What is a DIAC - Tutorial

- the DIAC, Diode AC bi-directional switch is widely used with TRIACs to improve operation of alternating current power switching systems.

DIAC Tutorial includes

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The DIAC is a full-wave or bi-directional semiconductor switch that can be turned on in both forward and reverse polarities.

The DIAC gains its name from the contraction of the words DIode Alternating Current.

The DIAC is widely used to assist even triggering of a TRIAC when used in AC switches. DIACs are mainly used in dimmer applications and also in starter circuits for florescent lamps.


Circuit symbol

The DIAC circuit symbol is generated from the two triangles held between two lines as shown below. In some way this demonstrates the structure of the device which can be considered also as two junctions.


Circuit symbol for the DIAC

The two terminals of the device are normally designated either Anode 1 and Anode 2 or Main Terminals 1 and 2, i.e. MT1 and MT2.


Operation

The DIAC is essentially a diode that conducts after a 'break-over' voltage, designated VBO, is exceeded.

When the device exceeds this break-over voltage, it enters the region of negative dynamic resistance. This results in a decrease in the voltage drop across the diode with increasing voltage. Accordingly there is a sharp increase in the level of current that is conducted by the device.

The diode remains in its conduction state until the current through it drops below what is termed the holding current, which is normally designated by the letters IH.

Below the holding current, the DIAC reverts to its high-resistance (non-conducting) state.

Its behaviour is bi-directional and therefore its operation occurs on both halves of an alternating cycle.


DIAC applications

Typically the DIAC is placed in series with the gate of a TRIAC. DIACs are often used in conjunction with TRIACs because these devices 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.

The typical circuit configuration with DIAC placed in series with the TRIAC gate to improve its switching performance
Typical DIAC / TRIAC circuit configuration

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 its 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.


Structure

The DIAC can be fabricated as either a two layer or a five layer structure. 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.

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

Diagram showing how a DIAC is fabricated and the different regions within the device
The structure of a DIAC

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.

By Ian Poole


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