SCR Applications
The ability of an SCR to control large currents to a load by means of small gate current
makes the device very useful in switching and control applications. A few of the possible
applications for the SCR are listed in theintroduction to SCR blog post.
Here we will consider six applications of SCR like power control, switching, zero-voltage
switching, over-voltage protection, pulse circuits and battery charging regulator.
1. Power Control.
SCR Power Control Circuit
Because of the bistable characteristics of semiconductor devices, whereby they can be
switched on and off, and the efficiency of gate control to trigger such devices, the SCRs
are ideally suited for many industrial applications. SCRs have got specific advantages
over saturable core reactors and gas tubes owing to their compactness, reliability, low
losses, and speedy turn-on and turn-off.
The bistable states (conducting and non-conducting) of the SCR and the property that
enables fast transition from one state to the other are made use of in the control of power
in both ac and dc circuits.
SCR Phase Control
In ac circuits the SCR can be turned-on by the gate at any angle α with respect to
applied voltage. This angle α is called the firing angle and power control is obtained by
varying the firing angle. This is known asphase control. A simple half-wave circuit is
shown in figure a. for illustrating the principle of phase control for an inductive load. The
load current, load voltage and supply voltage waveforms are shown in figure b. TheSCR
will turn-off by natural commutation when the current becomes zero. Angle β is known
as the conduction angle. By varying the firing angle a, the rms value of the load voltage
can be varied. The power consumed by the load decreases with the increase in firing
angle a. The reactive power input from the supply increases with the increase in firing
angle. The load current wave-form can be improved by connecting a free-wheeling diode
D1, as shown by the dotted line in fig-a. With this diode, SCR will be turned-off as soon as
the input voltage polarity reverses. After that, the load current will free wheel through
the diode and a reverse voltage will appear across the SCR. The main advantage of phase
control is that the load current passes through a natural zero point during every half
cycle. So, the device turns-off by itself at the end of every conducting period and no other
commutating circuit is required.
Power control in dc circuits is obtained by varying the duration of on-time and off-time
of the device and such a mode of operation is called on-off control or chopper
control. Another important application of SCRs is ininverters, used for converting dc
into ac. The input frequency is related to the triggering frequency of SCRs in the
inverters. Thus, variable frequency supply can be easily obtained and used for speed
control of ac motors, induction heating, electrolytic cleaning, fluorescent lighting and
several other applications. Because of the large power-handling capacity of the SCRs, the
SCR controlled inverter has more or less replaced motor-generator sets and magnetic
frequency multipliers for generating high frequency at large power ratings.
Operation of Power Control in SCR
A commonly used circuit for controlling power in load RL using two SCRs is shown in
figure. Potentiometer R controls the angle of conduction of the two SCRs. The greater the
resistance of the pot, lesser will be the voltage across capacitors C1 and C2 and hence
smaller will be the time duration of conduction of SCR1 and SCR2 during a cycle.
During positive half cycle capacitor C2 gets charged through diode D1, pot R, and diode
D4. When the capacitor gets fully charged, (charge on the capacitor depending upon the
value of R) it discharges through Zener diode Z. This gives a pulse to the primary and
thereby secondary of the transformer T2. Thus SCR2, which is forward biased, is turned
on and conducts through load RL. During negative half cycle similar action takes place
due to charging of capacitor C1 and SCR1 is triggered. Thus power to a load is controlled
by using SCRs.
2. Switching.
Thyristor, being bistable device is widely used for switching of power signals owing to
their long life, high operation speed and freedom from other defects associated with
mechanical and electro-mechanical switches.
AC Circuit Breaker using SCR
Figure shows a circuit in which two SCRs are used for making and breaking an ac circuit.
The input voltage is alternating and the trigger pulses are applied to the gates of SCRs
through the control switch S. Resistance R is provided in the gate circuit to limit the gate
current while resistors R1 and R2 are to protect the diodes D1and D2 respectively.
