What is a Diac?

- summary or overview describing what is a diac, and detailing ways of using diacs and how a triac and diac are used together.

Triac, Diac, SCR Tutorial Includes:

Thyristor basics     What is a triac     Diac overview    

A diac is a full-wave or bi-directional semiconductor switch that can be turned on in both forward and reverse polarities. The name diac comes from the words Diode AC switch. The diac is an electronics component that is widely used to assist even triggering of a triac when used in AC switches and as a result they are often found in light dimmers such as those used in domestic lighting. These electronic components are also widely used in starter circuits for fluorescent lamps.

Although the term is not often seen, they may also be called symmetrical trigger diodes - a term resulting from the symmetry of their characteristic curve.

Diac symbol

The diac symbol used to depict this electronic component in circuit diagrams can be remembered as a combination of what may appear to be two diodes in parallel with each other but connected in opposite directions.

Circuit symbol for the diac

Circuit symbol for the diac

Owing to the fact that diacs are bi-direction devices the terminals cannot be labelled as anode and cathode as they are for a diode. Instead they may be labelled as A1 and A2 or MT1 ("Main Terminal") and MT2.

Diac operation

Diac circuits use the fact that a diac only conducts current only after a certain breakdown voltage has been exceeded. The actual breakdown voltage will depend upon the specification for the particular component type.

When the diac breakdown voltage occurs, the resistance of the component decreases abruptly and this leads to a sharp decrease in the voltage drop across the diac, and a corresponding increase in current. The diac will remain in its conducing state until the current flow through it drops below a particular value known as the holding current. When the current falls below the holding current, the diac switches back to its high resistance, or non-conducting state.

Diacs are widely used in AC applications and it is found that the device is "reset" to its non-conducting state, each time the voltage on the cycle falls so that the current falls below the holding current. As the behaviour of the device is approximately equal in both directions, it can provide a method of providing equal switching for both halves of an AC cycle, e.g for triacs.

Most diacs have a breakdown voltage of around 30 volts, although the exact specifications will depend upon the particular type of device.. Interestingly their behaviour is somewhat similar to that of a neon lamp, although they offer a far more precise switch on voltage and thereby provide a far better degree of switching equalisation.

Diac applications

One of the major uses of diacs within triac circuits. The diac is placed in series with the gate of a triac to provide a more symmetrical switching characteristic. It is found that 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 device fires, the greater the level of harmonics produced. It is generally undesirable to have high levels of harmonics in a power system.

To help in overcoming this problem, a diac 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. In view of their usefulness, diacs may often be built into the gate terminal of a triac.


Diacs are a widely used electronic component. The chief application of diacs is for use in conjunction with triacs to equalise their switching characteristics. By equalising the switching characteristics of these triacs, the level of harmonics generated when switching AC signals can be reduced. Despite this, for large applications, two thyristors are generally used. Nevertheless the diac / triac combination is very useful for lower power applications including light dimmers, etc.

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