The DIAC (DiodeAlternating CurrentSwitch) 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. The diac has a high impedance
when not switched and alow impedance
when switched. It operates bidirectionally, equally in both directions, and switches when the breakdown voltage is exceeded. Switching
If the applied voltage exceeds the breakdown voltage, the junction operated in the reverse direction switches to its low-impedance state, causing both junctions to conduct. If the applied voltage is reduced and falls below the holding 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.