Power Electronics Lab Experiments

AC Regulators using Triac, Anti parallel thyristor and Triac & Diac

Nov 27 2023

Aim

Apparatus Required:

S.No.	Apparatus/Software Used	Specification	Range/Rating	Quantity
1
2
3
4
5

Theory & Formulae Used:

Diac and Triac are used AC Power control circuit. The firing of a triac can be phase controlled. Since the exercised in both directions the power in the load can be varied from full value to almost zero. Fig. shows a RC phase shift circuit applied as control signal to the gate. The Diac in the circuit gives a wide range of summetrical firing of the Triac. The resistance R1 control the Gate current by controlling the voltage across C. Here the voltage across C first turns on the diac which in turns applies enough voltage to the Gate to turn on the triac. By property matching the diac to the triac symmetrical triggering of the triac in both the positive and negative alternations is accomplished. Morever, the range of firing control is much wider so that a turn on delay of almost 180 degree is possible in each alternation. The circuit however shows some hystersis i.e. the power applied to the load does not vary in the same manner when the direction of rotation of R1 is reversed.

AC Phase Control by RC Triggering:In above figure we control circuit in series with load it is meant for comfortable commutation of Triac and save the extra transformer cost.Initially the triac is in commuted position, so 230 V will reach at resistance R1, VR1and charged the capacitor C1. Capacitor C2 will charged through resistance through Resistance R2. The circuit will allow passing both the AC power cycles. By varying the variable resistance VR1 we can change the fire angle of AC power. We can use this circuit to control AC power from 0 to 100 %. Diac is a device which gives minimum resistance after certain value of potential difference so it helps us smooth changing of AC phase control.