In a thyristor, the magnitude of anode current will

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not change with any variation in gate current.

Explanation: A thyristor (SCR) is a latching device. Once the device is turned on by a gate current pulse exceeding the threshold, it enters a state of conduction determined by the external circuit parameters (anode voltage and load resistance) rather than the gate current magnitude.

Working Principle: The thyristor acts as a bistable switch. After the regenerative turn-on process (triggered by the gate current), the internal feedback mechanism keeps the device in the ON state. Increasing the gate current further does not influence the anode current magnitude because the internal junction is already fully saturated; the current is governed solely by $I_A = \frac{V_S - V_{AK}}{R_L}$.

Key Formulas: $I_A = \frac{V_{source} - V_{AK}}{R_{load}}$ — Anode current is defined by circuit parameters; $I_{holding}

Key Points: The gate is only required to initiate the turn-on process.; Once $I_A$ exceeds the latching current ($I_L$), the gate pulse can be removed.; Variation in $I_G$ after turn-on does not affect the anode current flow.; The thyristor is a current-controlled latching device.

Advantages: High power handling capability; Efficient switching with low gate power requirements

Disadvantages: Cannot be turned off via the gate terminal; Requires external commutation circuits to turn off

Applications: AC voltage controllers; Phase-controlled rectifiers; Static switches

Additional Info: The gate current pulse duration must be sufficient to ensure $I_A$ reaches the latching current.. Once the thyristor is triggered, the gate loses control over the anode current, which is why options A, B, and C are incorrect.

Answer: D is correct — Once a thyristor is turned on, the gate loses control, and the anode current is determined solely by the external circuit parameters.

Exam Tip: Always remember that SCRs are latching devices; in exams, if the question implies the SCR is already ON, gate variations are irrelevant to the main current magnitude.

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