The DIAC(Diode Alternating Current Switch) is a bidirectional semiconductor switch for switching low currents. It is a three- layer diode, usually pnp, with one pn junction and one np junction. One of the two pn/np junctions is always operated in the forward direction, the other in the reverse direction.
Switching is triggered by applying a voltage. If the applied voltage exceeds the breakdown voltage, then the junction operated in the reverse direction switches to its low resistance state, making both junctions conductive. If the applied voltage is reduced and falls below the hold voltage, then one of the two layers flips to its reverse state and the diac is high impedance.
The voltage and current values are over a hundred volts and a few amps.