Triac

- the bidirectional semiconductor switch

The triac is a three terminal semiconductor for controlling 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 biriectional device. As such 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.

Circuit symbol for the triac

There are three terminals on a triac. The Gate that acts as the trigger to turn the device on, and two anodes or main terminals. These are usually designated Anode 1 and Anode 2 or Main Terminal 1 and Main Terminal 2 (MT1 and MT2).

It can be imagined from the circuit symbol that the triac consists of two thyristors back to back. The operation of the triac can be looked on in this fashion, although the actual operation at the semiconductor level is rather complicated. When the voltage on the MT1 is positive with regard to MT2 and a positive gate voltage is applied, one of the SCRs conducts. When the voltage is reversed and a negative voltage is applied to the gate, the other SCR conducts. This is provided that there is sufficient voltage across the device to enable a minimum holding current to flow.

Equivalent circuit of a triac

Structure
The structure of a triac may be considered as a p-n-p-n structure and the triac may be considered to consist of two conventional SCRs fabricated in an inverse parallel configuration.

In operation, when terminal A2 is positive with respect to A1, then a positive gate voltage will give rise to a current that will trigger the part of the triac consisting of p1 n1 p2 n2 and it will have an identical characteristic to an SCR. When terminal A2 is negative with respect to A1 a negative current will trigger the part of the triac consisting of p2 n1 p1 n3. In this way conduction on the triac occurs over both halves an alternating cycle.

The structure of a triac

Operation
Triacs do not fire symmetrically as a result of slight differences between the two halves of the device. This results in harmonics being generated, and the less symmetrical the triac fires, the greater the level of harmonics produced. It is generally undesirable to have high levels of harmonics in a power system and as a result triacs are not favoured for high power systems. Instead two thyristors may be used as it is easier to control their firing.

To help in overcoming this problem, a device known as a diac (diode AC switch) is often placed in series with the gate. This device helps make the switching more even for both halves of the cycle. This results from the fact that the diac switching characteristic is far more even than that of the triac. Since the diac prevents any gate current flowing until the trigger voltage has reached a certain voltage in either direction, this makes the firing point of the triac more even in both directions.