UNIT-5

1. UNIT-5
DIAC

2. DIAC DIAGRAM

3. CONSTRUCTION
• A diac is a two-terminal, three layer bidirectional
device which can be switched from its OFF state
to ON state for either polarity of applied voltage
the diac can be constructed in either npn or pnp
form.
• The DIAC is a diode that conducts electrical
current only after its breakover voltage, VBO, The
DIAC is a five layers and contains two terminal,
anode A1 (or main terminal MT1) and anode A2
(or main terminal MT2) like a TRIAC just without a
gate terminal.

4. OPERATION
• When a positive or negative voltage is applied across
the terminals of a diac, only a small leakage current
IBO will flow through the device.
• As the applied voltage is increased, the leakage current
will continue to flow until the voltage reaches the
breakover voltage VBO.
• At this point, avalanche breakdown of the reverse-
biased junction occurs and the device exhibits negative
resistance i.e. current through the device increases
with the decreasing values of applied voltage.
• The voltage across the device then drops to ‘breakback’
voltage VW.

5. CHARACTERISTICS OF DIAC