Diac

A diac is a form of solid-state switch used to switch AC voltage; it belongs to the class of switches known as thyristors. It is like a junction transistor without a base lead (it is a two-lead device) and accomplishes its switching action by breakdown at a certain voltage. There are also four layer devices with a similar mode of operation known as four-layer diodes.

Fig 1. Diac Symbol

This is a bi-directional trigger diode used mainly in firing Triacs and Thyristors in AC control circuits. Its circuit symbol (shown in figure 1) is similar to that of a Triac, but without the gate terminal, in fact it is a simpler device and consists of a PNP structure (like a transistor without a base) and acts basically as two diodes connected cathode to cathode.

The DIAC is designed to have a particular break over voltage, typically about 30 volts, and when a voltage less than this is applied in either polarity, the device remains in a high resistance state with only a small leakage current flowing.

Once the break over voltage is reached however, in either polarity, the device exhibits a negative resistance as can be seen from the characteristic curve in Figure 2.

Simplified AC Power Control Circuit using a Diac

Fig 2

Diac Characteristics

When the voltage across the diac exceeds about 30 volts (a typical break-over voltage) current flows and an increase in current is accompanied by a drop in the voltage across the Diac. Normally, Ohm's law states that an increase in current through a component causes an increase in voltage across that component; however the opposite effect is happening here, therefore the Diac exhibits negative resistance at break-over.

In the simple power control circuit in Figure 2 the Diac is used to trigger a Triac by the "Phase Control" method. The AC mains waveform is phase shifted by the RC circuit so that a reduced amplitude, phase delayed version of the mains waveform appears across C. As this wave reaches the break over voltage of the Diac, it conducts and discharges C into the gate of the Triac, so triggering the Triac into conduction. The Triac then conducts for the remainder of the mains half cycle, and when the mains voltage passes through zero it turns off. Some time into the next (negative) half cycle, the voltage on C reaches break over voltage in the other polarity and the Diac again conducts, providing an appropriate trigger pulse to turn on the Triac.

By making R a variable value, the amount of phase delay of the waveform across C can be varied, allowing the time during each half cycle at which the Triac fires to be controlled. In this way, the amount of power delivered to the load can be varied.