Chapter 7: THYRISTORS
The Silicon-Controlled Switch (SCS)
If we take the equivalent circuit for an SCR and add another external terminal, connected to the base of the top transistor and the collector of the bottom transistor, we have a device known as a silicon-controlled-switch, or SCS: (Figure below)
The Silicon-Controlled Switch(SCS)
This extra terminal allows more control to be exerted over the device, particularly in the mode of forced commutation, where an external signal forces it to turn off while the main current through the device has not yet fallen below the holding current value. Note that the motor is in the anode gate circuit in Figure below. This is correct, although it doesn't look right. The anode lead is required to switch the SCS off. Therefore the motor cannot be in series with the anode.
SCS: Motor start/stop circuit, equivalent circuit with two transistors.
When the "on" pushbutton switch is actuated, the voltage applied between the cathode gate and the cathode, forward-biases the lower transistor's base-emitter junction, and turning it on. The top transistor of the SCS is ready to conduct, having been supplied with a current path from its emitter terminal (the SCS's anode terminal) through resistor R2 to the positive side of the power supply. As in the case of the SCR, both transistors turn on and maintain each other in the "on" mode. When the lower transistor turns on, it conducts the motor's load current, and the motor starts and runs.
The motor may be stopped by interrupting the power supply, as with an SCR, and this is called natural commutation. However, the SCS provides us with another means of turning off: forced commutation by shorting the anode terminal to the cathode.  If this is done (by actuating the "off" pushbutton switch), the upper transistor within the SCS will lose its emitter current, thus halting current through the base of the lower transistor. When the lower transistor turns off, it breaks the circuit for base current through the top transistor (securing its "off" state), and the motor (making it stop). The SCS will remain in the off condition until such time that the "on" pushbutton switch is re-actuated.
- A silicon-controlled switch, or SCS, is essentially an SCR with an extra gate terminal.
- Typically, the load current through an SCS is carried by the anode gate and cathode terminals, with the cathode gate and anode terminals sufficing as control leads.
- An SCS is turned on by applying a positive voltage between the cathode gate and cathode terminals. It may be turned off (forced commutation) by applying a negative voltage between the anode and cathode terminals, or simply by shorting those two terminals together. The anode terminal must be kept positive with respect to the cathode in order for the SCS to latch.