I successfully made an LED dimmer, except it doesn't work when I connect an AC motor to it. Actually, what happens is the motor/fan just turns on on its own.

I was told that a snubber circuit is to be used for inductive loads, so I added that; 100 nF and 100 Ω in series going across the TRIAC's main terminals. Then I started to see smoke coming from a TRIAC leg when no "on" signal was given by MCU.

Then, when I did give a dimmer signal, the motor spun, but always at the same speed, irrespective of phase angle control signal from MCU. Current through motor more or less the same. I checked.

Then, when I had the dimmer turned on at low power (10%), I saw some arcing between the main terminals of my TRIAC. After a few minutes of experimentation, the TRIAC was completely dead. This whole time, my waveforms were correct. Oscilloscope confirms.

Here's my setup: Unit test

Here's the circuit for each dimmer (works great for my infrared heater):

enter image description here

And here's the only documented project I could find online for a digital AC motor controller. It works great. I wonder if the difference is the chunky transformer I have on my motor, but I don't think so.

Project found online

  • \$\begingroup\$ That's not a transformer on the motor. It's the motor magnetising coil. It has only two terminals. It will have a high inductance and this is probably generating a very high voltage when the triac tries to turn off. \$\endgroup\$
    – Transistor
    Apr 18, 2023 at 17:06

2 Answers 2


Motors come in many types.

  • A "universal AC/DC" motor is physically designed in such a way that it is speed-controlled via voltage (or average voltage, something a TRIAC dimmer can do.) This is because it has brushes and armature windings. The fact that it has brushes means that it can rotate at any speed - the speed is not limited to the design. The more Volts it is given, the faster it will spin. Note that DC motors cannot be used with AC; a DC motor has to be designed and wired in a special way to make it a universal motor. If you could change the AC frequency going to this motor, it would make little difference in operation.

  • A "shaded pole" motor is what is depicted. There is one magnetizing coil, and a copper rod (or two) welded near the top. These copper rods form the "shaded pole", which causes the armature of this type to spin. On some, the copper rods are omitted and a weld is made through the laminations in the place where the copper rods would have went (cost-cutting measures.) The armature in this motor is nothing but a chunk of metal; the shaded poles are required to induce a force in it and get it spinning. (Cut a shaded pole and the motor won't work anymore.) This also means that the rotational speed is locked to the AC frequency - reducing the voltage only serves to reduce the amount of torque this motor generates, not its speed. If you could somehow change the AC frequency, this motor would be speed-controllable that way but not with voltage changes.

  • \$\begingroup\$ better answer than mine \$\endgroup\$ Apr 18, 2023 at 18:20
  • \$\begingroup\$ Dunno about that; I just work with motors often. And yes, I actually cut a shaded pole to see what it would do (it destroyed the motor.) :) \$\endgroup\$
    – rdtsc
    Apr 18, 2023 at 18:44
  • \$\begingroup\$ We build motors here. It's a sickness \$\endgroup\$ Apr 18, 2023 at 18:47
  • \$\begingroup\$ Thanks for the input! Slapping together a stripboard VFD isn't something I'm too keen on at the moment, so I'll look for a brushed metal motor/fan. Still crazy to me that the fan would just turn on on its own no matter what, but I won't jump in the rabbit hole. Interestingly, it worked when I connected it to a potentiometer circuit. Magic. \$\endgroup\$
    – Popeye
    Apr 19, 2023 at 11:04

I believe the project you referenced is for a universal motor, which runs from either AC or DC and has a speed dependent on amplitude. Your motor appears to be an AC motor whose speed is related to frequency.

  • \$\begingroup\$ Thanks for the input! \$\endgroup\$
    – Popeye
    Apr 19, 2023 at 11:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.