What Is SCR In Electronics[Silicon Controlled Rectifier]



SCR In Elec­tron­ics SCR is a three ter­mi­nal (anode, cath­ode and gate), three junc­tion and four lay­er semi­con­duc­tor switch­ing device. 

Principle of operation

In the nor­mal oper­at­ing con­di­tions, a pos­i­tive volt­age is applied to the anode (A) and a small pos­i­tive volt­age is applied to the gate (G) with respect to the cath­ode (K)

When the gate is kept open Vg — 0, the SCR is sim­i­lar to a PNPN diode. The junc­tions J. and 3 oper­ate in for­ward bias and the junc­tion J, oper­ates in reverse bias. So no cur­rent flows through the SCR. There­fore the SCR is in OFF state. The SCR now offers high resistance.When the anode volt­age is grad­u­al­ly increased, the junc­tion J, attains break­down at a par­tic­u­lar voltage. 

Now the SCR con­ducts heav­i­ly going to ‘ON’ state. The anode volt­age, at which the SCR con­ducts heav­i­ly (ON state) with­out gate volt­age is called Breakover voltage.

‘When a small pos­i­tive volt­age is applied to the gate, the junc­tion Jz is in for­ward biased and the junc­tion J, is in reverse biased by this voltage. 

Now the elec­trons move from N type lay­er (cath­ode) to P type lay­er, through the junc­tion Jz. The elec­trons in junc­tion J, are also attract­ed by junc­tion J„ and gate cur­rent starts flowing. 

The holes also move from P lay­er (G) to N‑layer (K) which in turn increas­es the anode cur­rent, and makes more elec­trons avail­able at junc­tion J, This cumu­la­tive process make to break­down the junc­tion J in a short time. Now the SCR con­ducts heavily. 

once an SCR is turned ON the gate los­es its con­trol. Even if the gate volt­age is removed the SCR does not go to OFF state. 

To turn the device OFF is only done by low­er­ing the anode volt­age and makes the cur­rent less than hold­ing current 


The SCR has only two states, name­ly ON s. and OFF state. There is no state in between the states. When appro­pri­ate gate cur­rent is passed, the son starts con­duct­ing heav­i­ly and remains in this sta def­i­nite­ly, even if gate volt­age is removed. 

This cor­re­sponds to ON condition.wins in this state inHow­ev­er, when the anode cur­rent is reduced to the hold­ing cur­rent, the SCR turned OFF. 

This behav­iour of SCR is sim­i­lar to a mechan­i­cal switch an SCR is an elec­tron­ic device, ther­fore it is also called elec­tron­ic switch.

When a suf­fi­cient pos­i­tive volt­age is applied to its gate ter­mi­nal, the SCR con­ducts and anode cur­rent flows to a max­i­mum level. 

Now the SCR goes in to the ON state, worked as a closed switch. Its out­put volt­age is low.When a prop­er neg­a­tive volt­age is applied to its gate ter­mi­nal, the SCR goes into OFF state, Now no cur­rent flows through the SCR, worked as an open switch. It’s out­put volt­age is high. 


1)It has no mov­ing parts, so noise­less oper­a­tion at high efficiency. 

2)High speed operations 

3)It can con­trol large cur­rent in the load by mean­sof small gate current. 

4) Small size and give trou­ble free service.

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