AC Motor Speed Controller

This AC motor speed controller can handle most universal type
(brushed) AC motors and other loads up to about 250W. It works in much
the same was a light dimmer circuit; by chopping part of the AC waveform
off to effectively control voltage. Because of this functionality, the
circuit will work for a wide variety of loads including incandescent
light bulbs, heating elements, brushed AC motors and some transformers.
The circuit tries to maintain a constant motor speed regardless of load
so it is also ideal for power tools. Note that the circuit can only
control brushed AC motors. Inductive motors require a variable frequency


Schematic of the AC Motor Controller


Total Qty.
R1 1 27K 1W Resistor
R2 1 10K 1/4W Resistor
R3 1 100K 1/4W Resistor
R4 1 33K 1/4W Resistor
R5 1 2.2K 1/4W Resistor
R6 1 1K 1/4W Resistor
R7 1 60K Ohm 1/4W Resistor
R8 1 3K Linear Taper Trim Pot
R9 1 5K Linear Taper Pot
R10 1 4.7K Linear Taper Trim Pot
R11 1 3.3K 1/4W Resistor
R12 1 100 Ohm 1/4W Resistor
R13 1 47 Ohm 1W Resistor (See Notes)
C1, C3 2 0.1uF Ceramic Disc Capacitor
C2 1 100uF 50V Electrolytic Capacitor
D1 1 6V Zener Diode
Q1 1 2N2222 NPN Transistor 2N3904
SCR1 1 ECG5400
TR1 1 TRIAC (See Notes)
U1 1 DIAC Opto-Isolator (See Notes)
BR1, BR2 2 5A 50V Bridge Rectifier
T1 1 Transformer (See Notes)
MISC 1 PC Board, Case, Line Cord, Socket For U1, Heatsinks


  1. TR1 must be chosen to match the requirements of the load.
    Most generic TRIACs with ratings to support your load will work fine in
    this circuit. If you find a TRIAC that works well, feel free to leave a
  2. U1 must be chosen to match the ratings of TR1. Most generic
    DIAC based opto-isolators will work fine. If you have success with a
    specific part, feel free to leave a comment.
  3. T1 is any small transformer with a 1:10 turns ratio. The
    circuit is designed to run on 120V so a 120V to 12V transformer will
    work. Alternately, you can wind T1 on a transformer core using a primary
    of 25 turns, a secondary of 200 turns, and 26 gauge magnet wire.
  4. R9 is used to adjust motor speed. R10 is a trim pot used to
    fine tune the governing action of the circuit. R8 fine tunes the
    feedback circuit to adjust for proper voltage at the gate of SCR1. It
    should be adjusted to just past the minimum point at which the circuit
    begins to operate.
  5. R13 must be chosen to match the load. Generally, larger loads will require a smaller value.
  6. Since this circuit is not isolated from mains, it must be built in an insulated case.

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