thick and 18 cm square will have a
thermal resistance of around 3°C/W, while a plate of the same size, but made
of steel, will have a higher thermal resistance of around 4°C/W under identical
conditions.
It should be noted that the rules presented above regarding the correction
applicable in the event of a change in orientation also apply here, despite
some corrections which we consider to be minor.
N.B. : Frequently in industry, there is a sheet-metal mounting plate at the back
of the control unit which can be used as a dissipator, or even the rear plate of
the cabinet itself may be used. In this case, to increase thermal performance,
special care must be taken to remove entirely all traces of the paint which is
usually applied to the unit elements.
36
37
Using One Dissipator for Several Power Sources_
The characteristics of dissipators are generally given considering that a single
heat source is situated in the centre of the cooling device.
Where several SSRs are mounted on one dissipator, it is easier to define the
dissipator by dividing it into sections of equal size for each SSR. As an initial
approach, each section can be considered to be independent.
However, this method is only valid if the relays are sufficiently far apart. For
single-phase relays, this method can only be considered valid where the
spacing between two SSRs is at least 10 cm.
For a multiple-source application with increments of less than 10 cm/SSR, the
thermal resistance is calculated using the equivalent thermal diagram for the
assembly. The example below presents three relays mounted on one
dissipator. The thermal diagram can be represented as shown in the Figure
below :
TJ(n)
Junction temperature of each heat source
TC(n)
Housing temperature of each heat source
TS
Temperature of the hottest point on the heat sink
RθJC(n)
RθCS(n)
Thermal resistance of the junction - housing for each relay
Thermal resistance of the relay housing - heat sink for each heat source
RθSA
Thermal resistance of the heat sink.
To calculate the optimum RθSA , run the following equation on each heat
source and select the dissipator with the lowest resistance RθSA.
RθSA
= TJ1 – (RθJC(n) + RθCS(n)) P1 - TA
P1 + P2 + P3
To account for the fact that the sources are distributed at different points of
the heat sink, thus ensuring better thermal dissipation, the following may be
used:
RθSA =
TJ1
RθJC1
TC1
Examples
RθCS1
TJ2
TJ3
RθJC2
RθJC3
TC2
calculated RθSA/0.7
RθJC2
TC3
TS
RθSA
TA
RθCS3
Thermal Circuit with Several Heat Sources on One Heat
Sink
SSR
Power Dissipated
Max Tj (°C)
RθJC
#1
10 W
120
1
#2
20 W
120
1
#3
10 W
120
1.2
RθSA1 =
RθSA2 =
RθSA3 =
RθSA =
38
120 – (1 + 0.1) 10 – 40
= 1.725 °C/W
40
120 – (1 + 0.1) 20 – 40
= 1.45 °C/W
40
120 – (1.2 + 0.1) 10 – 40
40
1.45/0.7 = 2.07 °C/W
39
= 1.675 °C/W
Mechanical Design of Solid-State Relays
Special Mounting Precautions_
In addition to any theoretical considerations addressed above, it is essential
that a minimum of additional elementary precautions be taken when mounting the relays.
The ability of an SSR - dissipator assembly to dissipate best the heat
generated by Joule's law when a current passes through the junctions of a
semiconductor, plays a critical role in the operating reliability of the relay.
In particular, when using
a dissipator - but also in
all other mounting configurations
natural
convection must be able
to occur freely without
external
interference.
Consequently, be sure to
install the heat sink fins
vertically.
This ability to dissipate heat depends of course on the size of the dissipator,
on the ambient temperature, on the quality of the SSR - dissipator assembly
… all of which were addressed in chapter "Thermal Characteristics of SolidState Relays" above.
The equation for the thermal diagram relating to this type of assembly is:
RθJA = RθJC + RθCS + RθSA
}
Not only should the
elements be positioned so Mounting a Solid-State Relay
that they do not obstruct
convection, but also, heatsensitive elements (electronic devices, etc.) should not be located in the
convection airflow.
When mounting a relay on a heat sink or any other metal support used as a
thermal dissipator, heat transfer compound should be used to ensure
optimum, very low, thermal resistance ( approx. 0.1°C/W) and even contact
between the base plate of the SSR and the support or heat sink.
To avoid distortion in the SSR base plate - which may be aluminium, or
copper, or steel, tightening torque on the mounting screws should not exceed
around 1.1 to 1.3 Nm. This torque value will ensure even distribution of the
heat transfer compound without air bubbles and without distortion. Like
CROUZET, other manufacturers now offer a thermal interface consisting of an
aluminium plate with a coating of special thermal compound in the form of a
paste on both sides; this paste liquefies at 50°C and above, thus ensuring
very low thermal resistance. This solution is extremely effective from the
thermal point of view, and also offers the advantage of being clean and easy
to use.
Depends on the
user's choice
Defined by design
and RθJA consists of two elements, one of which defines the ability of the
relay to transfer to the base plate the heat given off as a result of the Joule
effect at the junctions. From the user's viewpoint, for a given assembly, this
equation results in the need for an external heat sink whose size and
performance increase with RθJC thermal resistance. So manufacturers have
tried to reduce this resistance as far as possible to improve the dissipation of
Joule-effect heat in relay power elements. This is why CROUZET has
introduced the latest generation of "DCB" technology (Direct Copper
Bonding) into its solid-state relays.
DCB Technology
Assembling a chip on an electric circuit support of the ceramic type (BeO or
AI203) usually involves sandwiching several different layers of material,
depending on the need and the performance required.
Traditional assembling usually used for low and medium power chips,
consists of a series of layers held together either simply by glue, soldering or
vacuum evaporation.
The number of interfaces in the assembly can increase rapidly, with limited
thermal characteristics.
In DCB technology, the sandwich is achieved using a hot compression
technique, which causes the copper to diffuse into the upper layers of the
ceramic; this renders the assembly thermally uniform and virtually insensitive
to differential thermal expansion between the various materials.
40
41
Load Switching on Solid-State Relays
DCB Assembly
To optimise still further the thermal
characteristics of our relays, while
guaranteeing a minimum electric isolation of
4 KV between the load circuit and the SSR
housing, CROUZET has elected to reduce
the thickness of AI203, ceramic - a material
which, contrary to BeO (Beryllium oxide), is
not poisonous - to 0.380 mm instead of
0.63 mm.
For high-power SSRs, inserting a thick
copper plate in the base of the housing, in
particular the 75, 100 et 125 A versions,
improves heat diffusion to the heat sink.
Among the major advantages of this
technology, the following deserve special mention :
•
•
•
•
•
Improved thermal resistance,
Higher electric load capacity,
Operating temperature up to 800°C (excluding the electronic chip),
Improved reliability since there are fewer interfaces,
Lower assembly cost and material savings.
If, as a general rule, using an EMR relay does not cause particular problems
in the majority of cases, the difficulties with installation which are often
encountered in solid state relay applications basically arise from lack of
familiarity with the operating conditions for the SSR when connected to the
various types of load.
Resistive loads
In an electrical circuit with purely resistive load, the instantaneous current in
the load is always proportional to the instantaneous voltage at its terminals.
In this case, current and voltage are in phase and are linked by the equation:
V=R.I
where
R is the resistance of the circuit.
If the switching of such a load occurs by means of an SSR, use of a relay with
synchronous switching is strongly recommended:
When the current is in phase with the voltage, the relay will open and close
when voltage and current pass through zero. In these conditions, relay
operation will be such that :
• dv/dt will be limited by the power supply (105,000 v/s for a 50 Hz 230 V
supply, ie 0.1 v/µs),
• di/dt will be limited by the impedance of the load and of the output circuit.
1
1 power supply voltage
Diffusing copper into the upper layers of the ceramic also endows the
assembly with a mechanical resistance which is much better than that of the
traditional, soldered assembly. So the relay output terminals can be
connected directly to the chip, thus significantly improving the electrical
capabilities and thermal dissipation.
This technology enables Crouzet to lay claim to having the best RθJC thermal
performance of all the solid-state relays currently available on the market.
2 control pulse
3 load current/voltage
4 load
5 voltage at the SSR terminals
2
4
Synchronous switching on resistive load
3
Technology
10 Amp
25 Amp
50 Amp
75 Amp
100 Amp
125 Amp
Traditional
1.48
1.02
0.63
0.31
0.28**
0.22
Crouzet GN DCB
0.40
0.40
0.25
0.155
0.155
0.15
42
5
Electro-magnetic emission is then limited both on closing and on opening
because of the low power which is present during switching.
