Electromechanical or solid state relay? We can help you choose the

Electromechanical or solid state relay?
We can help you choose the right relay for your application
Let’s connect.
Relay applications
Relay applications
Electromechanical and solid state relays
Selecting the most appropriate relay for an application is often met with confusion or even overlooked
altogether. To address this issue we need to gain an understanding of the capabilities and limitations of
each popular type of relay available to industry.
Electromechanical relays
The first relays were electromechanical, that is they
comprised an electrical component, the coil and a
mechanical component, the contacts. This type of relay is
still by far the most common type in use today. The inherent
simplicity and low cost has resulted in the electromechanical
relay being used in many switching processes for inputs and
outputs of control systems.
Coil voltages are available for all industrial requirements
from 5V DC to 240V AC. Contacts are typically available to
switch up to 16A. Contacts can be arranged as normally
open, normally closed or as a changeover, and in groups of
1 to 6 poles. In some cases electromechanical relays can
be left to operate for many years without malfunction. The
“Achilles Heel” of electromechanical relays is the fact that
being partly mechanical they wear out, especially when they
are required to switch at high speeds. High speed switching
leads to deformation of the contact and overheating of the
coil. Typical switching rates of more than a few operations
per minute will require a more specialised product such as a
Solid State Relay.
Often when selecting a relay we only think about the
maximum current and voltage a relay can switch.
These are necessary but equally as important are the
minimum switching values. As we will see the type of
contact material and plating have a major influence on
switching characteristics.
Take for example the Weidmüller Termseries miniature
rail mounted relays. These have contacts made of the
alloy Nickel Silver. This is a very hard wearing material
but is designed for minimum switching levels of at least
5 V at 100mA.
The reason for this is that over time a thin layer of oxide or
tarnish will develop on the face of the contacts. This is not
a problem if the switching levels are above the minimum
specified as the tarnish is removed by the small arc that is
developed. If you were to switch a much smaller signal the
contact resistance could be too great and the reliability of
the switching action not maintained.
To overcome this situation Weidmüller offers gold plated
versions of their relay products. These versions use the
same base contact material but with a plating of 5um of
pure gold. This will allow signals down to 1V at 1mA to be
reliably switched.
For even smaller switching levels the MC5R Normally Open
range of relays can switch down to 100mV at 100uA. These
relays use a special bifurcated (two part) gold contact for the
ultimate in low switching level reliability.
WAUS Electromechanical and Solid state relays application brochure 2013
Solid state relays
N/O Output
Electromechanical diagram
Some relays are produced with a
gold flash over the base contact
material. This is designed to give the
contacts protection against airborne
contaminants until such time that
the relay is put into service. During
switching operations the gold flash is
usually burnt off leaving clean nickel
silver contacts. The low level switching
capabilities of gold flash contacts are
therefore the same as nickel silver.
Ever since the first industrial relays
were produced there has been
a concerted effort to overcome
the deficiencies present in
electromechanical relays ie. life
constraints and switching frequency
limitations. The invention of the Triac
and the power MOSFET have enabled
solid state relays to be developed
and optimised for reliable industrial
use. The coil and contacts have been
replaced by an optical switch. In
essence this is simply a light emitting
diode (LED) adjacent to a photo
sensitive transistor.
DC output: Solid state relay circuit
AC output: Solid state relay circuit
When the LED is energised it produces
light that falls on the transistor and
causes it to switch on. Depending
on the output configuration the
transistor drives a triac to switch AC or
a MOSFET to switch DC. No moving
parts are required so there is nothing
to wear out. Solid state relays are
also sometimes referred to as “optoisolators”, “opto-couplers” or
“Solenoid Drivers”.
Solid state relays are very reliable and
should last the life of the installation.
Versions are available specifically as
input devices with switching capacities
of up to 100mA, or as output devices
switching up to 40A inductive loads.
Power solid state relay
Other features of relays
Relays not only provide a remote
switching function but have several
other important operational features.
Relays can be used to interface
different voltage levels or to enable
large currents to be switched from low
level signals.
One point that is often overlooked is
the electrical isolation intrinsic to the
design of both electromechanical and
solid state relays.
Isolation is the ability to prevent the
flow of current from the input to the
output (or vice versa). For example a
relay may be used to switch a 240V AC
load but the input or coil voltage may
only be 24V DC. We need to ensure
the potentially dangerous voltage
on the output can never arc over or
allow current to flow to the input. This
“isolation” is usually expressed
in kilovolts.
