Diacs And Triacs 1

DIACS DIACS AND AND TRIACS TRIACS

DIACS • A diac is a 2-terminal 4-layer semiconductor device (thyristor) that conduct current in either direction when properly activate. • Conduction occurs in diac when the breakover voltage is reached with either polarity is across the 2 terminal.

Commonly used symbols

CONSTRUCTION

CONSTRUCTION • Neither terminal is referred to as cathode • Contain 2 anodes, anode 1(electrode 1) and anode 2(electrode 2) • When anode 1 is positive, the applicable layers are p1n2p2 and n3 • When anode 2 is positive, the applicable layers are p2n2p1 and n1

DIAC CHARACTERISTICS

• Once breakover occur, current in direction depending on the polarity across the terminal. • It turn OFF when the current drop below the holding current(IH)

DIAC BASIC OPERATION From A1 to A2 • Q1 & Q2 forward-biased • Q3 & Q4 reverse-biased • Operate on the upper right portion of the characteristic curve.

DIAC BASIC OPERATION From A2 to A1 • Q3 & Q4 forward-biased • Q1 & Q2 reverse-biased • Operate on the lower left portion of the characteristic curve.

APPLICATION • Trigger circuit for the Triac • Proximity sensor circuit

TRIAC • The triac is like a diac with a gate control. • It can be turned on by a pulse of gate current. • It does not require the breakover voltage to initiate conduction. • It can conduct current in either direction when it is trigger ON (depending on polarity of voltage across it terminal A1 and A2).

Commonly used symbol

CONSTRUCTION • Neither terminal is referred to as cathode • Contain 2 anodes, anode 1(electrode 1) and anode 2(electrode 2) & 1 gate terminal.

TRIAC CHARACTERISTICS

• • •

Current in direction depending on the polarity across the terminal. It turn OFF when the current drop sufficient low level. Breakover potential decrease as the gate current increase.

TRIAC BASIC OPERATION • Terminal A1 is biased positive with respect to A2

TRIAC BASIC OPERATION • Terminal A2 is biased positive with respect to A1.

Triac Pin Identification

APPLICATIONS AC power control circuits

Summary Diac

Triac

2 terminal

3 terminal

Contain 2 anodes

Contain 2 anodes &

•OFF when the current drop below the holding current(IH)

1 gate OFF when the current drop sufficient low level

Similarity • Both conduct current in either direction when properly activate. • Both cease to conduct when the anode drops below specified value of holding current(IH). • Both have same characteristic curve.