Single Phase AC Voltage Controller of SCR

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AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value of the alternating voltage applied to a load circuit by introducing Thyristors between the load and a constant voltage ac source. The RMS value of alternating voltage applied to a load circuit is controlled by controlling the triggering angle of the Thyristors in the ac voltage controller circuits.

In phase control the Thyristors are used as switches to connect the load circuit to the input ac supply, for a part of every input cycle. That is the ac supply voltage is chopped using Thyristors during a part of each input cycle.

The thyristor switch is turned on for a part of every half cycle, so that input supply voltage appears across the load and then turned off during the remaining part of input half cycle to disconnect the ac supply from the load.

By controlling the phase angle or the trigger angle ‘α’ (delay angle), the output RMS voltage across the load can be controlled. The trigger delay angle ‘α’ is defined as the phase angle (the value of ωt) at which the thyristor turns on and the load current begins to flow.


To control ac load voltage by phase control technique by using two& SCR’s with R and RL Load.

Apparatus required

AC Voltage Controller kit– 1Nos

☞Patch cards- Required Nos

☞Power card- 1Nos

☞CRO with 1:10 probe- 1Nos

☞100W Incandescent Lamp- 1Nos

Circuit Diagram for Single Phase AC Voltage Controller of SCR



CRO Settings

☞Time (X) Axis: 5ms/div

☞Voltage (Y) Axis: 20V/div

☞Probe: 1:10 CRO Probe is suitable one.

Initial Settings

☞Ensure the CRO is working properly with probe checking and proper ground line axis.

☞Ensure the 230V Power Supply is in proper with Tester.

Mimic Diagram for Single Phase AC Voltage Controller of SCR


Experiment Steps

☞The connections are made as per the circuit diagram given above.

☞The R Load must be 60W bulb or 200ohm/2A Rheostat. The Load is 120mH/2A.

☞When the load is Rheostat, then it must be in maximum position.

☞Turn on the SW2 and observe the waveform across between G1&K1 and G2&K2. And ensure it is like triggering pulse given in model graph.

☞Turn ON SW1 and observe the waveform across the R load and RL Load.

☞Measure the X-axis time interval of ON time of Load Voltage and OFF Time of Load Voltage.

☞Adjust the value of Firing angle and note down the load voltage and current

☞Repeat the step 6.

☞Repeat the experiment for different values of VGS and note down Vo.


☞If the connections are wrong the kit may get damaged.

☞Please switch off the kit when not in use.

Model Waveform

Across Load

Single Phase AC Voltage Controller of SCR

Input voltage and Load voltage

input-voltage-and-load-voltage.jpg (285×275)


S.NOOFF Time(ms)ON Time(ms)Alpha(α in Degree)Vo(Volts)

Model Calculation

1. Total Time Period


Output Voltage



Thus AC voltage controller using SCR in anti-parallel connection was constructed and the performance was also analyzed.

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