What is a TRIAC - Tutorial

- this overview or tutorial details basic TRIAC - TRIode for Alternating Current - operation, structure, circuit symbol, and all the required relevant details.

The TRIAC is a three terminal semiconductor device for controlling current. It gains its name from the term TRIode for Alternating Current.

It is effectively a development of the SCR or thyristor, but unlike the thyristor which is only able to conduct in one direction, the TRIAC is a bidirectional device.

TRIAC / thyristor comparison

The TRIAC is an ideal device to use for AC switching applications because it can control the current flow over both halves of an alternating cycle. A thyristor is only able to control them over one half of a cycle. During the remaining half no conduction occurs and accordingly only half the waveform can be utilised.

A comparison between the typical switching waveforms of the TRIAC and Thyristor showing how the TRIAC can control both halves of the waveform
Typical / idealised TRIAC & thyristor switching waveforms

The fact that the TRIAC can be used to control current switching on both halves of an alternating waveform allows much better power utilisation. However the TRIAC is not always as convenient for some high power applications where its switching is more difficult.

TRIAC symbol

The circuit symbol recognises the way in which the TRIAC operates. Seen from the outside it may be viewed as two back to back thyristors and this is what the circuit symbol indicates.

TRIAC symbol for circuit diagrams
TRIAC symbol for circuit diagrams

On the TRIAC symbol there are three terminals. These are the Gate and two other terminals are often referred to as an "Anode" or "Main Terminal". As the TRIAC has two of these they are labelled either Anode 1 and Anode 2 or Main Terminal, MT1 and MT2.

TRIAC basics

The TRIAC is a component that is effectively based on the thyristor. It provides AC switching for electrical systems. Like the thyristor, the TRIACs are used in many electrical switching applications. They find particular use for circuits in light dimmers, etc., where they enable both halves of the AC cycle to be used. This makes them more efficient in terms of the usage of the power available. While it is possible to use two thyristors back to back, this is not always cost effective for low cost and relatively low power applications.

It is possible to view the operation of a TRIAC in terms of two thyristors placed back to back.

TRIAC equivalent as two thyristors
TRIAC equivalent as two thyristors

One of the drawbacks of the TRIAC is that it does not switch symmetrically. It will often have an offset, switching at different gate voltages for each half of the cycle. This creates additional harmonics which is not good for EMC performance and also provides an imbalance in the system

In order to improve the switching of the current waveform and ensure it is more symmetrical is to use a device external to the TRIAC to time the triggering pulse. A DIAC placed in series with the gate is the normal method of achieving this.

DIAC and TRIAC connected together
DIAC and TRIAC connected together

Advantages and disadvantages

When requiring to switch both halves of an AC waveform there are two options that are normally considered. One is to use a TRIAC, and the other is to use two thyristors connected back to back - one thyristor is used to switch one half of the cycle and the second connected in the reverse direction operates on the other half cycle.

As there are two options the advantages and disadvantages of using a TRIAC must be weighed up.


Advantages Disadvantages
  • Can switch both halves of an AC waveform
  • Single component can be used for full AC switching
  • A TRIAC does not fire symmetrically on both sides of the waveform
  • Switching gives rise to high level of harmonics due to non-symmetrical switching
  • More susceptible to EMI problems as a result of the non-symmetrical switching
  • Care must be taken to ensure the TRIAC turns off fully when used with inductive loads

Despite what may seem like a number of disadvantages, it is still the best option for many circumstances. However when using a TRIAC, it is necessary to be aware of its limitations so that these can be satisfactorily addressed and overcome should they affect the operation of the overall circuit in any significant way.

Applications

TRIACs are used in a number of applications. However they tend not to be used in high power switching applications - one of the reasons for this is the non-symmetrical switching characteristics. For high power applications this creates a number of difficulties, especially with electromagnetic interference.

However TRIACs are still used for many electrical switching applications:

  • Domestic light dimmers
  • Electric fan speed controls
  • Small motor controls
  • Control of small AC powered domestic appliances

The TRIAC is easy to use and provides cost advantages over the use of two thyristors for many low power applications. Where higher powers are needed, two thyristors placed in "anti-parallel" are almost always used.

The TRIAC is an electronic component that is widely used in many circuit applications, ranging from light dimmers through to various forms of AC control. It is generally only used for lower power applications, thyristors generally being used for the high power switching circuits. . . . . . . . .

By Ian Poole


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