TRIAC (TRIODE FOR ALTERNATING CURRENT)

INTRODUCTION 

Triacs are electronic components that are widely used in AC power control applications. They are able to switch high voltages and high levels of current, and over both parts of an AC waveform. This makes triac circuits ideal for use in a variety of applications where power switching is needed.

One particular use of triac circuits is in light dimmers for domestic lighting, and they are also used in many other power control situations including motor control and electronic switches.

As a result of their performance, triacs tend to be used for low to medium power electronic switching applications, leaving thyristors to be used for the very heat duty AC power switching applications.

HOW DOES A TRIAC WORKS?

For the operation of the triac, it can be imagined from the circuit symbol that the triac consists of two thyristors in parallel but around different ways. The operation of the triac can be looked on in this fashion, although the actual operation at the semiconductor level is rather more complicated.

The triac structure is shown below and it can be seen that there are several areas of N-type and P-type material that form what is effectively a pair of back to back thyristors.

The triac is able to conduct in a number of ways - more than the thyristor. It can conduct current irrespective of the voltage polarity of terminals MT1 and MT2. It can also be triggered by either positive or negative gate currents, irrespective of the polarity of the MT2 current. This means that there are four triggering modes or quadrants:

• I+  Mode  MT2 current is +ve, gate current is +ve
• I-  Mode   MT2 current is +ve, gate current is -ve
• III+  Mode:   MT2 current is -ve, gate current is +ve
• III- Mode:   MT2 current is -ve, gate current is -ve

It is found that the triac trigger current sensitivity is greatest when the MT2 and gate currents are both of the same polarity, i.e. both positive or both negative. If the gate and MT2 currents are of the opposite polarity then the sensitivity is typically about half the value of when they are the same.

The typical IV characteristic of a triac can be seen in the diagram below with the four different quadrants labelled.

Triac circuit examples

There are many ways in which triacs can be used. The two examples below give a taste of what can be done with these semiconductor devices.

• Simple triac electronic switch circuit:   The triac can function as an electronic switch - it could enable a trigger pulse of a low power switch to turn the triac on to control a much higher power levels that might be possible with a simple switch.

Triac variable power or dimmer circuit:   One of the most popular triac circuits varies the phase on the input of the triac to control the power that can be dissipated into load.

There are many more triac circuits that can be used. The device is very versatile and can be used in a variety of circuits, typically to provide various forms of AC switching.

Triac applications

Triacs are used in many applications. These electronic components are often used in low to medium power AC switching requirements. Where large levels of power need to be switched, two thyristors / SCRs tend to be used as they can be controlled more easily.

Nevertheless triacs are widely used in many applications:

• Lighting control - especially domestic dimmmers.
• Control of fans and small motors.
• Electronic switches for general AC switching and control

There are naturally many other triac applications, but these are some of the most common.

In one specific application, triacs can be included in modules called solid state relays. Here an optical version of this semiconductor device is activated by an LED light source turning the solid state relay on according to the input signal.

Typically within solid state relays, the LED light or infrared source and the optical triac are contained within the same package, sufficient isolation being provided to withstand high voltages which may extend to hundreds of volts or possibly even more.

Solid state relays come in many forms, but those used for AC switching may use a triac.