Diac is a device that has two electrodes. It is a member of the thyristor family. It is mainly used in thyristor firing. The advantage of using this device is that it can be turned on or off simply by reducing the voltage level below its avalanche breakdown voltage.
In addition, it can be activated or deactivated for the polarity of the voltages. This device works when an avalanche fault occurs.
Advantages of TRIAC:
o The TRIAC needs a single fuse for protection.
o It can be activated with positive or negative polarities of the gate pulses.
o It is possible to perform a safe interruption in any direction, but for SCR protection the protection must be provided with parallel diodes.
o You need only a single heatsink of a slightly larger size where, as for SCR, two smaller heatsinks should be required.
o When the voltage is reduced to zero, the TRIAC goes OFF.
Disadvantages of TRIAC:
o You can shoot in any direction, so we must be careful with the circuit shot.
o Compared to the SCR (silicon controlled rectifier) it has low classifications.
o TRIACs are not very reliable compared to SCR.
o This is not suitable for DC applications.
o The dv / dt classification is very low compared to SCR.
o It has a very high switching delay.
Diac application
It can be used mainly in the triac trigger circuit. The diac is connected to the triac door terminal. When the voltage across the gate decreases below a predetermined value, the gate voltage will be zero and, therefore, the triac will turn off. The main applications are-
1. Can be used in the dimmer circuit of the lamp.
2. It is used in the heat control circuit.
3. It is used in the speed control of a universal motor.
It is used with triac in combination of series for activation. The triac door is connected to a diac terminal. When the voltage applied through diac increases above the avalanche fault, it can only do so. However, when the voltage in diac decreases below its avalanche breakdown voltage, it will turn off and, therefore, the triac will also remain in the deactivated state.