WHAT IS DIAC?
A diac belongs to thyristor family. Once it is triggered, it starts conducting in both directions. It is similar to diode but there is a main difference between diode and diac. The diode conducts in one direction, when it is forward biased. On the other hand, diac conducts in both directions or we can say that diac conducts both half cycle of the alternating current. That is why the diac is also known as a bidirectional avalanche diode. It is known as bidirectional avalanche diode because it is conducts only when applied voltage becomes equal or more the forward breakover voltage.
Basically, diac is a two terminal bidirectional semiconductor device that can conduct alternating current.
Di means (two terminal device)
Ac means conduct alternating current.
Construction of Diac
The material used for manufacturing of diac is silicon like thyristor. Silicon materials have many advantages.
A diac is a four layer (PNPN), two junctions (J1, J2) and two terminal (MT1 and MT2) semiconductor device.
MT1 is known as Main Terminal One.
MT2 is known as Main Terminal Two.
There is no control terminal in this device.
Symbol of Diac
Working of Diac
AC supply is given to the input of circuit. Diac is connected in series with load. The current does not flow through the load because the voltage applied to the diac is less than breakover voltage and at this stage diac remains in non-conducting state.
When input voltage exceeds the breakover voltage, the diac starts conduct and current start flowing through the load.
I-V Characteristics of Diac
The device can be triggered by either positive or negative half of an ac cycle. When the supply voltage is less than the breakover voltage, a very small amount of current called the leakage current flows through the device as shown in figure.
Leakage current is not sufficient to conduct the device. The device remains practically in non-conducting mode.
When the voltage level reaches the breakover voltage, the device start conducting. The current increased exponentially at this stage.
For positive half cycle the characteristics obtained in the first quadrant.
Negative half cycle the characteristics obtained in the third quadrant.
Diac does not give controlled output like Triac.
Applications of Diac
Diac are basically used to triggering of triac. Triac and diac pairs are available in market.