AC Phase Controller Using TRIAC

AC Phase Controller Using TRIAC

Tags: Single Phase AC Voltage Controller using TRIAC, Single Phase AC Voltage Controller, Single Phase AC Voltage controller with Lamp Load using TRIAC, Fixed AC voltage to controlled AC voltage Controller using TRIAC, Circuit Diagram for Single phase AC Voltag, Circuit Diagram for Single phase AC Voltage controller with RLLoad using TRIAC, Single phase AC Voltage controller Waveform for RLoad using TRIAC, Single phase AC Voltage controller Waveform for RLLoad using TRIAC, AC Motor Control using Single phase AC V
Overall rating
Single Phase AC Voltage Controller


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 brief, an ac voltage controller is a type of thyristor power converter which is used to convert a fixed voltage, fixed frequency ac input supply to obtain a variable voltage ac output. The RMS value of the ac output voltage and the ac power flow to the load is controlled by varying (adjusting) the trigger angle ‘α’


Figure 1. Block Diagram of AC Voltage Controller

There are two different types of thyristor control used in practice to control the ac power flow

  • On-Off control
  • Phase control

These are the two ac output voltage control techniques.
In On-Off control technique Thyristors are used as switches to connect the load circuit to the ac supply (source) for a few cycles of the input ac supply and then to disconnect it for few input cycles. The Thyristors thus act as a high speed contactor (or high speed ac switch).


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 ‘’ (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 t) at which the thyristor turns on and the load current begins to flow.

Thyristor ac voltage controllers use ac line commutation or ac phase commutation. Thyristors in ac voltage controllers are line commutated (phase commutated) since the input supply is ac. When the input ac voltage reverses and becomes negative during the negative half cycle the current flowing through the conducting thyristor decreases and falls to zero. Thus the ON thyristor naturally turns off, when the device current falls to zero.

Phase control Thyristors which are relatively inexpensive, converter grade Thyristors which are slower than fast switching inverter grade Thyristors are normally used.

For applications upto 400Hz, if Triacs are available to meet the voltage and current ratings of a particular application, Triacs are more commonly used.

Due to ac line commutation or natural commutation, there is no need of extra commutation circuitry or components and the circuits for ac voltage controllers are very simple.

Due to the nature of the output waveforms, the analysis, derivations of expressions for performance parameters are not simple, especially for the phase controlled ac voltage controllers with RL load. But however most of the practical loads are of the RL type and hence RL load should be considered in the analysis and design of ac voltage controller circuits.


The ac voltage controllers are classified into two types based on the type of input ac supply applied to the circuit.

  • Single Phase AC Controllers.
  • Three Phase AC Controllers.

Single phase ac controllers operate with single phase ac supply voltage of 230V RMS at 50Hz in our country. Three phase ac controllers operate with 3 phase ac supply of 400V RMS at 50Hz supply frequency.



  • Lighting / Illumination control in ac power circuits.
  • Induction heating.
  • Industrial heating & Domestic heating.
  • Transformers tap changing (on load transformer tap changing).
  • Speed control of induction motors (single phase and poly phase ac induction motor control).
  • AC magnet controls.

Lab Experiment


To control ac load voltage by phase control technique by using TRIAC.

Apparatus Required

  • Trainer kit- AC voltage controller -1 No
  • Patch Chords - Sufficient Nos
  • Load
    a. 100Watts Lamp Load (or) External Rheostat-200ohm/2A
    b. 120mH/2A
  • CRO - 1 No
  • 10:1 CRO Probe - 1 No

Circuit diagram


Single Phase AC Fully Controlled Converter with R Load

Mimic Diagram


CRO Settings

  • Ensure the 230V Power Supply is in proper with Tester.
  • Ensure the CRO is working properly with probe checking and proper ground line axis.

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 for triggering pulse.
  • Turn ON SW1 and observe the waveform across the 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  and note down Vo.

Note: If the connections are wrong the kit will get damage. Please switch off the kit when not in use.

Model Waveform



OFF Time(ms)

ON Time(ms)

Alpha(α in Degree)



























Model Calculation

Total Time Period

Output Voltage

Model graph


Join the World's Largest Technical Community

we respect your privacy.