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:
- 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.
- 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.
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