43
This peculiarity of operation in synchronous SSRs on resistive loads makes
them particularly suitable for convector heater type applications where the
switching frequency can be very high.
Connection to a 3-phase supply
The same connection rules as for the single phase supply apply to each
phase of the 3-phase supply.
In the case of a resistive load, the simple acceptance of maximum current and
voltage parameters results in trouble-free SSR operation in the majority of
applications.
In the case of a balanced 3-phase supply without neutral (star or delta
connection), switching can occur on 2 phases instead of 3.
Attention should still be paid to the maximum voltages which can arise
following a short-circuit or supply imbalance at the SSR terminals. As one
phase is then permanently connected to the device, it is important to check
that measures have been taken to ensure the safety of personnel.
Exception concerning incandescent lamps
Incandescent lamps represent the worst example of resistive loads due to the
low resistance, particularly of the tungsten filaments in cold state. This may
cause overloads on ignition of up to 10 or 20 times the current in steady state.
3
These wiring methods apply to resistive loads, and also inductive loads.
Current profile for an
incandescent lamp
18
L1
16
1 12
8
4
L2
2 ignition
L3
3 permissible voltage surge
(1 cycle 20 ms)
4
0
1 amps
ms
0
10
20
30
40
50
60
70
80
90
100
CROUZET
84 068 453
L1
400Va
400Va
CROUZET
84 068 453
L2
400Va
400Va
L3
N
2
1
L1
L2
Depending on the characteristics of the lamp, the nominal current may only
be reached after a number of cycles. It is then possible to check that the
characteristics of the selected SSR allow the corresponding overload to be
tolerated.
L3
400Va
400 V
If IRMS is the rms load current in steady state, the max current on ignition will
be:
IMAX = 20 .
2 . IRMS
If a synchronous SSR is used, the di/dt ratio can then be limited and the
lifetime of the lamp will be greatly prolonged. To avoid possible destruction of
the SSR, it is a good idea to place a fuse in series with the lamp, which will
not only protect the SSR at the moment of ignition, but also in the event of the
power supply wires accidentally short-circuiting, if the filament breaks.
44
1 230 V a max.
2 230 V normal state
400 V unbalanced state
4 current in steady state In = 2A
2
Wiring on a 3-phase supply
45
The problem most commonly encountered with an inductive load concerns
the dv/dt parameter at the time of breaking. At this moment the load voltage
is applied to the SSR terminals for an instant. Dephasing may lead to the dv/dt
gradient then being very high, which would turn the relay back on (see
Figures 3.16 and 5.5). The use of filters mounted in parallel on the SSR output
can limit or even eliminate this phenomenon.
Inductive loads
The principal effect of an inductive load on an AC circuit is that dephasing
occurs between the voltage and the current. This delay is expressed as a
phase angle of between 0 and 90° or even as a power factor Pf of between 1
and 0 respectively. This power factor is also equivalent to the equation:
This problem is particularly important in SSRs with Triac outputs, as their
dv/dt characteristics are more critical than those obtained with SCR outputs.
Pf = R/Z = cos ϕ
where
R is the resistance of the circuit
1 supply voltage
Z its impedance
1
ϕ phase angle
2 control pulse
7
For a purely resistive load Pf = 1
3 load current and voltage
4 voltage
5 current
a
6 dv/dt >>
1
R
7
2
L
7 t
a
3
Z = R 2 + X2
X = Lω
1 AC supply
4
5
6
Fp = Cos ϕ = R/Z
7
Inductive load on solid state relay
and circuit phase diagram
Z = R 2 + L 2 ω2
Lω
Cos ϕ = FPpf
ϕ
Synchronous switching on an
inductive load 0.7 < Pf < 1
As with resistive loads, the use of an SSR with synchronous switching
is recommended in the case of a load with a power factor Pf between
0.7 and 1.
R
Example of loads with Pf < 0.7
Synchronous solid state relays are designed for operation in normal
conditions with loads which have a power factor of between 1 and 0.5 (phase
angle = 60°). This range of power factors, with just a few exceptions,
encompasses practically all inductive loads found in actual applications.
Nonetheless, it is advisable to limit the use of synchronous SSRs to
applications with a cos ϕ ≥ 0.7 (resistive load with low inductive impedance).
Take care to avoid saturation of inductive loads, as impedance may then
become extremely low (residual ohmic value) and lead to overloads or even
to the destruction of the solid state relay.
46
Where the power factor < 0.7, a synchronous relay may not turn on in spite of
a control signal being applied at the gate. If the impedance of the load is too
inductive, the load will strongly oppose the establishment of a current in the
load. If the minimum holding current has been unable to establish itself during
the active switching zone, the output element cannot switch, or only switches
partially for half a period. This phenomenon often occurs in systems with low
load currents.
47
This problem can be resolved by placing a "snubber" type filter or shunt
resistances in parallel with the load to allow faster establishment of a
minimum current in the SSR.
B 3
1 M curve
4
4
1
1 minimum holding current
3 (induction)
5
Br
Synchronous switching on an
inductive load Pf << 0.7
2 hysteresis cycle
6
1
5 S curve
6 H magnétique field
2
V
7 voltage at transformer
terminals
2 load current
3 command
5
4 saturation zone
7
4 I < holding current
➡ no activation
2
3
5 active zone
In this case, an SSR with asynchronous switching is preferable so as to
allow the current the time to establish itself correctly. In addition, as
with a higher power factor, attention must be paid to the dv/dt
parameter.
Example of switching on a transformer
The transformer is a special, extreme case of inductive loads which can
become saturated with residual magnetism.
The magnetic characteristic of a transformer can be described by its B/H
curve (Induction / magnetic field) as illustrated below. H is the magnetic field
with the same sign as the voltage applied to the transformer terminals and B
is the induction present in the transformer.
In steady state AC, the magnetic cycle of the transformer describes the M
contour. If the voltage at the transformer terminals is broken following a final
positive half-wave, the residual magnetic state of the transformer will be fixed
at point BR.
Magnetic cycle of a transformer
T
If the first half-wave is positive at the next power-up, the magnetic cycle of the
transformer will follow on the first half-wave of the S curve leading to high
saturation and a drop in the impedance of the primary coil, resulting in a high
voltage surge. This saturation will tend to diminish during subsequent cycles
until normal operation is resumed (in line with the M curve) in steady state, as
described earlier.
During saturation, and depending on its intensity, the circuit impedance may
be equal to just the ohmic impedance, which is generally very low, of the
transformer primary with additional external resistors on the circuit.
Example:
Supply voltage
:
230 VAC
Coil resistance
:
1Ω
External resistance
(wire + connection)
:
0.5 Ω
Overload peak current:
IS = 230 .
2 / 1.5 = 216 Amps
Placing a low-level resistor in series with the transformer will limit the
overload. This resistor should be chosen on the basis of not disturbing normal
transformer operation.
48
49
Current across the
transformer
2
3
1
PUSABLE :
usable power
η
:
efficiency
cos ϕ
:
power factor
1 current
U
:
supply voltage
2 current surge
I
:
current per phase
3 current in steady state
PUSABLE, η and cos ϕ are usually indicated on the motor name plate.
Besides, the power factor for an asynchronous motor is a function of
the mechanical load and may be considerably less than 0.4 or 0.5 during
no-load operation.
In this example, the preferred option is an SSR which can at least
tolerate a current surge during 1 cycle of at least 200 A.
Consequently, the use of asynchronous relays is recommended for
switching motors.
If a synchronous relay is chosen, there is a 50-50 chance of transformer
saturation occurring, resulting in an overload on the first half-waves.
When a transformer is not charged, it constitutes a pure inductance. The
current across it is then the magnetising current which is in quadrature with
the voltage. In these conditions maximum voltage switching then limits the
overload of the magnetising current by starting at current zero. But this option
can be varied according to the load on the secondary, which will impose zero
switching if it is capacitive.
In these conditions, instantaneous switching, although not ideally
suited to all situations, is the solution which suits most transformer
applications.
Switching motors
When determining which relay to use, both the size of this starting overload
and that of the heatsink associated with the relay should be taken into
account, especially if the overload occurs repeatedly following numerous
stops and starts (repetitive cycles).
