Basic Power Electronics

Lab report # 08

Phase and Power Control using Diac and Triac

Apparatus:

·         Diac

·         Triac

·         Resistor

·         Potentiometer

·         Capacitor

·         Voltage Source (AC)

·         Oscilloscope

Circuit Diagram:

Procedure:

As the AC supply voltage increases at the beginning of the cycle, capacitor, C is charged through the series combination of the fixed resistor, R1 and the potentiometer, VR1 and the voltage across its plates increases. When the charging voltage reaches the breakover voltage of the diac (about 30 V), the diac breaks down and the capacitor discharges through the diac, producing a sudden pulse of current, which fires the triac into conduction. The phase angle at which the triac is triggered can be varied using VR1, which controls the charging rate of the capacitor.

Once the triac has been fired into conduction, it is maintained in its “ON” state by the load current flowing through it, while the voltage across the resistor–capacitor combination is limited by the “ON” voltage of the triac and is maintained until the end of the present half-cycle of the AC supply.

At the end of the half cycle the supply voltage falls to zero, reducing the current through the triac below its holding current, IH turning it “OFF” and the diac stops conduction. The supply voltage then enters its next half-cycle, the capacitor voltage again begins to rise (this time in the opposite direction) and the cycle of firing the triac repeats over again.

Calculations:

I_T= 0.2mA

I_D= 0.1mA