DIAC Diode :Basics, Characteristics, Applications

DIAC - Diode on AC: Tutorial

The DIAC is basically a two-terminal parallel-inverse combination of semiconductor layers that permits triggering in either direction.
There is no control terminal on this device. Physically it looks like a regular semiconductor diode.

  • As the voltage is increased from zero in either direction, a small amount of leakage current occurs, as shown in characteristic curve.
  • When VBR is reached in either direction, the diac fires (ie, starts to conduct).
  • There is then a negative-differential-resistance region, similar to that of UJT.
  • After the breakover voltage is reached, the diac conducts current easily and has very little internal resistance.
  • The diac looks like a semiconductor diode. It is not coded for polarity because it acts the same in both directions.

The symbol is shown below:

Symmetrical Diac:

  • The device is rated by current and voltage.
  • When both forward and reverse break-over voltages are almost same value it is known as Symmetrical Diac.

Asymmetrical Diac: 

  • In this type, the voltage ratings are mentioned as 20/18V.
  • It means in one direction breakdown voltage is 20V, and in the other it is 18V.
  • A circle is marked on high voltage side of the device.

Applications of DIAC:

  • It can be used in an oscillator circuits.
  • It is low power triggering device.


  • The basic arrangement of the semiconductor layers of the diac is shown in Figure.
  • Note that neither terminal is referred to as the cathode.
  • Instead, there is an anode 1 (or electrode 1) and an anode 2 (or electrode 2).
  • When anode 1 is positive with respect to anode 2, the semiconductor layers of particular interest are p1n2p2 and n3.
  • For anode 2 positive with respect to anode 1, the applicable layers are p2n2p1 and n1.

What is Quadrac? 

  • The diac is a two-way breakover triggering device.
  • The triac is a two-way SCR with one gate terminal.
  • The diac and the triac are used together in a variety of applications, such as the room incandescent-light dimmer switch.
  • A diac-triac combined package is called as Quadrac.

Read More: 

TRIAC - Basics, Operation, Applications, Structure, Testing 
Gate Turn Off Thyristor(GTO) Tutorial 
Power Diode Structure 
UJT UniJunction Transistor Tutorial 

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2 Responses

  1. Sagar patil says:

    Are the switching and v-i characteristics same?

  2. friend says:

    No. both are different... VI char is static characteristics..... the graph which shows the details about the waveforms when the device is turn on or turn off is switching char...

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