Many days ago, I published a question here asking about dimming an AC LED using and Arduino and TRIAC, and I was told that most LEDs that support dimming should work with the circuit I linked to them.

Today, I came across another method to use AC PWM to dim a light bulb. The link can be found here. So my question is, would this circuit work if I use it to dim an LED lamp instead? Also, I was told that AC LED normally don't play well with dimming. If I get a regular LED lamp that has an AC/DC adapter, the I modify use it with the PWM circuit, would it work?



1 Answer 1


In ac circuits, you can't use PWM similar to DC. In ac you need to do a phase control which involves identification of zero crossings in ac sine wave and then accordingly firing up a triac after a delay.

In most cases, there is some circuitry (capacitors, inductors, diodes etc) inside the LED bulb which will interfere with the phase controlled signal generated by the triac and thus you won't get expected results.

If however you have a lamp that uses an external DC adapter for powering up, there is a good chance that you will be able to implement direct PWM on that supply voltage. To do an initial check, try to reduce the voltage and see if brightness is decreasing.

Once you have verified this, you can use a mosfet to generate a PWM on adapter supplied DC voltage. You can use an arduino to generate the PWM.

  • \$\begingroup\$ "In ac circuits, you can't use PWM similar to DC." - Yes, you can. See the second link in the OP for an example. \$\endgroup\$
    – marcelm
    Apr 12, 2016 at 14:01
  • \$\begingroup\$ What about my rotary dimmer switch for my (AC) dining room light, which uses a "pot?" That's nothing fancy. \$\endgroup\$ Apr 12, 2016 at 15:09
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    \$\begingroup\$ @TimSpriggs your dimmer switch probably does exactly what WhiskeyJack describes in his first paragraph. Whether or not you consider that fancy is up to you... \$\endgroup\$
    – marcelm
    Apr 12, 2016 at 16:14
  • \$\begingroup\$ @marcelm - Sorry for replying too late. The link you posted uses a bridge rectifier which effectively converts the ac into pulsating dc on which a PWM is applied using a mosfet. By ac, I meant pure ac with positive and negative regions. As far as I know a straightforward PWM will not work on that ac. Also, the link will be good for resistive loads only. I don't know whether it will work with fans. :) \$\endgroup\$ Aug 16, 2016 at 10:14
  • \$\begingroup\$ @TimSpriggs - Rotary dimmer doesn't use a pot directly. It contains a triac, a few capacitors, an inductor and a diac as well. Here is the barebone schematic \$\endgroup\$ Aug 16, 2016 at 10:20

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