Nov 302020

Dimmer / AC motor speed controller circuit using TRIAC

If you want to control the light level of a room or if you want to control the speed of a drill or a fan (devices that use AC motors), this is the Dimmer / AC motor speed controller circuit you are looking for.

If you need to automatically control an AC fan, you may be interested in a thermistor controlled fan circuit that only use 3 bipolar transistors.

Many of these regulatory power circuits have an activation point and a deactivation point that do not match. This phenomenon is called “hysteresis” and is common on TRIACs and other electronic components.

How a dimmer / AC motor speed controller works?

Resistor R3 and capacitor C3 are used to filter the high voltage transients that may appear on the circuit. The potentiometer P and capacitor C2 are elements necessary to trigger the triac. (using phase control)

Dimmer / AC Motor Speed Controller Circuit using TRIAC

The TRIAC controls the flow of alternating current to the load, switching between the conduction state and the cut-off state, during the positive and negative half cycles of the power supply (110/220 VAC that comes from the electric outlet of our homes).

It should be clarified that the capacitor, in an AC circuit, has an out of phase voltage with respect to the original signal. The following image shows the moments the TRIAC is triggered (small red triangles) and the voltage waveform that reaches the load.

Phase control waveforms using a TRIAC

Michael Schmid [CC BY-SA 3.0], via Wikimedia Commons

Phase control waveforms using a TRIAC

Moving the potentiometer knob, we can control the amount of current flowing to the load and thus the electrical power dissipated in it. To correct the hysteresis of the TRIAC, resistors R1 and R2 and capacitor C1 are used.

BT137 TRIAC PinOut

BT137 TRIAC (triode for alternating current)

List of circuit components

  • 2 47 K resistors (R1, R2)
  • 1 100 ohms resistor (R3)
  • 1 100 K potentiometer (P)
  • 3 0.1 uF capacitors (uF = microfarads) (C1, C2, C3)
  • 1 BT137 TRIAC (T) (depends on the load, 2 amps for common applications like this dimmer)
  • 1 commonly used plug (110/220 Volts)

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