In thyristor terminology (SCR), “closing” the device means turning it OFF (commutating it). What is the only reliable way to force an SCR to turn off in normal operation?

Introduction / Context:
The silicon-controlled rectifier (SCR) is a unidirectional PNPN thyristor that latches ON after it is triggered. Unlike a transistor or a MOSFET, an SCR cannot be driven OFF by its gate once it is conducting. This question checks whether you know the correct mechanism for turning an SCR OFF (often called “closing” it or commutation).

Given Data / Assumptions:

• Device: SCR (thyristor) operating in its normal region.
• Gate can trigger conduction but does not provide a turn-OFF path.
• Standard definitions: latching current and holding current.

Concept / Approach:
An SCR turns ON when the anode-cathode current exceeds the latching current after a gate trigger or breakover. Once ON, it remains conducting until the anode current falls below the device’s holding current. This forced reduction of current—often by natural AC current zero or by commutation networks—is called low-current dropout or current starvation below the holding current.

Step-by-Step Solution:
Recognize: Gate triggering enables ON-state; it cannot force OFF-state.Identify OFF condition: I_AK (anode current) must drop below the holding current I_H.Methods: natural commutation in AC at current zero, or forced commutation using external circuitry.Therefore, the only reliable “close” method is low-current dropout below I_H.

Verification / Alternative check:
Datasheets specify I_L (latching current) and I_H (holding current). Practical circuits for DC use explicit commutation (capacitor/inductor networks) to interrupt anode current, confirming that OFF control is by reducing current, not by the gate.

Why Other Options Are Wrong:

• a) Gate trigger: Turns ON, not OFF.
• b) Forward breakover: Another turn-ON mechanism at high forward voltage.
• d) Valley voltage: A term more relevant to UJT characteristics, not SCR turn-OFF.
• e) Reverse-biasing the gate only: Gate control cannot unlatch an SCR once conducting.

Common Pitfalls:

• Assuming gate works like a transistor base for turn-OFF—it does not.
• Ignoring holding current; even small load currents can keep the SCR latched.

Final Answer:
low-current dropout