Once SCR starts conducting a forward current, its gate loses control over

Question

Examoogle · Accepted Answer

anode circuit voltage and current

Explanation: Once a Silicon Controlled Rectifier (SCR) is in the ON state, the internal regenerative feedback mechanism maintains conduction even after the gate signal is removed. Consequently, the gate loses control over both the anode current magnitude and the anode-to-cathode voltage across the device.

Working Principle: An SCR consists of a p-n-p-n structure which functions as two coupled transistors. When the gate current exceeds the threshold, the regenerative action drives both transistors into deep saturation. Once latched, the internal junction potentials are maintained by the anode current itself, rendering the gate terminal incapable of turning the device OFF or modulating the conduction state.

Key Formulas: $I_L = I_H + I_{gate}$ — where $I_L$ is Latching current and $I_H$ is Holding current; $V_{AK} \approx V_T$ — The ON-state voltage drop, independent of gate once triggered

Key Points: SCR is a semi-controlled device: controllable turn-on, uncontrollable turn-off.; Latching current ($I_L$) is the minimum anode current required to maintain the device in ON-state after gate signal removal.; Holding current ($I_H$) is the minimum anode current to maintain conduction before it reverts to blocking state.; Gate signal is only a 'triggering' pulse, not a continuous modulation signal.

Advantages: High current handling capability; Efficient switching for high-power rectification

Disadvantages: Cannot be turned off via gate terminal; Requires complex commutation circuits for AC-to-DC inversion

Applications: Phase-controlled rectifiers; AC voltage regulators

Additional Info: The transition from forward blocking to conduction occurs when the internal regenerative process self-sustains.. Option A is incorrect because the gate also loses control over the current; Option B is incorrect as it implies control over voltage is retained.

Answer: C is correct — Once the SCR is in the ON state, the internal regenerative action sustains conduction, making the device immune to further gate control over the anode voltage or current.

Exam Tip: In competitive exams, remember that the SCR is a 'semi-controlled' device; it is controlled for turn-on but requires external forced commutation for turn-off.

Related Questions

Discussion (0)