TRIAC: Empowering AC Switching for Efficient Control

Solid-state bidirectional thyristor: A triac is a semiconductor device that acts like a switch for AC current. Unlike a transistor that controls DC current, a triac can handle current flowing in both directions (bidirectional).

AC Switching: Triacs are designed to operate on AC power lines, typically 120V or 240V. They can turn ON and OFF the current flow during both positive and negative half cycles of the AC waveform.

Synchronous Switching:

The triac is triggered ON/OFF at a specific point in the AC cycle, typically right after the voltage crosses zero (zero-crossing point). This synchronized switching minimizes RFI because the switching event happens at a consistent point in the waveform. Synchronous switching circuits are not included in the article.

Choosing the Right Method:

 Asynchronous switching is simpler to implement but can cause RFI issues. It's suitable for basic ON/OFF control where RFI is not a major concern.

Synchronous switching is preferred for applications where minimizing RFI is crucial, like dimming lights or controlling motors. It requires more complex circuitry.

Triggering Methods:

Synchronous/Asynchronous Activation: The triacs can be activated synchronously or asynchronously. Synchronous triggering refers to turning the triac ON/OFF in sync with the AC waveform's zero crossings. Asynchronous triggering can happen at any point in the AC cycle. The specific method depends on the application and desired control.

Holding Current:

·       Triac Latch: Once triggered, a triac latches ON and continues conducting even if the gate current signal stops. This latching behavior persists until the main current flowing through the triac falls below a certain level called the holding current.

·       Turn-off at Half-cycle Completion: If the gate current falls below the holding current level at the end of an AC half-cycle (180 degrees), the triac will automatically turn OFF and wait for the next trigger signal.

How to Connect a Triac

A triac has 3 terminals, which are MT1, MT2, and the Gate. The MT stands for main terminal. Therefore, the main terminals MT1 and MT2 are used for switching heavy AC mains operated loads, through 220V or 120V AC mains supply. This switching happens in response to a small DC voltage applied to the gate terminal of the Triac.