Operation of SCR - Deepakkumar Yadav

Operation of SCR :

Operation of SCR can be studied under two different operating conditions. They are as follows :

SCR Operation without gate current :

Refer to Figure (a) The gate terminal of SCR is left open so that I_G= 0. SCR is forward biased by applying a positive voltage to anode with respect to cathode.
Out of the three junctions, the junctions J₁ and J₃ are forward biased and junction J₂ Is reverse biased. Therefore current does not flow through the SCR. The entire applied voltage appears across the junction J₂
As the anode to cathode voltage is increased, the voltage across junction J₂ increases.
At a certain voltage this junction will breakdown and SCR will start conducting. This voltage is called as forward breakover voltage (V_BO). Thus it is possible to turn on an SCR without any gate current by exceeding its forward voltage beyond V_{BO.

Figure (a) :- SCR Operation without gate current}

SCR Operation with gate current :

Now refer to Figure (b). It shows that SCR is forward biased as before and the gate cathode junction also is forward biased using an external power source.
The gate current I_G starts flowing which can be adjusted by the resistance R_G.
The value of gate current will decide breakover voltage of SCR. As I_G increases, breakover voltage will decrease i.e. SCR will turn on at lower and lower voltages.

Figure (b) :- SCR Operation with gate current