Definition
Three-terminal solid-state semiconductor devices used to control alternating current (AC). A triac acts as an electronic switch: when a small signal is applied to its gate terminal, it allows current to flow in either direction through its two main terminals. It remains conducting until the AC current naturally drops to zero, at which point it switches off until triggered again.
Plain English
A small electronic component that turns AC power on and off, or controls how much of it flows through a circuit. Think of it as a fast, silent switch with no moving parts that works in both directions of an alternating current.
Context Anchor
Seen in aircraft electrical and maintenance discussions, especially in power-control circuits such as lighting dimmers or other alternating-current control equipment.
Derivation
The name comes from 'triode' (a three-terminal device) plus 'AC' (alternating current). So 'triac' literally means 'three-terminal AC device.' Knowing this helps you remember both what it looks like (three connections) and what it does (handles AC).
Why Pilots Care
Triacs are part of the solid-state circuitry that has replaced older mechanical relays and switches in modern aircraft. When troubleshooting AC-powered systems or reading avionics documentation, you may see triacs referenced as the component handling load switching or dimming.
Intuition Check
Do not think of a triac as a normal cockpit switch with a handle. It is an internal electronic switch inside a circuit.
Example Sentence 1
The cabin lighting dimmer uses a triac to vary the AC power delivered to the bulbs.
Example Sentence 2
Triacs in the environmental control system allow variable fan speeds without mechanical contacts.