DIAC (Bidirectional Thyristor Diode)

• Bidirectional Thyristor Diode.
• The term diac means, Diode that can work on A.C.
• The diac. is a two terminal device .

CONSTRUCTION:

The diac is a two terminal device . Which can pass current in either direction when the breakover voltage is reached in either polarity across the two terminals. .The basic construction of a diac is similar to a TRIAC, But without a gate terminal as shown in figure.

It may be noted from fig. 1.26(a) that a diac consists of two four layer diodes connected in parallel but in opposite direction. The four layer diodes are P 1 N 1 P 2 N 2 and P 2 N1 P1 N3 as shown by the broken lines in fig. Since the diac conducts In both directions therefore the terminals ate designated by numbers instead of anode and cathode. The diac has two main terminals namely M.T.1 and M.T.2 fig. Shows the equivalent circuit of a diac which consists of two four layer diodes connected in parallel but in opposite direction. Fig shows the schematic symbol of the diac.

DIAC OPERATION:

The diac can pass current in either direction depending upon the polarity of voltage across its main terminals. it can be turned ON only When the applied voltage across it’s main terminals reaches the breakover voltage. When the applied voltage make M.T 2 positive with respect to M.T.1 the diac passes current through the diode p1 N1 p2N2 from M.T.1 .However if the applied voltage makes M.T.2 negative with respect to M.T.1, the diac passes current through the diode P 2 N1 P1 N 3 from M.T.1 to M.T.2 as shown in fig. The diac turns when the current drops below the holding value.

APPLICATIONS:

• It is used in a proximity detector.
• It is used for triggering of the triac in the adjustable phase control of AC mains power.
• It is used in the circuit of lamp dimmer.
• It is used in heat control circuits.
• Universal motor speed control circuits utilise this device.
• Diacs are used as triggering Device in light dimming circuits in heat control circuits in motor control circuits.