Silicon Controlled Switch (SCS) Symbol

A silicon-controlled switch (SCS) is a type of thyristor device with four terminals used in switching applications. For the most part, the gate junction operates as a SCR does, except this device has a fourth gate known as "Anode Gate", which also controls the flow of current from anode (A) to cathode (C). A negative voltage at the anode gate triggers the device ON, however a positive voltage triggers it OFF.