Silicon Controlled Rectifier ( SCR) is a semiconductor device used in switching applications. Unlike Transistors, SCR latches once triggered and remain on even if the triggering voltage at the gate is removed. The device can be switched off by removing the Anode current through a Push- to- off switch or by disconnecting the power supply.
Careful designing is needed to make an SCR switch. Like an NPN transistor, SCR has three pins. Its Anode ( A ) corresponds to the Collector, its Cathode ( K ) corresponds to the Emitter and its Gate ( G ) corresponds to the Base of the transistor. So when a positive voltage is applied to its gate, SCR triggers and current flows from Anode to Cathode.
Fig. 1 SCR Relay Driver Circuit
Fig. 1 shows the Relay switch using an SCR. It is designed to prevent accidental false triggering which is a common problem in SCR. Capacitor C1 and resistor R2 are included to ensure that the SCR is only triggered when the input is made positive deliberately. Resistor R3 is provided to ensure that minimum current always flows into the Anode, even if the device connected to Anode causes current to fluctuate. Such fluctuations can cause the SCR unlatch accidentally. Diode D1 removes high voltage ( e.m.f ) developed in the relay coil ( or any inductive device such as Motor)thus protecting the SCR. This diode is not necessary if the load is LED, Buzzer etc.