How to choose the right relay for your application
Many engineers and technicians can be left perplexed by the variety and diversity of relays available
today. The selection criteria can differ from application to application but some common guidelines to
assist in this decision can be addressed.
Electromechanical may be appropriate
Solid state relay may be appropriate
•• Switching at less than 5 times per minute
•• Standard industrial use ie PLC/ DCS output
0.5 to 10A resistive load
•• Isolation up to 4kV required
•• Switching at high speed, more than 5 times per
minute up to 100kHz
•• Industrial use as PLC/DCS input 5mA to
100mA switching
•• Industrial use as an output device for inductive
load switching up to 40A
•• Critical switching where long life and reliable
operation are paramount.
Weidmüller has been manufacturing electromechanical and solid state relay products for over thirty
years. Our vast installed base across all industries attests to our dedication to ensuring the most
appropriate “fit for purpose” relay solutions are available to meet the demands of industry.
WAUS Electromechanical and Solid state relays application brochure 2013
Common issues when using relays
Electrically noisy industrial environments
Low Hysteresis
Narrow Hysteresis band of only 3.7V.
Ensures maximum rejection of industrial
noise and interference
ON 17.5V
13.8V OFF
MC5R – NEO is the perfect solution for all industrial switching needs.
Silver contact: 7940009903
Gold contact: 7940013020
Today’s industrial plants are radiating electrical noise in
every direction. Control systems are designed to reject this
interference. Unfortunately miniature relays with their low
input power consumption can be affected to the point that
they will not switch off!
One new solution to this problem is the Weidmüller’s
MC5R – NEO noise rejecting relay.
Another problem is the LED indication found on most relays
may indicate the relay is energised. When this situation
occurs it can be very difficult to find a solution. It is not
uncommon to see external resistors hanging from the relay
coil terminals to try and increase the input power and hence
lessen its susceptibility to noise. This can work to a point but
is not good engineering practice.
Standard relays can fail to switch off with a coil voltage of
only 4V but the MC5R NEO will reliably switch even when
extreme electrical interference and noise are present.
The MC5R – NEO rejects noise and switches reliably in the
most arduous of conditions. With low hysteresis the 24VDC
relay is unaffected by up to 13V of noise.
•• High Noise Immunity
•• Rejects Interference
•• Change Over Contact
•• 6A / 240VAC/ 24VDC
Electro-mechanical and solid-state relays – Comparison
Relay modules and solid-state relays – Comparison
Electromechanical relay
Solid-state relays
Fixed contacts
Movable contact
Advantages of electromechanical relay modules
+ AC and DC operation in load circuit possible
Versatile (advantage as interface between different
plant equipment)
+ No leakage current in the load circuit
A semi-conductor does not achieve 100 % isolation
+ Low residual voltage in the load circuit
Low voltage drop
+ No power loss in the load circuit
In contrast to the semi-conductor in opto modules
there is no electrical resistance in the contacts of
the electromechanical relay modules that can lead
to a rise in ­temperature when under load. Therefore,
heat sinks are not necessary.
+ Multiple contacts possible
A single control signal can switch several
load circuits.
+ Control circuit less sensitive to transients
Unwanted switching operations caused by voltage
fluctuations are prevented by the make capacity of
the magnetic coil.
Transmitter (LED)
Depending on the requirements, the choice between
electromechanical and solid-state relays is made based on
the different advantages that the different versions offer:
Advantages of solid-state relays (SSR)
+ Long operational lifetime and reliability
No moving parts or contact wear
+ Small dimensions
Saves space on the PCB and mounting rail
+ Low control power
An LED is activated - no mechanical parts
are moved
+ Fast response times
Fast switching, which allows high frequencies to
be achieved
+ No contact bounce
Reduces switching delays
+ No switching noise
Suitable for use in noise-sensitive environments
+ Not susceptible to shock and vibration
Prevents unwanted switching statuses
+No electromagnetic radiation due to switching sparks
or coils
No interference of adjacent assemblies or
electronics components
WAUS Electromechanical and Solid state relays application brochure 2013
Selection table
Selection table
TERMSERIES - Electromechanical relay modules
TRS 24V UC 1 C/O
TRS 230V AC RC 1 C/O
TRS 24V-230V UC 1 C/O
TRS 24V DC 2 C/O
TRS 24V UC 2 C/O
TRS 230V AC RC 2 C/O
TRS 24V-230V UC 2 C/O
Input (coil) voltage
24V DC +/- 20%
24V AC/DC +/- 10%
230V AC +/- 10%
24-230V AC/DC +/- 10%
24V DC +/- 20%
24V AC/DC +/- 10%
230V AC +/- 10%
24-230V AC/DC +/- 10%
TERMSERIES - Solid state relay modules
Output (contact type)
1 CO AgNi
1 CO AgNi
1 CO AgNi
1 CO AgNi
2 CO AgNi
2 CO AgNi
2 CO AgNi
2 CO AgNi
Max Switching
6A 30V DC / 250V AC
6A 30V DC / 250V AC
6A 30V DC / 250V AC
6A 30V DC / 250V AC
8A 30V DC / 250V AC
8A 30V DC / 250V AC
8A 30V DC / 250V AC
8A 30V DC / 250V AC
Switching Frequency
0.1Hz (6 ops/min)
0.1Hz (6 ops/min)
0.1Hz (6 ops/min)
0.1Hz (6 ops/min)
0.1Hz (6 ops/min)
0.1Hz (6 ops/min)
0.1Hz (6 ops/min)
0.1Hz (6 ops/min)
Order No.
