Power electronics — a triac differs from an SCR (silicon-controlled rectifier) primarily because a triac ________.

Introduction / Context:
SCRs and triacs are thyristor-family devices used for power control in AC applications such as lamp dimmers and motor speed controls. Understanding their conduction properties and gating behavior is fundamental for safe and effective circuit design. This question asks for the key difference that most directly impacts AC control topologies.

Given Data / Assumptions:

• SCR: conducts after being triggered and continues in one polarity until current falls below the holding current.
• Triac: essentially two SCR structures in inverse parallel within one package.
• Gate turn-off capability is not implied for either device in their classic forms.

Concept / Approach:
An SCR is a unidirectional device: once gated, it conducts only in the forward direction (anode to cathode) and turns off when line current crosses zero (or is forcibly commutated). A triac, by contrast, can conduct in both half-cycles of AC because it is bidirectional. This allows phase-angle control with a single device for AC loads, simplifying designs compared with using two antiparallel SCRs.

Step-by-Step Solution:

Verification / Alternative check:
AC dimmer circuits commonly employ a single triac plus an RC snubber and a diac trigger network, leveraging bidirectional conduction for symmetrical control of positive and negative half-cycles.

Why Other Options Are Wrong:

• Large current/voltage handling: SCRs typically outperform triacs at high power for the same silicon area.
• Gate turns off current: standard SCRs/triacs are not gate turn-off devices; commutation is required.

Common Pitfalls:
Thinking triacs are “better SCRs” in all metrics; in fact, they are convenient for AC but can be less rugged for heavy inductive loads.

Final Answer:
Allows current flow in both directions.