Starting curve
4
1
1 motor current
2
A motor is an inductive load with its own laws, which is a function of the
mechanical load with which it is associated. On starting, a 3–phase motor can
therefore have a current across it of 6 or 8 times the nominal current in steady
state. For some single phase motors, this overload may be as high as 10 IN.
The current absorbed by a motor is defined by:
I=
Starting state
2 I surcharge
3 I Num.
3
T
t1
t2
t3
t4
t5
PUSABLE
Approximating the current to square root signals, the rms current can be
defined by:
U . η . cos ϕ
for a single phase motor.
or:
I=
4 current approximation
IRMS =
I2(load) . t1 + I2(nominal) . t2 + ...
PUSABLE
t1 + t2 + t3 + ...
3 .U.η.cos ϕ
t1 starting time
t2 nominal operating time
for a three-phase motor.
t3 pause time
50
51
This value can be transferred onto thermal charts in order to determine the
size of the heatsink, using the method described above.
Reversing motor
1
L1
The relay is determined by the value of the starting current and the operating
voltage.
L2
2 . Valim
C
V2
R1
Some motors incorporate a starting winding which is switched by centrifugal
power, which has the effect of increasing the current noticeably on power-up.
To limit electro-magnetic radiation, this winding can be switched by a second
SSR.
1 rotor
R2
2 supply (V power supply)
V1
V1 = Valim
2
Motor with starting winding
1
2
The voltages at the terminals of L1 and L2 are in quadrature and the voltage
VSSR at the terminals of the open relay is the same as the voltage at the
terminals of the capacitor C, ie as in the corresponding phase diagram:
1 switch commuté par effet centriguge
2 enroulement de démarrage
3
VSSR =
3 rotor
4
4 réseau
2 VSUPPLY
If L1 and L2 are not totally in quadrature, as when stopping, this voltage may
be even higher. For a 230 V motor, choose an SSR with a nominal voltage of
400 V.
Single phase reversing motor
3-phase motor
This type of motor can be controlled by two relays, taking care not to shortcircuit the dephasing capacitor C by simultaneously or accidentally switching
both SSRs. If this condition cannot be absolutely guaranteed, a resistance
should be defined which limits the discharge current of C in such a way that:
Whereas a 3-phase load with neutral will require a control on each phase, a
load without neutral can be switched with only 2 relays.
L1
R≥=
VMAX
ITSM
=
2 VSUPPLY
ITSM
ITSM : maximum SSR current overload
L1
GA3
M
L2
M
L2
L3
Connecting 3-phase
loads without neutral
L3
and the power of the resistance is defined by:
P = I2R
This value of R and P can be spread over two resistances R1, and R2 ie:
R1 = R2 = R/2
In cases where control occurs on three phases, once the supply voltage has
been disconnected but before stopping completely, the motor generates
slightly less EMF than the supply voltage, decreasing to zero. If the
mechanical load is high on rapid stopping, the voltage surge may be as high
as:
VRMS =
V AC
= V AC . 1.5
Cos ϕ 30°
When two motor phases are disconnected, the EMF voltage generated by the
motor, with the phase which has not been disconnected, can create a voltage
at the SSR terminals which is slightly less than twice the supply voltage.
52
53
On a 230V AC supply, it is possible to estimate a maximum value for the dl/dt
parameter by considering the following calculation:
Examples of motor applications
3-phase motor with 2 poles, 3000 rpm, ∆ 400 V AC, starting time ≤ 3 s.
on passage to zero of the dv/dt sine wave = 100000 V/second
PN
(Kw)
Cos ϕ
IN (A)
400 V
Efficiency
η
Id/IN
Id (A)
(D.O.L.
starting)
In a capacitor with the value C, the current I is defined by:
Relay
calibration
0.18
0.50
0.80
67
5.5
2.75
0.25
0.66
0.78
71
6.8
4.48
0.37
0.95
0.83
71
4.8
4.60
0.55
1.35
0.85
75
4.9
6.60
0.75
1.90
0.83
71
5.8
11.00
1.10
2.60
0.82
76
6.4
16.60
1.50
3.30
0.82
79
7.7
25.40
2.20
4.40
0.89
82
6.8
29.90
3.00
6.30
0.83
80
7.6
47.80
45 A
5.50
10.90
0.88
83
8.6
93.70
7.50
15.50
0.85
82
8.3
128.00
1-PHASE SSR
90 A
I
ie
10 A
=
C . dv/dt
I (amp) =
C (Farad) . 105
I
0,1 A/µF
=
10 A
corresponding to the max dl/dt which can
appear in this type of circuit.
25 A
We recommend placing shock inductances in series in a highly capacitive
circuit.
25 A
45 A
Various SSR connections
D.O.L STARTING
STAR OR DELTA STARTING I/V33
Parallel / series connection
The SSR inputs can be connected either in parallel or in series, whereas the
outputs can only be combined in a serial arrangement, apart from DC outputs
with an FET transistor. When the outputs of 2 relays are connected in series,
the maximum output current will correspond to the most limited characteristic
of the two SSRs, while the maximum applicable voltage will equal the sum of
the voltages applicable to each of the SSRs taken individually.
3-phase asynchronous motor, ∆ 400 V AC, D.O.L. starting, starting time ≤ 3 s
3-
PN
(Kw)
4-POLE
IN (A)
ID (A)
0.25
0.8
3.6
0.55
1.5
5.5
0.75
2.1
9.5
1.10
2.5
14.0
1.50
3.2
21.2
2.20
5.3
28.0
3.00
7.0
35.0
5.50
12.0
64.0
7.50
15.6
118.0
11.00
20.9
120.0
6-POLE
Relay
10 A
25 A
45 A
1-PHASE
SSR 90A
IN (A)
ID (A)
1.0
4.0
1.6
8.2
1.9
8.6
2.4
3.4
8-POLE
Relay
IN (A)
ID (A)
1.1
3.6
1.5
6.9
2.2
8.1
12.2
3.0
12.1
14.8
3.9
15.8
10 A
6.5
31.5
25 A
7.0
37.2
45 A
12.9
68.5
15.0
90.3
21.5
119.0
1-PHASE
SSR
90 A
7.2
9.0
26.0
29.6
15.3
42.0
18.6
52.0
25.6
95.0
Relay
10 A
Series output - parallel input
2
25 A
1
45 A
1
2
1
2
1 AC supply
4
2 load
SSR 1-PHASE
90A
3
4
3
Capacitive loads
Loads which are purely capacitive are not very common, but they can
nonetheless exist in combination with other loads (inductive or resistive) in
some electrical installations. With high value capacities (low impedance), it
will then be necessary to pay particular attention to the dl/dt parameter.
Switching of this type of load is always performed by a synchronous
relay.
54
55
Protecting SSRs against transient phenomena
Increasing the output current on a DC SSR
The output current of DC SSRs can seem limited, especially in the versions
with transistor output. It is possible to minimise this drawback by adding an
external power circuit.
Increasing the load on a DC SSR
DC +
3
1
The weakest point of solid state relays, compared with EMR relays, is
probably their sensitivity to electrical interference and transients as well as
voltage and current surges.
Transient phenomena can have two origins:
●
an electro-magnetic radiated origin whose interference mainly affects the
low-voltage structure of the SSR, such as the input circuit comprising the
optocoupler.
●
an electrical origin conducted by the power supply wires. As the I/O
capacitive coupling is weak, these transients mainly affect the power output
circuit.
1 DC SSR
2 load
3 transistor or DARLINGTON transistor
2
However, whatever the phenomenon may be, it is often caused by lightning,
switching on inductive loads (motors, etc), distribution equipment placed on
the electrical supply (micro power cut, etc). These phenomena exist on all
industrial electrical supplies, and their incidence can in many cases be
reduced by simply being aware of them and taking elementary precautions.
OV
Summary
TYPE OF LOAD
SWITCHING TO BE CHOSEN
Resistive load
Synchronous switching
Motor load
Asynchronous switching
Inductive load 0.7 ≤ Pf ≤ 1
Synchronous switching
Inductive load Pf < 0.7
and transformer
Asynchronous switching
Capacitive load
Synchronous switching
A vital basic safeguard against all these phenomena is to select the
correct size of SSR in relation to its application in order to take
advantage of all the SSR characteristics with an adequate safety
margin.
