Thyristor control: A silicon-controlled rectifier (SCR) can be turned off (commutated) by which method under normal operation?

Introduction:
SCRs latch on after being triggered and conduct until current falls below the holding current. Turning them off requires line commutation (in AC) or explicit forced commutation in DC circuits. This question checks understanding of valid SCR turn-off mechanisms.

Given Data / Assumptions:

• Conventional SCR (not a GTO or IGCT).
• Gate used only for turn-on; device is latched once conducting.
• DC or controlled AC applications are considered.

Concept / Approach:
In DC or when natural current zero is unavailable, auxiliary networks or switches momentarily divert or reverse the current (or apply reverse voltage) to commutate the SCR—this is forced commutation. A negative gate pulse cannot turn off a standard SCR; exceeding breakover voltage turns it on, not off; a generic “off switch” statement is not a technical turn-off method without forcing the current below holding current.

Step-by-Step Solution:

Verification / Alternative check:
Standard commutation classes (A–F) describe practical forced-commutation topologies using capacitors/inductors or auxiliary switches to create current zero or reverse bias.

Why Other Options Are Wrong:

• Negative gate pulse: Ineffective on standard SCRs; only GTO devices accept turn-off gate drive.
• Off switch: Vague; unless it forces current below holding current, the SCR remains on.
• Breakover exceeded: Causes unintended turn-on, not turn-off.

Common Pitfalls:
Assuming the gate can both turn on and off an SCR; confusing SCR with GTO; ignoring holding and latching current specifications.

Final Answer:
by forced commutation