TRIAC triggering — bidirectional conduction control: “The gate of a TRIAC can turn it on so that current flows in either direction.” Determine whether this description accurately reflects TRIAC operation.

Introduction / Context:TRIACs are bidirectional thyristors used widely for controlling AC power to lamps, heaters, and small motors. The core idea is that a gate pulse can trigger conduction in both polarities, simplifying hardware compared to two antiparallel SCRs.

Given Data / Assumptions:

• TRIAC conduction can be triggered in all four gate quadrants, with varying sensitivity.
• Once latched, the TRIAC conducts until current falls below the holding current, typically at the AC zero crossing.
• Load and supply are AC; device ratings are respected.

Concept / Approach:The TRIAC can be triggered to conduct on either the positive or negative half-cycle. After the gate current initiates conduction, the device latches and continues to conduct for that half-cycle. This property provides bidirectional control with a single device and explains its dominance in single-phase phase-control applications. While a DIAC often improves trigger symmetry, the TRIAC itself is inherently bidirectional once properly gated.

Step-by-Step Solution:

Verification / Alternative check:Phase-angle control waveforms show conduction windows in both half-cycles after each trigger event; appliance dimmers confirm bi-directional current through the load.

Why Other Options Are Wrong:

• Incorrect: Ignores the defining bidirectional nature of TRIACs.
• Valid only with a DIAC: A DIAC is not strictly required, though it improves symmetrical triggering.
• True only for positive cycles: Not true; both half-cycles are controllable.
• Depends only on heat-sink size: Thermal limits constrain power, not the bidirectional principle.

Common Pitfalls:Assuming TRIACs behave like single SCRs; overlooking holding and latching current requirements; neglecting triggering quadrant sensitivities.

Final Answer:Correct