Silicon Controlled Switch (SCS): Basics, Operation, Application

Silicon Controlled Switch (SCS): Basics, Operation, Application

This post will explain about one of the newly developed Power Semiconductor Device known as Silicon Controlled Switch [SCS]. It acts as a switch in the power electronic circuits and its switching operation (both turn on and turn off) can be controllable. That is the reason this device is called as Silicon Controlled Switch.

  • The Silicon Controlled Switch (SCS) is a power semiconductor device similar to an SCR, but it is designed to turn off when a positive voltage/input current pulse is applied to an additional "Anode gate" terminal.
  • The SCS can also be triggered into conduction mode by applying a negative voltage/output current pulse to the same lead.
  • In addition to this, the Silicon Controlled Switch behaves just like an SCR ( ie, by applying positive voltage to the "gate" terminal we can turn on the device).
  • In summary, SCS is a four layer, four terminal PNPN semiconductor device. The four terminals are Anode(A), Cathode (K) and two gates. The two gate terminals are namely anode gate(AG) and cathode gate(KG). The device has one anode gate(AG) similar to PUT and another cathode gate(KG) similar to SCR. As it has four terminals, it is named as tetrode (four electrode thyristor).
  • The symbol of the Silicon Controlled Switch (SCS) is shown below

SCS Equivalent circuit:

The following figure shows a basic model of an Silicon Controlled Switch (SCS), along with its bipolar equivalent circuit. The equivalent circuit looks a lot that the SCR equivalent circuit, with the exception of the anode gate connection.

Silicon Controlled Switch Applications:

Silicon Controlled Switch (SCS) is used in pulse generators, voltage sensors, counters, timing, logic and triggering circuits.

The device is used in lamp drivers, power-switching circuits, and logic circuits as well as in essentially any circuit that requires a switch that can be turned ON and OFF by two separate control pulses.

How SCS Works:

  • When a positive pulse of current is applied to the gate terminal, the NPN transistor turns ON.
  • This allows current to exit the PNP transistor’s base, hence turning the PNP transistor ON.
  • Now that both transistors are ON, current can flow from anode to cathode—the SCS is turned on.
  • The Silicon Controlled Switch (SCS) will remain ON until we remove the anode-to-cathode current, reverse the anode and cathode polarities, or apply a negative voltage to the anode gate.
  • The negative anode gate voltage removes the transistor’s self-sustaining biasing current.

When buying an silicon controlled switch (SCS), make sure to select a device that has the proper breakdown voltage, current, and power-dissipation ratings. A typical specification table will provide the following ratings: Collector to Base Breakdown Voltage (BVCB) Emitter to Base Breakdown Voltage (BVEB) Collector to Emitter Breakdown Voltage (BVCE) Emitter Current (IE) Collector Current (IC) Holding Current (IH)  and Power Dissipation.

Read More:
Diac: Basics, Characteristics, Operation, Applications
Triac: Basics, Operation, Applications
IGCT Insulated Gate Commutated Thyristor Tutorial
GTO Gate TurnOFF Thyristor Tutorial
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5 Responses

  1. chigbo says:

    thanks fir that post.. do u know the kind of transistors that would be suitable for the sca circuit ? thanks again

  2. Victor Victor says:

    I would like to know where can I purchase this product in my home country

  3. amir says:

    Did scs have another name like gcs ? Are they have same function ? Im confuse of gcs. So can u help me in this situation ?

  4. Atom says:

    how is an SCS useful in voltage sensors???

  5. NIYONGABO Olivier says:


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