Diac vs Triac: Understanding the Key Differences

This page compares Diac and Triac. It explains the basics of each and highlights the key differences between them.

As we know, the thyristor is a half-wave device, similar to a diode, and therefore only provides half the power.

Triac

A Triac consists of two thyristors connected in parallel but in opposite directions, all controlled by the same gate. It’s essentially a two-dimensional thyristor that can be triggered on both halves of an AC cycle using either positive or negative gate pulses.

Triac connecting terminals are labeled MT1, MT2, and gate (G), as shown in the image below. The triggering pulses are applied between the G and MT1 terminals.

The gate current needed to handle 100A from a triac is typically no more than 50mA or so.

Diac

A Diac is constructed from two Zener diodes connected back-to-back.

One common application for Diacs and Triacs is a lamp dimmer circuit, as shown in the figure below. The Diac conducts in either direction when the voltage across capacitor C1 reaches its breakdown value of approximately 30 volts. This current pulse from C1 triggers the Triac, supplying power to the load (in this case, a lamp).

The conducting period of the Triac depends on the value of resistor R1. The higher the value of R1, the slower C1 will charge. This causes the Triac to turn on later in the cycle, resulting in a dimmer light.

After studying Diacs and Triacs, you might also find it helpful to explore other electronic terminologies and the differences between Diacs, Triacs, SCRs, and UJTs, as mentioned below.