AC Lamp Dimmer Circuit Working Explanation

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Power controllers are different where Dimmers belong to the category of phase pulse power controllers.

Although, a triac may be fired into the conducting state by a simple resistive triggering circuit, a more reliable and faster turn-on may be had if a switching device is used in series with the gate. One of the switching devices that can trigger a triac is the diac.

They have an extremely simple circuit that is universal and is included in a variety of devices plus they are very cheap which's why they are very popular.

Dimmers can be found in household equipment, for example in a.c. lamps for adjusting the brightness of a light bulb, in power tools for adjusting engine speed, in heaters for adjusting the power heating element, and so on.



AC Lamp Dimmer Circuit Diagram

Schematic of an ac lamp dimmer circuit shown below.


Components Used

  • Q1 - BTA41-600
  • D1 - DB3
  • R1 - 4k7/1W
  • R2 - 500k POT
  • R3 - 100R
  • C1 - 0.1uF/600V
  • C2 - 0.1uF/400V

Working Principle of Lamp Dimmer

The above schematic shows a typical circuit that may be used for smooth control of a.c. power fed to a lamp.

This permits to control of the light output from the lamp, where the basic control is by an RC variable gate voltage arrangement.

The main component of the lamp dimmer is a TRIAC, it consists of two thyristors connected in parallel but in opposite directions.


The series R3 - C1 circuit across the triac Q1 is designed to limit the rate of voltage rise across the device during switch off.

The working principle of the AC Lamp dimmer circuit is simple.

As the input voltage increases positively or negatively, C1 and C2 charge at a rate determined primarily by R2.

When the voltage across C2 exceeds the breakover voltage of the diac D1, the diac is fired into the conducting state.

The capacitor C2 discharges through the conducting diac D1 into the gate of the triac Q1.

Hence, the triac Q1 is turned on to pass the a.c. power to the lamp.

By adjusting the value of R2, the rate of charge of capacitors and hence the point at which triac Q1 will trigger on the positive or negative half-cycle of input voltage can be controlled.

However, triacs in DC circuits can be opened by applying a trigger signal to their control pin.

But even if you remove this signal, the triac will not close, even if you close the control pin to the power supply ground it also will not close.

You can close the track in the DC circuit if you remove voltage from the power pin or turn OFF the power and turn it ON again.

Therefore, triac regulators can't work in the dc circuit.

The graph shows the waveforms of AC supply voltage with a frequency of 50Hz/60Hz and load voltage in the diac-triac control circuit.


The firing of triac can be controlled up to a maximum of 180°.

In this way, we can provide continuous control of load voltage from practically zero to full r.m.s. value.

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