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 the introduction to SCR blog post.
1. Power Control.
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.
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 as phase 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. The SCR 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 in inverters, 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.
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.
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 D1 and 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
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 SCR2 and 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.
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 half-cycle 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.
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 half-cycle 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.
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 current becomes zero, it will turn-off. The output circuit is designed to have discharge current of less than a milli-second 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.
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 turn-on 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.