Working of SCR - Silicon Controlled Rectifiers

In solid state S/C silicon controlled rectifier , anode terminal is always kept at positive potential w.r.t cathode terminal. The load is connected in the series with the anode .

The working of SCR circuit can be grouped as followings:

When gate is open:In the SCR circuit with no voltage is applied to the gate i.e open gate,junction J2 is reverse biased whereas J1 and J3 is forward biased connection . Therefore , the condition in the junction J1 and J3 is as similar as npn transistor with base open .As a result no current flows via the load resister RL.,at that time the SCR is cut off state.If the applied voltage in the circuit is slightly increased ,a stage is reached when junction J2 breakdown because of reverse biased . Now the SCR conducts rapidely and at that time SCR side to be ON state .The amount of the applied voltage at which SCR conducts rapidely with open gate is called break over voltage.

When gate voltage is applied : When the gate terminal is positive w.r.t cathode , here junction J1 and J3 is forward biased where as the junction J2 is reverse biased.When small gate voltage is applied the SCR conduct heavily. In proper biasing condition ,the electron starts to move from n-type material to cross junction J3 towards left and holes from p-type material towards right .As a result ,electrons attracted across the junction J2 and gate current flows ,the anode current heavily increase .Consequently more electron available at junction J2 .This process run continues ,So junction J2 breaks down at an extremely small time and the SCR starts conducts heavily in the circuit .Once the SCR starts to conducts ,the gate loses its control properties .If gate voltage is removed ,even the anode current conducts heavily in the circuit. To stop current conduction , the applied voltage is reduced to zero voltage.

Conclusion:The following conclusions are drawn out from the working principles of SCR:

1.An SCR has two state ,one is ON state and other is OF state(i.e either is conducts heavily or does not conducts ). So SCR behaves like electronics switch.
2.On the SCR ,there are two ways to run the SCR as electronics switch.The first ways is to keep the gate open and make applied voltage equal to the break over voltage .The second ways is to apply the gate voltage ,and supply voltage is less than break over voltage.
3.When the gate voltage is applied then the break over voltage is always much greater than supply voltage .
To makes SCR non-conducting (ie open the SCR),reduce the supply voltage to zero.