Thread: Diac-the follower triacs

Diacs are bidirectional trigger diodes designed to trigger triacs or SCRs. These, like four-layered diodes, remain non-conducting until their break-over voltage is reached. At this point, they turn on fully and remain 'on' until the applied voltage or circuit current is reduced below the holding values.

Basically, a diac, when connected across a voltage source with a current-limiting resistor, acts like a high impedance until the applied voltage rises to above 35 volts. Thereafter it acts as a 30V zener diode and DCV conducts. The remaining 5 volts develop- across the current-limiting resistor.



In the avalanche state, the diac exhibits negative resistance characteristics and the voltage across it snaps back typically about 5 volts, which is sufficient to trigger the triac or SCR. If the forward current falls below the minimum holding value of 30 volts (typical), the diac will turn off. The main draw¬back of the device is that it cannot be triggered at just any point in the AC power cycle. It triggers at its present break-over voltage only.

Diacs are used in AC power control circuits to provide trigger pulse to triacs for their proper operation. As shown in, a diac-triac combina¬tion can be used to switch on a lamp. During each half cycle of the waveform, capacitor CI charges de¬pending on the value of RI. When the voltage in the capacitor rises to the breakdown voltage of the diac, it conducts sending a positive pulse to the gate of triacs. The triac and lamp then turn 'on and remain' on' until the waveform crosses through zero voltage again.