What is a DIAC? Operation Applications Circuits

A DIAC is a bi-directional semiconductor switch that can be turned on in both forward and reverse polarities above a certain voltage: it is often used to provide defined switching for a triac.

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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, DIACs may also be called symmetrical trigger diodes - a term resulting from the symmetry of their characteristic curve.

DIACs come in a variety of formats. As discrete components they may be contained in small leaded packages, they can be obtained in surface mount packages, in large packages that bolt to a chassis, or a variety of other packages. As they are often used as a DIAC TRIAC combination, they are often integrated into the same die as a TRIAC.

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
DIAC circuit symbol

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 and MT2, where MT stands for "Main Terminal."

DIAC operation

Electronic circuit designs incorporating DIACs use the fact that a DIAC only conducts current only after a certain breakdown voltage has been exceeded. This is marked as VBO on the diagram.

The actual breakdown voltage will depend upon a variety of factors in its fabrication, but it will be given in 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 to VF.

There is a corresponding increase in current. This can be clearly seen on the I/V characteristic for the device below.

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.

DIAC voltage- current characteristic
DIAC voltage- current characteristic

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 structure

The DIAC can be fabricated as either a two layer or a five layer structure - the difference between the two devices is relatively small in practiced.

In the three layer structure the switching occurs when the junction that is reverse biased experiences reverse breakdown. The three layer version of the device is the more common and can have a break-over voltage of around 30 V. Operation is almost symmetrical owing to the symmetry of the device.

Although the structure may appear to be similar to that of a bipolar transistor, the actual fabrication and layers, etc are rather different. The central region, for example is much thicker.

Diagram showing how a three layer DIAC is fabricated and the different regions within the electronic component
The structure of a three layer DIAC

A five layer DIAC structure is also available. This does not act in quite the same manner, although it produces an I-V curve that is very similar to the three layer version.

This structure can be considered as two break-over diodes connected back to back.

Diagram showing how a five DIAC is fabricated and the different regions within the electronic component
The structure of a five layer DIAC

In terms of its operation, when the top terminal is more positive with respect to bottom one on the diagram, the current does not flows through the corresponding N-layer but flows the path consisting of P2, N2, P1, N1. When potential is reversed the current flows through the path of the regions P1, N2, P2, N3.

For most applications a three layer version of the DIAC is used. It provides sufficient improvement in switching characteristics. For some applications the five layer device may be used.

DIAC applications

One of the major uses of DIACs within electronic circuit designs that utilise TRIACs. TRIACs do not fire symmetrically as a result of slight differences between the two halves of the device.

The non-symmetrical firing and resulting waveforms give rise to the generation of unwanted harmonics – the less symmetrical the waveform the greater the level of harmonic generation.

DIAC TRIAC combination
DIAC TRIAC combination

To resolve the issues resulting from the non-symmetrical operation, 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.

While the use of a DIAC improves the performance of the TRIAC, it is still not perfect and as a result TRIACs tend to be used within small current or power switching electronic circuit designs as some non-symmetry remains with the resulting harmonics and other issues are still present. These would be far more important within high current circuits and therefore two thyristors are generally used instead.

The use of two thyristors provides a far more satisfactory high current solution than a TRIAC, but at the cost of additional electronic components. The TRIAC is ideal for small power circuit designs because it reduces the electronic component count, the overall size and cost. It is for this reason that they are widely used in light dimmers where size and cost are very important parameters.

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 rather than a single triac. Nevertheless the DIAC / TRIAC combination is very useful for lower power applications including light dimmers, etc.

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