Question

Turn-off of an SCR, it is necessary to reduce its current to less than

• A. Trigger current
• B. Holding current
• C. Breakdown current
• D. Forward current

Hint:

SCR turns OFF only when current < holding current.

Answer 1

The Correct Option is B

Step 1: Explain why SCR stays ON after gate pulse.
Once an SCR is triggered ON, the two internal transistors sustain each other through regenerative feedback. The gate has no control once the SCR is latched ON.

Step 2: Identify the minimum current for latch to be sustained.
The latch condition requires $\alpha_{PNP} + \alpha_{NPN} \geq 1$. At very low anode currents, the individual $\alpha$ values drop below this threshold and regeneration fails. The minimum current at which latch is still sustained is the holding current $I_H$.

Step 3: State the turn-off condition.
To turn off the SCR, reduce the anode current below $I_H$. When $I_A < I_H$, regeneration collapses and the SCR turns OFF. \[ \boxed{\text{Holding current}} \]