TRIAC triggering polarity: assess the claim—“A TRIAC can be triggered by either a positive or a negative gate trigger input.”

Introduction / Context:
TRIACs (triode for alternating current) are bidirectional thyristor devices commonly used for AC power control in dimmers, motor speed controls, and heater regulators. Their key advantage over SCRs is bidirectional conduction and flexible triggering in multiple quadrants of the V-I plane. This question probes whether a TRIAC can respond to both positive and negative gate triggers.

Given Data / Assumptions:

• Standard TRIAC with main terminals MT1 and MT2 and a gate terminal G.
• Operation on AC mains or AC waveforms.
• Manufacturer-recommended gate drive currents are achievable in either polarity.

Concept / Approach:
A TRIAC can be triggered in four quadrants depending on the polarity of MT2 relative to MT1 and the gate current direction. Practical devices are often most sensitive in Quadrant I and III, but they can be triggered by either positive or negative gate pulses, enabling conduction in both positive and negative half-cycles. This is why a single TRIAC can replace back-to-back SCRs for many AC control applications.

Step-by-Step Solution:

Verification / Alternative check:
Device datasheets provide gate trigger current specifications for each quadrant (I–IV), confirming that both positive and negative gate currents can trigger conduction. The exact currents differ by quadrant but the bipolar triggering capability is fundamental.

Why Other Options Are Wrong:
Incorrect: contradicts TRIAC fundamentals.
Only Quadrant I or only with DC loads/MT2 positive: these are artificial restrictions not inherent to TRIAC operation, though sensitivity varies by quadrant.

Common Pitfalls:
Assuming a TRIAC behaves like a unidirectional SCR; forgetting that snubber networks and dv/dt limits can affect reliable triggering, but not the allowable gate polarity.

Final Answer:
Correct