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TRIAC Theory & Operation

- notes on the theory and operation of how a TRIAC works, and how this relates to its operation in electronics circuits.


TRIAC theory can be taken at several levels, and this can be used to show the TRIAC operation, and how it can be used in electronics circuits.

The TRIAC operation can be viewed on a number of levels, showing how the TRAIC operation in various circuits can be designed to overcome some of the issues.


TRIAC operation

It can be imagined from the circuit symbol that the TRIAC consists of two thyristors back to back but with a common gate terminal, and the cathode of one thyristor connected to the anode of the other, and vice versa. This configuration is more correctly termed antiparallel

On a basic level, the operation of the TRIAC can be looked on in the format of the antiparallel thyristors, 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
Equivalent circuit of a TRIAC

In terms of the structure of the device, and its more detailed operation, the main terminals MT1 and MT2 are both connected to p and n regions within the device. The current path depends upon the polarity of the voltage across the main terminals.

As there is considerable scope for confusion, the device polarity is normally described with reference to MT1.

In terms of its operation, the ON characteristics for a TRIAC in any direction are similar to that of a thyristor. However as a result of the physical structure of the TRIAC, the latching current, holding current, and gate trigger current vary according to the different halves of the cycle and which "thyristor" within the TRIAC is being used.


TRIAC switching issues

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.

DIAC and TRIAC connected together
DIAC and TRIAC connected together

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.

By Ian Poole

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