Thyristors (SCR) MCQ [Free PDF] – 6 Objective Questions with Answers

Commutation is the process of transferring current away from a conducting power semiconductor so that its current falls below the required holding value and the device turns off. In line-commutated circuits the AC source provides this naturally; in forced commutation an auxiliary network produces the required current/voltage condition.

D — Electrical isolation between control and power circuits. The opto-coupler passes the trigger signal as light, so the low-voltage control circuit is kept electrically separate from the high-voltage power circuit.

In the two-transistor equivalent of a GTO, anode shorts are introduced to alter regenerative action and improve turn-off behavior. The shorting path is represented between the emitter of T1 and the base region associated with T2, corresponding to the device's anode-short structure.

B — Direct light radiation. Light falling on the LASCR's gate-sensitive region generates carriers that turn it ON, giving optical isolation — useful in HVDC valves.

Latching current is the minimum anode current needed just after triggering so the device stays on when gate drive is removed. Holding current is the lower current threshold below which an already-conducting SCR turns off; latching current is normally higher, commonly a few times the holding current.

Exam focus: Gate pulse must persist until anode current exceeds latching current. Common trap: Do not substitute holding current for latching current during turn-on.

Thyristor protection separates dv/dt and di/dt problems. An RC snubber across the device slows the rise of off-state voltage and absorbs transient energy; a series inductance limits current rise during turn-on. Key relation: dv/dt protection → RC across switch; di/dt protection → series L.

Exam focus: Match the protection component to the stress being limited. Common trap: A capacitor alone can create severe discharge current without the snubber resistor.