Triggering of an SCR can be controlled by the gate signal applied:-

• SCR is an undirectional device which allows the current to flow in only one direction. It can be used as a switching device which can be made ON and OFF by controlling the input voltage at the GATE terminal.
• From the V-I characteristics of SCR we can observe that SCR conducts fastly when there is a gate current.

• When the gate is open and the anode is positive with respect to cathode the SCR does not conduct unless the forward voltage exceeds certain value, called the forward breakover voltage, VFB0. When the applied voltage is greater than VFB0 then at a particular value of applied voltage the reverse biased J2 breaks down and the SCR conducts heavily . Then the SCR is said to be ON.
• If a positive gate current is supplied, then the SCR can become conducting at a voltage much lesser than forward break-over voltage. The larger the gate current, lower the break-over voltage.