Transient phenomena on input / protection
The main problems encountered on an SSR input come from radiated or
conducted voltage surges. If such a voltage surge exceeds the minimum turnon voltage (1 to 3 volts) the SSR output circuit will open at least until the
output current next passes to zero. A voltage surge that is too high could
destroy the optocoupler if this is not adequately protected.
Input protection method
The input SSR can be protected against possible voltage surges by adding
an RC supply or a zener diode mounted in parallel on the input. These
components, often integrated directly within the SSR, delay the relay
switching for several micro-seconds (which will not have serious
consequences, particularly with AC) and reduces the effects of a radiated or
conducted stray impulse.
Furthermore, a synchronous relay is "naturally" protected against the effects
of a stray impulse on the input as long as the stray impulse occurs outside the
valid switching window.
56
57
Transient voltage phenomena on the output
Voltage surge on output
Output protection method (voltage)
RC supply - "Snubber"
If, following a voltage surge, the voltage at the output terminals of an AC SSR
exceeds the maximum permissible direct voltage or turnaround voltage, the
SCR or the triac will switch on until the current next passes to zero.
Mounting an RC supply in parallel on the output both reduces the dv/dt
gradient generated by a stray impulse and reduces the amplitude of this
impulse by filtering, as long as the impulses do not recur.
Because of the high switching speeds of SCRs or triacs, switching can be
triggered by a very short pulse of only a few µs.
Protection via "snubber"
1
L di/dt
Switching by exceeding the turnaround voltage
1
1 SSR
R
4
V
Vd
2
2 trigger circuit
3 RC filter
C
1 voltage
3
2 maximum direct voltage of SCR
2
5
3 output current
U
4 voltage surge
6
5 t
3
(resistive load)
Any variation in the voltage at the terminals of an RC supply results in a
current in the C capacitor, causing a voltage drop in the load such that:
5
Vd = V + L . di/dt - U
Increase in dv/dt direct voltage
This voltage drop will protect the output SCR.
This characteristic is linked to the physical structure of the output element and
in particular to the coupling capacities between anode and cathode in an SCR
or triac. If the variation of the voltage at the relay terminals is too rapid, this
can result in an uncontrolled turn-on.
The main disadvantage of this type of filter is the significant increase in the
SSR leakage current. Using a “snubber” can in fact, depending on the values
of the filter components, double the amount of leakage current.
The severity of the consequences of such a stray trip will obviously depend
on the application, but these can, in certain specific circumstances, lead
indirectly to the solid state relay being destroyed:
All SSRs usually have “snubber” filters, which improve their performance.
However, some SSRs use special SCRs which accept significant dv/dt
variations. These relays are known as “Snubberless”.
●
Very high energy impulse,
Typical value of a ”Snubber”:
●
Unexpected turn-on with short-circuiting of a relay acting as a reversing
motor.
Resistance :
33 Ω < R < 100 Ω
Capacity
0.1 µF < C < 0.47 µF
58
:
59
Protection using “Transil” (Surge-Suppressor) Diodes
Protection via Varistors
A “Snubber” filter alone is not usually sufficient to protect an SSR effectively,
particularly against high-energy stray pulses. Using a “Transil” (surge
suppression) diode improves this protection reliably.
To protect the SSR against high energy stray pulses, it is also possible to use
varistors.
The drawback with these is that they lose their characteristics over time as a
result of the stray pulses received. They must be replaced after each incident.
Surge-suppression diodes are intended to protection electrical equipment
which is sensitive to fast transients of low or medium energy levels. They are
designed to have very good performance where the needs of this type of
equipment are concerned, i.e. for overloads lasting close to one millisecond
or less. This type of component can also provide good protection against
electrostatic discharges (ESD).
The characteristic of a varistor is that with a voltage at its terminals less than
its nominal value, the impedance of the MOV is very great (several Mohms).
However, once that value is exceeded the impedance very quickly drops to
less than 1 Ohm, with the response time for the MOV being approximately 20
to 50 ns.
How to Select a Surge-Suppression Diode
The basic parameters of a varistor are:
On its ability to drain and on the two voltage characteristics: VBR and VRM.
VBR = reverse avalanche voltage or elbow voltage. This is the value above
which lth current in the diode rises very rapidly with a slight increase in
voltage.
VRM = standby voltage. This is the voltage which the diode can withstand with
constant load.
●
the voltage which the varistor must be able to tolerate permanently
(usually: Vvaristor = Vsupply x 1.15),
●
the peak voltage at which the phenomenon must be suppressed,
●
the energy (expressed in Joules) freed by the source of the transient
phenomenon (to be defined for each application).
Although the first two parameters can be found easily, choosing the correct
varistor requires, in addition, a minimum of knowledge about the source
impedance and the power of the stray pulse, as well as an approximate idea
of the supply quality, as the lifetime of an MOV is largely influenced by the
number of stray pulses and the power of each pulse received.
For applications requiring higher voltages, the diodes can be mounted in
series. It is not even necessary to balance them using RC supplies. However,
it is preferable to mount in series only diodes of the same type to distribute
the energy evenly.
Dans le domaine industriel, l'utilisation de varistances de moyenne puissance
pouvant absorber des impulsions d'énergie entre 50 et 130 Joules devraient
convenir.
Nota : Mounting in parallel in not usually possible.
N.B. : With an alternating current, use a bi-directional version or two “Transils”
mounted head to tail.
Varistor selection guide
Three-phase supply without neutral (load + SSRs connected between
phases)
Voltage
between
phases (V)
60
Peak limiting
Energy
voltage
(J)
(V) at 100 A (10-100µs)
Manufacturer references
HARRIS
SIEMENS
LCC
CROUZET
3 x 230 / 3 x 240
710
140
V275LA40A
SIOV-S18K275 VF20M10431K
26 532 740
3 x 400 / 3 x 415
1 100
160
V420LA40B
SIOV-S18K420 VF20M10681K
26 532 741
61
Three-phase and single phase supply with neutral connected (load + SSRs
connected between phases and neutral)
Single phase 3-phase supply Peak limiting
supply voltage voltage, neutral
voltage
(V)
connected (V) (V) at 100 A
Energy
(J)
(10-100µs)
Manufacturer references
HARRIS
SIEMENS
LCC
CROUZET
120
3 x 240
710
140
V275LA40A
SIOV-S18K275 VF20M10431K 26 532 740
230 - 240
3x400 / 3x415
710
140
V275LA40A
SIOV-S18K276 VF20M10431K 26 532 740
277
3 x 480
710
140
V275LA40A
SIOV-S18K277 VF20M10431K 26 532 740
380 - 415
3 x 690
1100
160
V420LA40B
SIOV-S18K420 VF20M10681K 26 532 741
Varistor connection mode
For optimum protection, a varistor is usually mounted in parallel with the
component it is to protect. However, in the case of a motor reverser, MOVs
should be mounted as described below to protect both directions of rotation
adequately.
Transient current phenomena
As with the voltage characteristics, it is essential to allow a sufficient safety
margin with regard to the current characteristics of the SSR. However, some
specific physical phenomena inherent in certain loads (inductances, motors,
etc), and also certain accidents, can lead to instantaneous or permanent
current overloads in the circuit.
The special characteristics of the main electrical loads are generally very well
known and it is possible to select an SSR on the basis of these
characteristics. A motor can have a starting current for a limited time of 10
times its nominal current. Similarly an inductance will have an impedance
equal to its ohmic impedance, therefore very low, in saturation phase.
Such overloads are acceptable for an SSR if this has been taken into account
when selecting the SSR. In contrast, the same motor accidentally blocked by
a mechanical overload will result in a permanent overload; this will initially
cause overheating of the output SCR, then its destruction possibly by shortcircuiting, which will destroy the motor if no preventive measures have been
taken.
An efficient protection system against overloads should, as a priority:
Connecting an MOV on a single phase SSR
1 V power supply
1
●
Restrict the duration of overloads,
●
Restrict the frequency of overloads.
2 varistors
Current limitation by fast-blowing fuse
2
L1
U
L2
V
PPR plate for GA0
Rather than use an unnecessarily large SSR, which will only protect the SSR
itself against current overloads, it is advisable to protect the whole installation
against accidental overloads by fitting fuses in the circuit which will protect
both the relay and the electrical systems connected to it. Such fuses should
be chosen from the range of fast-blowing fuses.
A fuse is equivalent to a low-level resistor which is capable of absorbing a
defined amount of energy in a given time. Once this has been exceeded, the
fuse blows.
