Unleashing the Power of the Silicon Controlled Switch (SCS)

SCS stands for Silicon Controlled Switch,SCS is a tetrode, Tetrodes are vacuum tubes with four electrodes thyristor. It has two gates, one anode gate (AG) like a PUT and another cathode gate (KG) like an SCR.
In other words, SCS is a four layer, four terminal pnpn device; with anode A, cathode K, anode gate AG and cathode gate KG.SCS can be turned on by either gate.
When a negative pulse is applied to gate AG, junction J1 is forward biased and SCS is turned on. A positive pulse at AG will reverse bias junction J1 and turns off the SCS. A positive pulse at gate KG turns on the device (just like an SCR) and a negative pulse at KG turns it off (just like a G.T.O.).
Its ratings are about 100 V and 200 mA. This can be operated like an OR gate.

Operation of a Silicon Controlled Switch

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 ﬂowfrom anode to cathode—the SCS is turned on.
The Silicon Controlled Switch (SCS) will remain ONuntil we remove the anode-to-cathode current, reversethe anode and cathode polarities, or apply a negative voltage to the anode gate.
The negative anode gate voltage removes thetransistor’s self-sustaining biasing current.

Silicon Controlled Switch (SCS) is its relatively slow turn-off time compared to other solid-state switches.
requirement for a gate current to trigger and control the switch.
Limited Turn-off Capability
High Voltage Drop
Limited Operating Frequency
Limited Voltage Ratings