For starting the circuit, when switch S is closed, SCR1 will fire at the beginning of the
positive half-cycle (the gate trigger current is assumed to be very small) because during
positive half cycle SCR1 is forward biased. It will turn-off when the current goes through
the zero value. As soon as SCR1 is turned-off, SCR2 will fire since the voltage polarity is
already reversed and it gets the proper gate current. The circuit can be broken by
opening the switch S. Opening of gate circuit poses no problem, as current through this
switch is small. As no further gate signal will be applied to the SCRs when switch S is
open, the SCRs will not be triggered and the load current will be zero. The maximum
time delay for breaking the circuit is one half-cycle.
Thus several hundred amperes of load current can be switched on/off simply by handling gate current of few mA by an ordinary switch. The above circuit is also called the
static contactor because it does not have any moving part.
DC circuit breaker
SCR Application-DC Circuit Breaker
As shown in figure, Capacitor C provides the required commutation of the main SCR
since the current does not have a natural zero value in a dc circuit. When the SCR1 is in
conducting state, the load voltage will be equal to the supply voltage and the capacitor C
will be charged through resistor R. The circuit is broken by turning-off SCR1. This is done
by firing SCR2, called the auxiliary SCR. Capacitor C discharges through SCR2and SCR1.
This discharge current is in opposite direction to that flowing through SCR1 and when
the two become equal SCR2 turns-off. Now capacitor C gets charged through the load
and when the capacitor C gets fully charged, the SCR2 tums-off. Thus the circuit acts as a
dc circuit breaker. The resistor R is taken of such a value that current through R is lower
than that of holding current.
3. Zero Voltage Switching.
SCR Switching Application
In some ac circuits it is necessary to apply the voltage to the load when the instantaneous
value of this voltage is going through the zero value. This is to avoid a high rate of
increase of current in case of purely resistive loads such as lighting and furnace loads,
and thereby reduce the generation of radio noise and hot-spot temperatures in the device
carrying the load current. The circuit to achieve this is shown in figure. Only half-wave
control is used here. The portion of the circuit shown by the dotted lines relates to the
negative half cycle. Whatever may be the instant of time when switch S is opened (either
during the positive or the negative half cycle), only at the beginning of the following
positive half-cycle of the applied voltage SCR1 will be triggered. Similarly, when switch S
is closed, SCR1 will stop conducting at the end of the present or previous positive halfcycle and will not get triggered again. Resistors R3 and R4 are designed on the basis of
minimum base and gate currents required for transistor Q1 and SCR1. Resistors Rl and
R2 govern rates of the charging and discharging of capacitor C1 Resistor R5 is used for
preventing large discharge currents when switch S is closed.
4. Over-Voltage Protection.
Over Voltage Circuit Protection
SCRs can be employed for protecting other equipment from over-voltages owing to their
fast switching action. The SCR employed for protection is connected in parallel with the
load. Whenever the voltage exceeds a specified limit, the gate of the SCR will get
energized and trigger the SCR. A large current will be drawn from the supply mains and
voltage across the load will be reduced. Two SCRs are used—one for the positive halfcycle and the other for negative half-cycle, as shown in figure. Resistor R1 limits the
short-circuit current when the SCRs are fired. Zener diode D5 in series with resistors
Rx and R2 constitutes a voltage-sensing circuit.
5. Pulse Circuits.
SCR-Pulse Circuit
SCRs are used for producing high voltage/current pulses of desired waveform and
duration. The capacitor C is charged during the positive half cycle of the input supply
and the SCR is triggered during the negative half-cycle. The capacitor will discharge
through the output circuit, and when the SCR forward currentbecomes zero, it will
turn-off. The output circuit is designed to have discharge current of less than a millisecond duration. The capacitor will again get charged in the following positive half-cycle
and the SCR will be triggered again in the negative half-cycle. Thus the frequency of the
output pulse will be equal to the frequency of the input supply. For limiting the charging
current resistor R is used. High voltage/current pulses can be used in spot welding,
electronic ignition in automobiles, generation of large magnetic fields of short duration,
and in testing of insulation.