The power dissipated by a current I in an electrical resistor R is shown as:
Connecting an MOV on a reversing motor
P=RI2
and the corresponding energy during that time is shown as:
Protecting DC SSRs
A diode mounted in parallel with an inductance and linked to a zener diode in
parallel on the SSR is the most efficient means of quickly eliminating high
voltage surges which can occur in an inductive DC circuit on breaking. These
voltage surges are the result of the energy 1/2 LI2 accumulated in the
inductance at the moment of breaking. This is the energy that must be
discharged via the diode. All these products in the CROUZET DC SSR have
an integrated protection diode which is suitable for most applications.
62
E=RI2t
63
For a fuse, R is constant and therefore the energy going through the fuse is
simply defined by the coefficient I2t.
For a relay, a coefficient I2t is calculated from the maximum peak current on
a half-period (10 ms) ie:
2
I t(10ms)
I2 max. peak .
0.010
=
2
3
4
1 Current
2 Fuse blowing time
3 Fuse starts to blow
5
4 Current witloof fuse
6
To protect an SSR a fuse with an I2t coefficient lower than that of the
relay should be selected.
The I2 t coefficient can be better adapted to the SSR to define the I2t
coefficient whatever the measurement time with the equation:
I2t (in time tx) = I2
The fuse corresponding to 1 ms will be:
I2t(1ms) =
365 . 0,001
0,01
= 1154 A2s
This calculation can be used, if necessary, to correlate the I2t values given by
some supplier catalogues at different times with those normally used for solid
state relays (10 ms).
In some cases the I2t value indicated by the fuse manufacturer should be
corrected to take account of the actual operating voltage, which may differ
from the manufacturer’s reference value.
The following should be taken into account when making the final selection of
a fuse:
●
the operating voltage of the fuse which should be at least equal to the
supply voltage,
●
the nominal current of the fuse which should be greater than the load
current in steady state,
●
the maximum current in the system,
●
the maximum permissible peak current for the fuse.
5 Current across the blowing fuse
6 Time
2
I2t(10 ms) = 300 A2s(10 ms)
(seconds)
Current limitation via a fuse
1
For example, a fuse could be selected with the value:
For the whole of its SSR range, CROUZET offer a selection of fuses from the
FERRAZ fuse range (see the section on “Accessories for solid state relays” in
the CROUZET catalogue).
t . tx
tx fuse reference time
Example:
GA3 25 A
Overcurrent during a cycle: 270 A
I2t
(10 ms)
=
2702 . 0.01
= 364.5 A2 s
2
(Typical catalogue value = 365 A2s at 10 ms)
64
65
Special precautions and help with circuit diagnostics for SSRs
A solid state relay is an active electronic component which it is wise to choose
carefully and avoid any chance of the SSR not being suitable for the
application. Detailed analysis of requirements must be performed before
choosing the SSR in order to define the essential characteristics which it
should meet.
Once the application parameters have been defined, an accurate choice can
then be made by using, if necessary, the guide drawn up in the form of a
flowchart which appears at the beginning of this manual. Particular attention
should also be paid to the selection of the heatsink, which is required when
the anticipated output current exceeds 5 amps.
Fault on closing
If the output circuit is not activated by a control pulse on the input, the first
thing to check is the output circuit wiring:
1 - When no control pulse has been applied to the input, the output voltage
should be the same as the load voltage.
●
If this voltage is zero, check that the load circuit is not open, and that the
power supply is correctly connected to the circuit (watch out for
intermediate switches which may not have been closed).
●
If the output voltage is the same as the power supply voltage, check that
the load has not short-circuited, which might have damaged the SSR output
in the open position.
Once installed, if the SSR malfunctions, two troubleshooting methods are
possible:
●
●
Replacement, pure and simple, of the SSR.
This method, although self-evidently the quickest troubleshooting method,
will not actually produce a positive result if the malfunction is due to a
phenomenon external to the solid state relay.
Preliminary analysis of the origin of the malfunction before replacement, if
necessary.
It should be remembered that generally, the reliability of a solid-state relay is
such that, if an solid-state relay fails after being installed according to
standard industry practices, it is highly probable that the cause of the failure
is external. Replacing the relay without first analysing the origin of the
malfunction or taking action to improve the external circuit is likely to solve the
problem only temporarily. Until the underlying reason for the fault is
eliminated, the fault is likely to recur. It is therefore essential to perform a fault
analysis.
The most common malfunctions noted in circuits with SSRs are of 2 types:
malfunctions on closing, or, malfunctions on opening.
These malfunctions may occur randomly in certain conditions and not recur.
They are then very difficult to correct; but by taking a minimum of precautions
and following a minimum of rules when wiring up the installation, such
malfunctions, including some total malfunctions, can very easily be avoided.
Take care, therefore:
●
●
●
●
●
●
to wire up the inputs and outputs separately to avoid the risk of interference.
to tighten the connection screws correctly.
to fit the recommended type of filters on the relay, so as to avoid transient
phenomena as far as possible.
to limit interference caused by the supply by fitting appropriate filters on all
the equipment (supply filter etc)..
to ensure correct ventilation in the control unit.
to mount an appropriate heat sink (+ heat transfer compound).
66
2 - When a control pulse has been applied to the input, the output voltage
should be approximately 1.5 V depending on the SSR. If this is not the case,
check the input.
●
●
Check the connections and polarity (DC SSR) of the control signal.
Measure the control voltages at states 0 and 1, and the corresponding
currents (ammeter or voltage measurement at the series resistor terminals
if this exists). If the current is zero, whatever voltage is applied, the input
circuit is faulty. If the voltage is outside the limits, check the external circuit.
And if the measured values are within the recommended limits but the
malfunction persists, replacement of the relay will probably be necessary.
Fault on opening
This malfunction can be confirmed by disconnecting the control wires:
1 - If the SSR opens, first check the switching thresholds on the input which
may be too low, meaning the SSR is faulty.
●
If the threshold voltages are correct, the external control circuit may be
suspect.
2 - If the SSR does not open after disconnecting the wires, check that the
power supply voltage does not exceed the maximum output voltage (see:
turnaround voltage on SCR or triac).
●
●
Check that adequate protective devices against transient phenomena are
present.
The load current must not exceed the maximum limits, as this will cause
overheating. Check the SSR load and output, which may have shortcircuited. If so, replace the relay.
This troubleshooting guide should help to resolve the majority of
problems encountered when using solid state relays. However, it should
not need to be used very often, given the reliability and long life of solid
state relays. Nevertheless, should you have particular difficulties when
using an SSR, do not hesitate to contact Crouzet Ltd for help and
advice, Nr 0 825 333 350.
67
Solid-State Relay Characteristics and Terms
A solid-state relay is obviously defined by a certain number of electric
parameters and characteristics, but also by its "packaging". The
manufacturers have developed several type of housing some of which are to
be considered to be standard.
The most wide-spread housing type is commonly known as the "Hockey
Puck" housing because of its shape. This housing is particularly well-suited to
medium and high-power applications, but nonetheless requires the user to
define the heat sink appropriate for the application. This type of housing
associated with an optimised heat sink, often represents the most costeffective alternative in applications involving large-scale production quantities.
To facilitate the implementation of solid-state relays, several years ago, the
manufacturers developed relays with integrated heat sinks corresponding to
the main relay characteristics in standard utilisation conditions. As a result,
the user no longer needs to select the heat sink, the mounting, the
attachment inside the control unit or electric cabinet. Moreover, this type of
relay also complies with the main size standards in industry, 22.5, 45 and 90
mm wide and capable of being mounted directly on DIN rails.
GN and GRD
Turn-Off Voltage
Value of the input voltage below which the OFF state of the relay is
guaranteed by the manufacturer. This value is usually very low (1 to 1.5 V for
DC, a few volts for AC). This characteristic is especially important where the
solid-state relay is to be controlled by a 2-wire element, with a high leakage
current. This case is dealt with in detail later.
Maximum Input Current
This value is usually specified at maximum control voltage and provides for
sizing the control circuit.
Input Impedance
This characteristic which is in principle given for SSRs with a passive input
circuit, provides for sizing the control circuit. The characteristic is useless for
input circuits with integrated current generators.
