SCR terminology — In thyristor (SCR) operation, what is the smallest anode–cathode current that will still sustain conduction once the device has been triggered on (i.e., the minimum current below which the SCR turns off)?

Introduction:
The silicon-controlled rectifier (SCR) is a latching device used widely in controlled rectifiers, motor drives, and power controllers. To apply it correctly, you must understand the specific current thresholds that define turn-on and turn-off behavior. One such key parameter is the holding current, which determines whether the SCR remains in conduction or reverts to its blocking state.

Given Data / Assumptions:

• Singly triggered SCR operating in its forward-conducting quadrant.
• Device has already been turned on via a gate pulse or by exceeding breakover.
• We consider steady conduction vs. turn-off after the trigger event.

Concept / Approach:

After triggering, an SCR continues to conduct as long as anode–cathode current exceeds a device-specific threshold called the holding current (I_H). If the current falls below I_H, internal carrier recombination reduces conductivity and the device switches back to its high-impedance, forward-blocking state. This is distinct from the latching current (I_L), which is the minimum anode current immediately after turn-on needed to “latch” conduction once the gate pulse ends; typically, I_L > I_H.

Step-by-Step Solution:

Verification / Alternative check:

Examine SCR datasheets: both I_L (latching) and I_H (holding) are specified. In practical AC circuits, the SCR commutates off naturally near current zero because load current dips below I_H, confirming the operational meaning of holding current.

Why Other Options Are Wrong:

• maximum forward current: A safe operating limit, not a sustaining threshold.
• maximum forward gate current: Limits gate drive but does not define conduction sustaining.
• reverse gate leakage current: A small undesired current with no bearing on conduction sustain.
• latching current: Required immediately after turn-on, not the minimum to maintain conduction thereafter.

Common Pitfalls:

• Confusing I_L and I_H; remember I_L ≥ I_H and applies only at the instant after triggering.
• Assuming the gate must be held on; once latched, gate current is not needed.

Final Answer:

holding current