SCR Turn Off Explained: Anode Current and Holding Current

A Silicon Controlled Rectifier (SCR), also known as a thyristor, is a semiconductor device used for controlling and switching electrical power. It acts like a switch that can be turned ON by a gate pulse but requires specific conditions to turn OFF.

Understanding SCR Turn-Off

An SCR has three terminals: Anode (A), Cathode (K), and Gate (G). Once an SCR is turned ON (latched), it continues to conduct current from anode to cathode even if the gate signal is removed. To turn the SCR OFF, the current flowing through it must be interrupted or reduced significantly.

The Role of Holding Current

Every SCR has a characteristic parameter called the holding current, denoted as $I_H$. This is the minimum anode current required to maintain the SCR in the ON state (conducting state). If the anode current drops below this specific value, the SCR will transition from its conducting state back to its blocking state (OFF state).

Condition for Turning OFF an SCR

For an SCR to turn OFF, the anode current ($I_A$) must fall below the holding current ($I_H$).

• When $I_A$ below $I_H$: The SCR turns OFF.
• When $I_A$ is equal to $I_H$: The SCR is at the threshold and will likely turn off.
• When $I_A$ is above $I_H$: The SCR remains ON (provided it was triggered).

Therefore, the critical condition for turning an SCR OFF is ensuring the anode current is reduced to a level below the specified holding current value.