The above figure shows the use of SCR to switch ON or OFF a.c. power to a load R L.Resistances R 1 and R 2 are for the protection of diodes D 1 and D2 respectively.Resistance R 3 is the gate current limiting resistor.To start the circuit, switch is closed.During the positive half-cycle of a.c. supply, end A is positive and end B is negative.Then diode D 2 sends gate current through SCR 1.
