TRIAC conduction over AC cycle: evaluate the statement—“A TRIAC will control AC voltage during the positive half-cycle only.”

Introduction / Context:A TRIAC is designed for bidirectional control of AC power. Unlike an SCR, which conducts only when forward biased and triggered, a TRIAC conducts in both polarities once triggered. This makes it ideal for phase-angle control across the entire AC cycle. The question challenges whether TRIAC control is limited to the positive half-cycle only.

Given Data / Assumptions:

• Standard TRIAC used with appropriate gate triggering circuitry.
• AC mains waveform with alternating positive and negative half-cycles.
• Basic phase-control or burst-firing applications.

Concept / Approach:Because the TRIAC is symmetrical, it can be triggered into conduction during either half-cycle, and once triggered above the latching current, it will remain on until the current falls below the holding current (usually at the next current zero crossing). Gate drive circuits deliver a pulse at the desired firing angle in each half-cycle, thereby controlling the portion of the waveform conducted and hence the RMS voltage delivered to the load across both halves of the cycle.

Step-by-Step Solution:

Verification / Alternative check:Oscilloscope traces of lamp dimmers show chopped sine waves in both half-cycles. Most application notes for TRIAC dimming explicitly present symmetrical firing for positive and negative halves.

Why Other Options Are Wrong:“Correct”: contradicts TRIAC operation. Conditional statements about series diodes, low firing angles, or resistive loads do not change the fundamental bidirectional control capability.

Common Pitfalls:Confusing TRIACs with SCRs; overlooking that inductive loads may require snubbers or special triggering but still allow bidirectional phase control; assuming firing only once per cycle.

Final Answer:Incorrect