6. Battery Charging Regulator.
Battery Charging Regulator
The basic components of the circuits are shown in figure. Diodes D1 and D2 are to
establish a full-wave rectified signal across SCR1 and the 12 V battery to be charged.
When the battery is in discharged condition, SCR2 is in the off-state as will be clear after
discussion. When the full-wave rectified input is large enough to give the required turnon gate current (controlled by resistor R1), SCR1 will turn on and the charging of the
battery will commence. At the commencement of charging of battery, voltage
VR determined by the simple voltage-divider circuit is too small to cause 11.0 V zener
conduction. In the off-state Zener diode is effectively an open-circuit maintaining
SCR2 in the off-state because of zero gate current. The capacitor C is included in the
circuit to prevent any voltage transients in the circuit from accidentally turning on of the
SCR2. As charging continues, the battery voltage increases to a point when VR is large
enough to both turn on the 11.0 V Zener diode and fire SCR2. Once SCR2 has fired, the
short circuit representation for SCR2 will result in a voltage-divider circuit determined by
R1 and R2 that will maintain V2 at a level too small to turn SCR1 on. When this occurs, the
battery is fully charged and the open-circuit state of SCR1 will cut off the charging
current. Thus the regulator charges the battery whenever the voltage drops and prevents
overcharging when fully charged. There are many more applications of SCRs such as in
soft start circuits, logic and digital circuits, but it is not possible to discuss all these here.
FET applications
JOJO
AUGUST - 30 - 2009
8 COMMENTS
FET has a very high input impedance (100 Mega ohm in case of JFETs and 10 4 to 109 Mega Ohm in
case of MOSFETs), the major shortcomings of an ordinary transistor i.e. low input impedance with
consequent of loading of signal source is eliminated in FET. Hence FET is an ideal device for use in
almost every application in which transistors can be used. FETs are widely used as input amplifiers in
oscilloscopes, electronic voltmeters and other measuring and testing equipment because of their
high input impedance.
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As a FET chip occupies very small space as compared to BJT chip, FETs are widely
used in ICs.
FETs are used as voltage-variable resistors (WRs) in operational amplifiers (opamps) and tone controls etc, for mixer operation on FM and TV receivers and in logic
circuits.
FETs are generally used in digital switching circuits though their operating speed is
lower.
Applications of FET:
1. Low Noise Amplifier. Noise is an undesirable disturbance super-imposed on a useful signal. Noise
interferes with the information contained in the signal; the greater the noise, the less the
information. For instance, the noise in radio-receivers develops crackling and hissing which
sometimes completely masks the voice or music. Similarly, the noise in TV receivers produces small
white
or
black
spots
on
the
picture;
a
severe
noise may wipe out the picture. Noise is independent of the signal strength because it exists even
when the signal is off.
Every electronic device produces certain amount of noise but FET is a device which causes very little
noise. This is especially important near the front-end of the receivers and other electronic
equipment because the subsequent stages amplify front-end noise along with the signal. If FET is
used at the front-end, we get less amplified noise (disturbance) at the final output.
2. Buffer Amplifier.
A buffer amplifier is a stage of amplification that isolates the preceding stage from the following
stage. Source follower (common drain) is. used as a buffer amplifier. Because of the high input
impedance and low output impedance a FET acts an excellent buffer amplifier, as shown in figure.
Owing to high input impedance almost all the output voltage of the preceding stage appears at the
input of the buffer amplifier and owing to low output impedance all the output voltage from the
buffer amplifier reaches the input of the following stage, even there may be a small load resistance.
3. Cascode Amplifier. Circuit diagram for a cascode amplifier using FET is shown in figure. A
common source amplifier drives a common gate amplifier in it.