Turn-On Response Time
Maximum time between the moment the control signal is applied to the input
and the moment the output switches. For an instantaneous relay, this time is
very short (< 0.1 ms). For a synchronous relay, depending on the control (AC
or DC) and the angular position of the control in relation to the load voltage,
this time may range up to 20 ms maximum (at 50 Hz).
Turn-Off Response Time
Maximum time between the end of the control signal and the moment the
relay switches to OFF state. As for turn-on/making response time, turn-off
response time can vary (up to 30 ms) depending on the type of relay, and its
control voltage. The fastest response times are provided by relays with DC
control voltage (10 ms 50 Hz).
This document would not be complete without mentioning the I/O-type
housings or input / output modules, which are simply low level AC or DC solidstate relays which represent a simple automation solution often used to
interface different voltage levels. These modules exist in several formats, the
most common of which are the industrial format and the modular format.
Input Characteristics_
Input Voltage
Voltage range which can be applied to the input and or which relay switching
is guaranteed. The upper value corresponds to the maximum voltage which
may be applied to the input without destroying the circuit. The lower value in
the range is defined based on the turn-on voltage (voltage at which the relay
switches from OFF to ON) and is usually 0.3 to 0.5 V (for DC) above this
value.
68
69
Typical Electric Specifications for Solid-State Relays / Manufacturer Data
70
71
Dv/dt
Output Characteristics_
Output Voltage (max. V RMS)
Voltage range which may be applicable continuously at the output and at
which the SSR will operate in accordance with the specifications.
Peak Voltage (1 min) or Peak Voltage When Open
This value corresponds to the turnaround voltage VBO of the relay SCRs. This
is the minimum voltage which can be applied to the relay terminals without
causing the SSR to turn-on by itself as a result of exceeding the VBO value.
This value i only significant for those SSRs without transil-type protection.
Voltage Drop When Closed
This characteristic is generally given when open (static / stable) and on
switching. The static dv/dt value defines the maximum voltage variation rate
on the SCR terminals when the SCR is open. A higher voltage variation than
this causes the output element to switch. The dv/dt value on switching
characterises the maximum voltage variation rate on the SCR terminals when
the relay opens. This characteristic may be exceeded when highly inductive
loads are switched, preventing the relay from closing.
Di/dt
This parameter characterises the ability of the solid-state relay to absorb a
current variation. Exceeding the limit value usually destroys the relay, as a
result of short-circuiting the SCR.
Utilisation Frequency
Voltage drop appearing at the relay terminals when a given current
(IMAX)passes through it. This value provides for calculating the power
dissipation of the relay.
Load Current / Maximum Current
Maximum current which can be switched by a relay in given conditions. This
value may be corrected as a function of external operating conditions
(temperature, heat sink…).
Leakage Current When Open
Leakage current passing through the relay and therefore through the load
when the relay is open. This value must be taken into account especially
where the relay load is another relay or any other electric device with high
input impedance(e.g. I/O module).
Operating frequency range. It should be noted that the leakage current on the
output is directly proportional to the operating frequency value.
Junction/ Housing Thermal Resistance
Expressed in degrees Celsius (or Kelvin) per Watt, this parameter
characterises the temperature gradient (whatever the power applied) between
the SCR power junction and the relay housing. This parameter is extremely
important in determining the optimal heat sink for a given application.
Dielectric Strength
Maximum voltage which may be applied between the relay terminals and the
base plate without causing a breakdown.
Input / Output Isolation
Holding Current
Minimum current necessary for the SSR output SCRs to remain closed when
a control signal is applied to the input.
Overcurrent : 1 Cycle / for 1 Minute
Maximum acceptable peak current over one cycle (10 ms for 50 Hz) for 1
minute. This non-repetitive current value decreases the longer the overcurrent
lasts. For 1 cycle this value is of the order of 10 times the nominal current. For
one second, the overcurrent may be 3 times the nominal current at the most.
I2t
Maximum potential which may be applied between the SSR inputs and its
outputs. An isolation fault may lead to serious damage in the electrical
installation in which the relay is installed.
Input / Output Capacity
The value of the coupling capacity between the input and output terminals.
Switching Mode
The solid-state relay may be switched in various ways depending on the type
of load. However, the mode most frequently used is the synchronous mode
which is suitable for many applications and generates the least interference.
This parameter represents the maximum energy that the solid-state relay can
withstand without damage during a short circuit or non-repetitive overcurrent.
This value is expressed in square amps for a given duration (usually 10 ms),
and is one of the main characteristics involved in selecting the protection fuse.
72
73
Standards Applicable to Solid-State Relays
For electric components and more generally, for automation components,
solid-state relays included, the following Directives are to be applied:
Solid-state relays are electric components intended for use in automation,
electric control, monitoring, … circuits. So it is subject on the one hand to
standards governing electric components themselves, but also in certain
cases to standards specific to the field in which it is used. So it is easy to see
that relays intended for use in hospitals may be subject to different regulations
than those in industry.
• Low Voltage Directive (LV) (73/23/CEE) applies to all electrical equipment
intended for use with voltages from 50 to 1000 VAC and 75 to 1500VDC.
The main task of various local or international commissions and organisations
is to prepare standards encouraging and improving international co-operation
in regulation matters. Although often complex by their organisation and
structure, standards represent a recognised and practical reference standard
whose main purpose and aim is always:
These two Directives are filled out by the Machine Directive (98/37/CE) which
applies to machinery or equipment incorporating moving elements (which
may represent a danger for the user) and to safety components.
• to guarantee peoples' safety,
• to define product quality levels,
• to participate in protecting the environment,
• product and component interchangeability.
As the market leader in solid-state relays and a manufacturer of various
automation components, Crouzet keeps a permanent watch on standards
and participates actively in the work of the various European or international
standardisation commissions.
• EMC Directive (Electro-Magnetic Compatibility) (89/336/CEE) applies to all
product likely to generate electro-magnetic interference… but also likely to
be susceptible to electro-magnetic radiation.
N.B.: Where an automation component such as a solid-state relay is intended
to be mounted in a machine, the component itself is not subject to the
Machine Directive - it is the responsibility of the manufacturer of the machine
to ensure that the equipment complies in its entirety with the Directive under
which it falls.
Each of these Directives is based on general or generic standards or where
they exist, on product- / application-specific standards (specifying for example
the maximum acceptable levels for the various parameters) and which refer
to various test standards.
Overall, the manufacturer is solely responsible for ensuring compliance with
these Directives. The manufacturer demonstrates that he fulfils the
requirements by applying the corresponding standards, which are published
by the European bodies (official gazette).
International Standards / Local Standards_
In the electronics and electro-technical fields, the main reference standard is
defined by the collection of IEC (International Electrical Committee)
standards, plus certain more local standards:
UL : American standards,
CSA : Canadian standards,
EN :European standards,....
Conversely, the IEC standards may be flown down locally to provide more
specific, trade- or application-oriented standards.
The European Directives_
In 1989 and 1991, the European Economic Community adopted several
Directives based on Article 10 A of the Treaty of Rome and which aim at
harmonising legislation in the Member States concerning health and safety
requirements imposed on certain products. These requirements relate to the
manufacture and commercialisation of these products while guaranteeing
sufficient protection for consumer safety and health.
74
CE Marking (93/68/CEE)_
CE marking on products (and machines) was initiated by European Law
based on the application of the European Directives issued to this end. This
marking is mandatory and guarantees the product concerned freedom to
circulate throughout the European Community.
Electric components and automation components whose main purpose is to
be mounted in equipment are principally subject to the Low Voltage Directive
guaranteeing that the component does not represent a danger, consequently
CE marking on these components is required per this Directive alone.
Throughout this reference document, the solid-state relay is considered to be
a component (and not a saleable end product as such) intended to be
connected to a low voltage electric supply. In this light, the solid-state relay
is only subject to compliance with the LV Directive for CE marking,
which ensures that it may circulate freely throughout the EC.
75
Beyond the strictly "standard" and regulatory aspect, the solid-state relay
remains first and foremost an electronic component, for rapid switching of
currents which may be high (up to 90 / 120 Amps), capable of causing severe
interference in its environment, and which can also be disturbed by its electromagnetic environment. To answer this need for reliability in a disturbed
electro-magnetic environment, Crouzet offers, over and above the CE
marking on relays per the LV Directive, a characterisation in relation to the
main standards on which the EMC Directive is founded.
Additionally, for those countries outside the EEC, all the Crouzet solid-state
relays comply with and are certified to various American and Canadian
standards, in particular UL 508 and CSA C22.2.
