The diac is a bidirectional trigger diode which is designed specifically to trigger a triac or SCR. Basically the diac does not conduct (except for a small leakage current) until the breakover voltage is reached. At that point the diac goes into avalanche conduction also at that point the device exhibits a negative resistance characteristic, and the voltage drop across the diac snaps back, typically about 5 volts, creating a breakover current sufficient to trigger a triac or SCR. A typical characteristic is shown below with its schematic symbol:

diac_triac_SCR_typical characteristic_schematic symbol

Although most diacs have symmetric switching voltages, asymmetric diacs are available. Typical diacs have a power dissipations ranging from 1/2 to 1 watt.


The main characteristics are:
(a) Breakover voltage
(b)Voltage symmetry
(c) Breakback voltage
(d) Breakover current
(d) Power dissipation



Typical diacs that are supplied by American Microsemiconductor


Breakover Voltage


Breakover Voltage SymmetryV Dynamic Breakback Voltage V

Breakover Current

Peak Pulse Current


Min  Typ  Max MAX MIN MAX MAX  
D30 28   32   36 3 5 100 2.0 
D40 35   40   45 3 5 100 2.0 
ST2 28   32   36 3.8 200 – 
HT32 27    –     37 3 5 50 2.0 
DB3 28   32   36 3 5 1.0