Diac

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 Diac characteristic is shown below with the Diac 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 Diac 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

ELECTRICAL CHARACTERISTICS (T=25oC)
DIAC NUMBERS Breakover Voltage

V

Breakover Voltage Symmetry

V

Dynamic Breakback Voltage

V

Breakover Current

µA

Peak Pulse Current

A

  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

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