Comments_
EN 60947-4-2 :
Low Voltage Devices. Semiconductor-based and starters
for AC motors.
EN 60947-4-3 :
Low Voltage Devices. Semiconductor-based and starters
for loads other than AC motors.
This standards is a specific standard applicable to solid-state relays in the
industrial field, and defining the "LV and EMC" limits for the products
concerned. Consequently, this standard replaces the following generic
standards as far as EMC requirements are concerned:
EN 50081-2 Emission in an Industrial Environment
Table of Standards Applicable to Solid-State Relays_
Environment
LV Directive
EMC Directive
Industrial
EN 60947-4-2 and -4-3 EN 60947-4-2 and -4-3
Commercial
Light Industry
EN 60335-1
EN 50081-1 (Emission)
EN 50082-1 (Immunity)
Information
processing
EN 60950
EN 50081-1 (Emission)
EN 50082-1 (Immunity)
Electro-medical
EN 60601-1
EN 60601-1-2
EN 50082-2 Immunity in an Industrial Environment
Machine Directive
Immunity:
The level required for industrial applications is tougher that
that for applications in the "domestic", "commercial", "light
industry ", and "electro-medical" fields.
Emission:
The maximum emission level accepted for industrial
applications (Class A) is lower than that allowed for the
other application fields (Class B).
EN 60204-1
The Crouzet solid-state relays fulfil all the provisions specified in these
reference standard.
LV Test Standards
Type of Test
Standards
EMC Test Standards
Type of Test
EN 60204-1 :
Safety of Machines.
Electric Equipment for Machines.
EN 60335-1 :
Safety in domestic electric and analogous equipment.
Standards
Immunity
Protection category
IEC 60529
Inflammability category
for plastic materials
IEC 60695-2-1
UL 94V0
Electro-static discharges
EN/IEC 61000-4-2
EN 60950 :
Safety in information processing equipment, including
electric office equipment.
Insulation voltage and
surge voltage
Electro-magnetic field
Amplitude modulation
EN/IEC 61000-4-3
EN60601-1 :
Safety in electro-medical equipment.
IEC 60664-1
Electro-magnetic field
Pulse modulation
ENV 50204
EN 50081-1 :
Fast transients
EN/IEC 61000-4-4
EMC/ Generic Emission Standards.
Domestic Commercial and Light Industry.
Radio Frequency
surge voltages
in common mode
EN/IEC 61000-4-5
EN 50082-1 :
EMC / Generic Immunity Standards.
Domestic Commercial and Light Industry.
EN 60601-1-2 :
Electro-medical equipment. EMC Requirements and Tests.
Magnetic Field
EN/IEC 61000-4-8
Dielectric strength
leakage current to
ground
EN 60950
EN/IEC 61000-4-6
Voltage drops and interruptions EN/IEC 61000-4-11
Emission
Conducted Emission (Mains/Line) EN55022/EN 55011
Radiated
76
EN55022/EN 55011
77
Applications
Although by their principle, solid-state relays are often compared to electromechanical relays, their implementation is nonetheless frequently more
complex and requires a minimum of precautions.
VAC
R1
In
AC
Out
As a result, solid-state relays cannot replace electro-mechanical relays easily
and directly, except for some specific relatively simple cases.
R2
Consequently, the examples presented below are to be considered as a
directory and application guide for the more complex cases and to orient the
designer in the circuit design, and in any precautions for use.
1
R1
Temperature Regulators _
R2
1
In
Temperature regulation is a typical application for solid-state relays. The
technical advantages of SSRs such as absence of noise when operating,
rapidity, longevity and lifetime are shown off to the full in this application.
load
Out
R2
Examples of switching high loads.
The solid-state relay could be replaced advantageously by an I/O model.
CTD
Resistor R1 protects the solid-state relay in the event of a short-circuit in the
load. The resistor must be selected so as to limit the current to the nominal
SSR value. In the same way, resistor R2 prevents external triacs or SCRs
from switching, which could be caused by excess leakage current from the
control solid-state relay, and I/O module.
R1 =
Vac
SSR nominal I
R2 =
0.7
SSR leakage current
The diodes mounted between Gate and Cathode provide for limiting their
reverse voltage which could damage the SCRs if it were to rise too high.
Reverser Mounting_
Controlling a Power Component by a Solid-State Relay (AC Circuit)_
Recent technological progress has led to the availability today of solid-state
relays capable of controlling currents up to 120 amps ; which is ample to
control most loads directly. However, it may be useful to control high loads
from I/O-type modules and power elements (SCRs or triacs).
78
A solid-state relay cannot be compared directly to an electro-mechanical
relay, but the combination of several relays enables very similar functions to
be achieved. A NO/NC reverser can be made (see diagram a) below) by using
four solid-state relays and a logic circuit. In diagram (b) below, we have simply
added an output buffer to control the solid-state relay inputs more reliably.
79
To fulfil this need, relay B can be triggered by a signal provided from the
voltage on the terminals at load A and slightly offset in time by an RC filter.
In
NC
Out
In
NO
Out
In this case, relay B can only be activated when load A is switched to the main
supply.
VAC
A
B
In
NC
Out
In
NO
Out
B
A
Auto-controlled
Switching Loop
R
B
C
In
B
Out
In
A
Out
(An MAS5 time-delay
relay could also be
used)
b. Equivalent reverser diagram
a. Wiring a reverser with a
solid-state relay
C
A
Reverser Mounting
VAC
C
Sequential Load Switching_
In some applications, it may be necessary to switch several power loads using
a single control signal and in a specific order. This function can be achieved
in two different ways:
Time-delay Switching
Time-delay Switching
Using two solid-state relays, two separate loads can be controlled in a similar
manner ; the state of load 1 controls the solid-state relay connected to load 2,
per the diagram below:
Here, solid-state relays A and B are switched by one and the same control
signal. When a control input is applied to points C - C’ ; (Figure 8) in the
circuit, this signal will be applied immediately to the inputs on relay A, while
being offset by the RC filter for relay B.
1
R
C
In
B
C
In
B
Out
In
A
Out
Time-delay Switching
for Load B
A
VAC
C'
(An MAS5 time-delay relay
could also be used)
Controlled Time-Delay Switching
In the case described in the previous Figure, should the circuit to load A be
interrupted, the A – B switching sequence will not be complied with.
Depending on the circumstances, this may lead for example to erratic or even
dangerous machine operating cycles.
80
AC
Out
In
AC
time delay
Out
1
Auto-controlled Switching Loop
Special attention must be paid to the characteristics of the source which must
be capable of withstanding the power of both loads because of the slight
overlap in simultaneous operation of the loads during phase changes.
Inserting time-delay circuits between relays 1 and 2 may provide for special
trigger functions and sequences.
81
AC ou DC
The output current on DC solid-state relays may appear limited for some
versions with a transistor output. This drawback can be alleviated by adding
an external power circuit per the diagram below.
+ Vcc
(An MXR1 time-delay relay
could also be used)
+
Increasing the Output Current on a DC SSR_
R2 =
Out
0.6
SSR leakage
R1
SSR
In
I
1
In
DC
Out
IB
IB =
load I
HFE(transistor)
R3 =
Vcc – 1,5
IB+SSR leakageI
R2
-
1 load
Augmentation du courant
de sortie d’un relais DC
As before, resistor R2 will be calculated to avoid the SSR leakage current
causing the external power transistor to start to conduct / become conductive.
Resistor R3 defines the base current of the external transistor. This current
should provide for saturating the transistor and thus limiting thermal
dissipation (0.4 w/amps or even less).
Pulse Control with Anti-Bounce Protection
Auto-Locking a Solid-State Relay_
Use of pulse pushbuttons is relatively widespread in industry to control
starting / stopping machines. The wiring schematic in Figure 9 provides for
auto-locking the solid-state relay based on pulses from two separate ON /
OFF pushbuttons.
10KΩ
1
ON
2
OFF
3
load
R
1
Where two control signals or two pulse buttons respectively and OFF are
available, this function can be achieved using two NAND gates wired as a
rocker switch with - in this case - anti-bounce protection. Remember that antibounce protection on the
input is only really necessary
for DC solid-state relays,
since AC solid-state relays
incorporate this protection by
design.
