What is Diac?

The DIAC is a bidirectional trigger diode designed  to trigger a Triac or SCR. The Diac
like the four-layered diode remains non-conducting  until its ‘break-over voltage’ is
reached. At this point, it turns on fully and remains on until the applied voltage or circuit
current is reduced below the ‘holding values’. The basic action of a diac is that, when
connected across a voltage source with a current limiting resistor, it act like a ‘high
impedance’ till the applied voltage rises to above 35 volts. Then the diac act as a 30 volt
Zener diode and conducts. The remaining 5 volts develop across the current limiting
resistor. At the avalanche state, diac exhibits ‘negative resistance characteristics’ and the
voltage across it ‘snaps back’ typically about 5 volts sufficient to trigger the Triac or
SCR. If the forward current falls below the minimum holding value of 30 volts (typical),
diac will turn off. The important draw back of the device is that, it cannot be triggered at
just any point in the AC power cycle. It triggers at its preset break over voltage only.

Diacs are used in AC power control circuits to provide trigger pulse to Triacs for its
proper operation.

During each half cycle of waveform, capacitor C1 charges depending on the value of R1.
When the voltage in the capacitor rises to the break down voltage of diac, it will conduct
sending a positive pulse to the gate of triac. The triac and lamp then turns on and remains
in the’ On state’ until the waveform crosses through zero voltage again