Cascode amplifier circuit
The cascode amplifier has the same voltage gain as a common source (CS) amplifier. The main
advantage of cascode connection is its low input capacitance which is considerably less than the
input capacitance of a CS amplifier. It has high input resistance which is also a desirable feature.
4. Analog Switch. FET as an analog switch is shown in figure. When no gate voltage is applied to the
FETi.e. VGS = 0, FET becomes saturated and it behaves like a small resistance usually of the value of
less than 100 ohm and, therefore, output voltage becomes equal to
VOUT = {RDS/ (RD + RDS (ON))}* Vin
JFET-analog-switch
Since RD is very large in comparison to RDS 0N), so Vout can be taken equal to zero.
When a negative voltage equal to VGS (OFF) is applied to the gate, the FET operates in the cut-off region
and it acts like a very high resistance usually of some mega ohms. Hence output voltage becomes
nearly equal to input voltage.
5. Chopper. A direct-coupled amplifier can be built by leaving out the coupling and bypass capacitors
and connecting the output of each stage directly to the input of next stage. Thus direct current is
coupled, as well as alternating current. The major drawback of this method is occurrence of drift, a
slow shift in the final output voltage produced by supply transistor, and temperature variations.
The drift problem can be overcome by employing chopper amplifier as illustrated in figure.
Chopper Amplifier
(a). Here input dc voltage is chopped by a switching circuit. The output of chopper is a square wave
ac signal having peak value equal to that of input dc voltage, VDC. This ac signal can be amplified by a
conventional ac amplifier without any problem of drift. Amplified output can then be ‘peak
detected’ to recover the amplified dc signal.
Square wave is applied to the gate of a FET analog switch to make it operate like a chopper, as
illustrated in other figure. The gate square wave is negative-going swing from 0 V to at least VGS (off)-
This alternately saturates and cuts-off the JFET. Thus output voltage is a square wave varying from
+VDC to zero volt alternately.
If the input signal is a low-frequency ac signal, it gets chopped into the ac waveform as shown in last
figure (c). This chopped signal can now be amplified by an ac amplifier that is drift free. The
amplified signal can then be peak-detected to recover the original input low frequency ac signal.
Thus both dc and low frequency ac signals can be amplified by using a chopper amplifier.
6. Multiplexer.
FET multiplexer
An analog multiplexer, a circuit that steers one of the input signals to the output line, is shown in
figure. In this circuit each JFET acts as a single-pole single-throw switch. When the control
signals (Vv V2 and V3) are more negative than VGS(0FF) all input signals are blocked. By making any
control voltage equal to zero, one of the inputs can be transmitted to the output. For instance, when
Vx is zero, the signal obtained at the output will be sinusoidal. Similarly when V2 is zero,
the signal obtained at the output will be triangular and when V3is zero, the output signal will be
square-wave one. Normally, only one of the control signals is zero.
7. Current Limiter.
JFET current Limiter
JFET current limiting circuit is shown in figure. Almost all the supply voltage therefore
appears across the load. When the load current tries to increase to an excessive level
(may be due to short-circuit or any other reason), the excessive load current forces the
JFET into active region, where it limits the current to 8 mA. The JFET now acts as a
current source and prevents excessive load current.
A manufacturer can tie the gate to the source and package the JFET as a two terminal device. This is
howconstant-current diodes are made. Such diodes are also called current-regulator diodes.
8. Phase Shift Oscillators.
FET-phase shift oscillator
JFET can incorporate the amplifying action as well as feedback action. It, therefore, acts well as a
phase shift oscillator. The high input impedance of FET is especially very valuable in phase-shift
oscillators in order to minimize the loading effect. A typical phase shift oscillator employing Nchannel JFET is shown in figure.
The Zener effect as embodied in the zener diode has many applications for control and regulation.
Applications
Zener Regulators
Zener Controlled Comparators
Zener Limiters
Role in Power Supplies