Switching Frequency
300Hz (18,000 ops/min)
100Hz (6,000 ops/min) DC input
3Hz (180 ops/min) AC input
3Hz (180 ops/min) AC input
3Hz (180 ops/min) AC or DC input
300Hz (18,000 ops/min)
10Hz (600 ops/min) DC input
3Hz (180 ops/min) AC input
3Hz (180 ops/min) AC input
3Hz (180 ops/min) AC or DC input
3Hz (180 ops/min)
3Hz (180 ops/min)
3Hz (180 ops/min)
3Hz (180 ops/min)
300Hz (18,000 ops/min)
3Hz (180 ops/min)
Order No.
TOS 24V DC 48V DC 0.1A
Input voltage
24V DC +/- 20%
Output (contact type)
Open collector bipolar transistor
Max Switching
100mA 3 - 48V DC
TOS 24V UC 48V DC 0.1A
24V AC/DC +/- 10%
Open collector bipolar transistor
100mA 3 - 48V DC
230V AC +/- 10%
24 - 230V AC/DC +/- 10%
24V DC +/- 20%
Open collector bipolar transistor
Open collector bipolar transistor
100mA 3 - 48V DC
100mA 3 - 48V DC
2A 3-33V DC
24V AC/DC +/- 10%
2A 3-33V DC
230V AC +/- 10%
24 - 230V AC/DC +/- 10%
24V DC +/- 20%
24V AC/DC +/- 10%
230V AC +/- 10%
24 - 230V AC/DC +/- 10%
24V DC +/- 20%
24 - 230V AC/DC +/- 10%
Open collector bipolar transistor
Open collector bipolar transistor
TOS 230V AC RC 48V DC 0.1A
TOS 24-230V UC 48V DC 0.1A
TOS 24V DC 48V DC 2A
TOS 24V UC 48V DC 2A
TOS 230V AC RC 48V DC 2A
TOS 24-230V UC 48V DC 2A
TOS 24V DC 230V AC 1A
TOS 24V DC 230V UC 1A
TOS 230V AC RC 230V AC 1A
TOS 24-230V UC 230V AC 1A
TOS 24V DC 24V DC 3.5A
TOS 24-230V UC 24V DC 3.5A
2A 3-33V DC
2A 3-33V DC
24-250V AC (min 20mA)
24-250V AC (min 20mA)
24-250V AC (min 20mA)
24-250V AC (min 20mA)
3.5A 3 - 33V DC
3.5A 3 - 33V DC
*For Solid state relays with switching outputs up to 10A 24V DC or 20A 230V AC please see catalogue below:
Relay modules and solid-state relays
Catalogue 2013/2014
Let’s connect.
Digital signal processing
For complete overview of our Relay modules and solid-state
relays catalogue. order number. 1427050000
Weidmüller – Partner in Industrial Connectivity.
As experienced experts we support our customers and partners around the world
with products, solutions and services in the industrial environment of power,
signal and data. We are at home in their industries and markets and know the
technological challenges of tomorrow. We are therefore continuously developing
innovative, sustainable and useful solutions for their individual needs.
Together we set standards in Industrial Connectivity.
For all Weidmüller Product Distibutors please visit www.weidmuller.com.au/au/distributors
Or contact your preferred supplier below:
Weidmüller Pty Ltd (Head Office)
43 Huntingwood Drive,
Huntingwood NSW 2148
Phone +61 (0) 2 9671 9999
Fax +61 (0) 2 9671 9900
Email [email protected]
Order Number: WAUS Electromechanical and Solid state relays application brochure 2013