D
In
SSR
Q
Out
Out
VAC
Where a D rocker switch§, is
used anti-bounce protection
can be provided by adding
an RC supply on the control
input.
Fig. 9 : Auto-lock Control on Separate ON/OFF
Resistors with a
Pushbuttons
value of 10K (or
more) are necessary
to avoid short-circuiting the power supply when the LON/OFF pushbuttons
are pressed. A current limiting resistor may be inserted in the power circuit to
protect the relay in the event of a short circuit.
Installing Solid-State Relays in Parallel_
Installing solid-state relays in parallel can be of interest where load currents
greater than the relay characteristics are encountered. This possibility can be
used with Dc relays and in particular with relays fitted with a MOSFET
transistor output, and which have the particularity of having a positive
temperature coefficient, resulting in the currents on the two relays being
automatically balanced.
Controlling an SSR by a D Rocker Switch
(An MUR1 time-delay relay configured for function B could also be used)
82
AC
10KΩ
In some cases, it may be necessary to control solid-state relays using pulses
of very short duration, or even on a rising or falling edge. This control pulse
can be memorised using a D, the control signal being provided by a pulse on
the clock input.
PR CLR
Q
CLK
In
230 VAC
Controlling a Solid-State Relay by a Pulse_
+
2
3
83
Installing DC relays with bipolar or AC output in parallel, although possible in
theory, is strongly advised against, even if the usual precautions to balance
the currents are taken.
In all cases, a fault on one of the relays will lead to the destruction of the
second if the latter has not been sized to withstand alone the entire load.
Inserting Function A-type timers in the control circuit between the relays'
simultaneous switching to ON and if necessary introducing a motor stop delay
before changing the direction of rotation.
Reversing the Direction of
Rotation on a Single-Phase
Motor
L1
M
L2
VAC
R1
In
Out
DC
1
1
R2
In
load
1
reverse contact
2
time delay on Direction 1
3
time delay on Direction 2
R1
+
R2
Out
Out
AC
AC
In
In
Out
DC
DC (MOSFET) Solid-State Relays Wired in Parallel
Reversing the Direction of Rotation of a Motor_
This "speed" monitoring
function can also be
achieved, but with less
safety using a Crouzet
FRL-type control block.
1
OFF
2
3
Reversing the rotation direction on a motor is one of the most common
applications for solid-state relays whether for DC or AC motors:
Single-Phase, AC Motor
Reversing the Direction of Rotation on a Three-Phase Motor
This type of motor can be controlled by two relays as long as care is taken not
to short-circuit the de-phasing capacitors C should both SSRs switch
simultaneously or inadvertently. If this condition cannot be totally guaranteed,
a resistor R shall be defined to limit the discharge current on C such that:
R ≥ VALIM
=
ITSM
√2 Vsupply
ITSM
With ITSM = the maximum overload current of the relay and the power of the
resistor will be defined by P = RI2. The resistor thus defined can be distributed
across the two solid-state relays such that RI1 = RI2 =R/2.
In these conditions, the solid-state relay shall be defined as a function of the
voltage which may appear on the motor terminals. The voltages on terminals
L1 and L2 (motor windings) are theoretically in quadrature and the voltage on
the terminals of the unswitched relay will be equal to the voltage on the
capacitor terminals, i.e. per the phase diagram below:
The direction of rotation of a three-phase motor can be reversed simply by
inverting two phases. This function can be achieved using four solid-state
relays in the case of a motor with no neutral. An additional relay will be
necessary for a motor with the neutral connected.
To avoid short circuits between phases when the direction of rotation is
reversed, FWD/BCKWD relays must not be closed simultaneously, and a time
delay of 100 to 150 ms is recommended. Complementary protection using a
resistor as calculated above and a fuse is strongly recommended. When
defining solid-state relay specifications, it may be of use to refer to the
Chapter "Switching motors" (see page 50).
R
S
T
AC
VALIM = √2 Vsupply
In
Out
AC
In
Out
AC
In
AC
Out
if L1 and L2 are not totally in quadrature, or when the motor is stopped, the
voltage on the relay terminals may be greater than this value. For a 230V
motor, a relay with a nominal voltage of 400V at the least, or even more,
should consequently be selected.
84
85
In
Out
Direction of Rotation Reverser using
Single-Phase Relays
L1
+
L2
L3
N
O I
O I
1
1
2
2
R1
GA0
R1
R2
L1
AIF
L1
U
L2
A2
U
L2
V
AIR
V
R2
+
2 phases cut off
(without neutral)
DC
Out
In
DC
Out
A
G
3
In
A
1
ON / OFF
2
fuse
3
varistor
M
+
B
In
DC
Out
In
DC
Out
B
3 phases cut off (with neutral)
Connecting a Direction Rotation Reverser using a GAO-Type Relay
N.B: The direction reversing function with a time delay is directly incorporated
in the GAO-type relays. Where reversing the direction of rotation is achieved
using single-phase solid-state relays, the control system must include an
inter-lock prohibiting giving the command for one direction while the other is
still active. Each relay shall be fitted with a varistor to absorb the surges on
the main supply.
Reversing the Direction of Rotation of a DC Motor
Reversing the direction of rotation of a DC motor requires 4 solid-state relays.
In particular, care must be taken when defining the control circuit which must
prohibit any attempt to switch to both directions simultaneously; this is likely
to create a short circuit which would destroy the solid-state relay.
Introducing a delay of a few milliseconds (depending on the motor) between
the orders to change direction, will allow the motor current to cancel out and
will avoid random overloads on the solid-state relays.
Care shall also be taken to protect the transistor or FET type outputs of the
SSRs, by locating "free wheel” diodes on the output terminals. While this
diode is often mounted as standard equipment by manufacturer on solid-state
relays, it is less often found on I/O modules which may be used to control
smaller motors.
Rotation Direction Reverser for a DC Motor
Braking Control for a DC Motor with Permanent Magnet
When the inertia on a motor is high with low friction, it may be necessary to
brake the motor during stop phases, when the motor will behave like a
generator (per Figure 10).
braking can be achieved by short-circuiting the motor terminals. Where a
reverser is used for ON - OFF/braking control, special attention shall be paid
to ensure that both relays are not conductive simultaneously, since this could
destroy them. An offset of 5 to 10 ms between the switching of the two relays
will guarantee that the first relay is totally non-conductive before triggering the
second relay.
This type of relatively brusque braking can be attenuated by installing a
resistor in series in the braking relay.
During the braking phase, the motor will behave like a generator and the
corresponding current may be as high as the start current. Consequently,
relays of similar characteristics should be selected for both the ON OFF/braking functions.
If additionally, the mechanical inertia of the motor is low, changing the motor
direction may lead to currents of up to twice the start current during the
braking phase.
86
87
Notes
In
DC
Out
+
Motor Control with
Integrated Braking
M
+
In
DC
Out
+
+
Fast Switching for an AC Load
Switching an AC load using SCRs does not provide for fast switching, in
particular when stopping conduction which may require up to 8.3 ms (at 60
Hz), 10 ms (50 Hz). A DC solid-state relay can switch an AC load via a diode
bridge thus avoiding the constraints specific to SCRs and triacs, in particular
the interruption at current zero and triggering with a high dv/dt.
This function can be achieved either with a DC solid-state relay and a diode
bridge (complete with the voltage drops which relate to it), or with two DC
solid-state relays and two diodes mounted in series on each SSR, reducing
voltage drops.
In
SSR
DC
Out
+
Diode Bridge Rectification
In
SSR
DC
Out
SSR
DC
Out
+
In
+
Full Wave Rectification using Two SSRs mounted in Parallel
Fast Switching ON to OFF for AC Loads
88
89
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Warning:
The product information contained in this catalogue is given
purely as information and does not constitute a
representation, warranty or any form of contractual
commitment. CROUZET
Automatismes and its
subsidiaries reserve the right to modify their products
without notice. It is imperative that we should be consulted
over any particular use or application of our products and it
is the responsability of the buyer to establish, particularly
through all the appropriate tests, that the product is suitable
for the use or application. Under no circumstances will our
warranty apply, nor shall we be held responsible for any
application (such as any modification, addition, deletion,
use in conjunction with other electrical or electronic
components, circuits or assemblies, or any other unsuitable
material or substance) which has not been expressly
agreed by us prior to the sale of our products.
Ref. 67 156 00 GW (Published January 2004) - Communication Crouzet Automatismes
AUSTRIA
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