SCR (Silicon Controlled Rectifier)

SCR-SILICON CONTROLLED RECTIFIER CONSTRUCTION

It is a four layer, three junction and three terminal device. The basic structure and circuit symbol shown in fig.

P₁ layer act as anode, N₂ layer act as cathode and N₁, P₂ layer junction act as

Operation

SCR is forward biased with a small voltage, it is in off and no current flows through the SCR. If applied forward voltage is increased, one particular voltage called forward breake over voltage and the junction J₂ break down. This causes the SCR switched to ON state

SCR is reverse biased, then the junction J₁ and J₂ are reverse biased and junction J₂ is forward biased and most of the voltage drop across junction J₁ only. When the applied reverse voltage is small, the SCR is OFF and there is no current flow through the device.

Characteristics of SCR

There are two types of V-I characteristics

(i) Forward characteristics

(ii) Reverse characteristics

Forward characteristics.

Current drawn between anode-cathode voltage (V_ak) and anode current (I_A) at different gate current. The applied voltage across the SCR increases in small suitable steps at each step

Reverse characteristics

Reverse characteristics is obtained by reversing the connections of the DC supplies. Adjust the gate current to suitable value. Increase the reverse applied voltage in suitable steps.

SCR act as a switch when it is forward biased. When the gate is kept open I_G = 0. When I_G<0, the amount of reverse biased applied to J₂ is increased. So the break over voltage V_BO is increased, when I_G>0 the amount of reverse bias applied to J₂ is decreased there by decreasing the break voltage.

The voltage at which SCR is switched ON can be controlled by varying the gate current I_G it is called as controlled switch.