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I have some experience with house wiring, and more recently gained some experience with 5mm RGB LEDs and an Ardiuno.

In that, I learned about PWM for dimming an LED.

I suspect that the standard wall dimmer an electrician installs is just a variable resistor. I could understand that doing weird stuff if there is some transformer or something in the non-dimmable led lamp.

I found that you can get solid state relays for an Ardiuno that can handle 250V AC and 2 amps. Since they are solid state, I suspect I could turn them on and off fast.

What happens if I PWM a standard "non-dimmable" LED lamp? Is there some complicated circuitry (that I don't yet understand or know of) that will interact poorly with a PWM power source?

(That does bring me to realize that for the PWM to work well, it should operate at a speed that doesn't have any weird "Lowest Common Multiple" effects with the AC supply that would be visible).

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    \$\begingroup\$ "... standard wall dimmer ... just a variable resistor" - Nope - at the very least it'll be a Triac-based circuit. "... some complicated circuitry ... interact poorly with a PWM" - Yep - there'll be a power supply which rectifies the mains AC into DC and then uses that to actually drive the LED through a constant-current switch-mode driver. \$\endgroup\$
    – brhans
    Commented Jun 13, 2017 at 18:34
  • \$\begingroup\$ That would depend on what "non-dimmable" means.... If its a constant current driver it may likely not work quite as you expect and may actually fail on you. \$\endgroup\$
    – Trevor_G
    Commented Jun 13, 2017 at 18:34
  • \$\begingroup\$ I would expect that "non-dimmable" fixtures have fixed-output drivers that expect full-sine AC. Feeding them PWM seems like it would be a more aggressive version of the Triac waveform, and probably cause more extreme versions of the problems people usually have trying to use a conventional dimmer with non-dimmable LED fixtures. \$\endgroup\$
    – Chris M.
    Commented Jun 13, 2017 at 18:48
  • \$\begingroup\$ here's an example of using "fast" (non-zero-cross) pwm on AC via IGBT \$\endgroup\$
    – dandavis
    Commented Jun 13, 2017 at 20:42

2 Answers 2

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I suspect that the standard wall dimmer an electrician installs is just a variable resistor.

Incorrect. That would get very hot and waste power. Dimmers use triacs to adjust the fraction of time that the AC is allowed through.

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Figure 1. Phase angle control of AC used in standard incandescent lamp dimmers. As switch-on point is delayed power is reduced.

... you can get solid state relays ... Since they are solid state, I suspect I could turn them on and off fast.

You could turn them on once per mains half cycle. They would then remain on until the next zero-cross.

What happens if I PWM a standard "non-dimmable" LED lamp?

It will probably fail as the internal power supply is not designed for chopped supply voltage.

Is there some complicated circuitry (that I don't yet understand or know of) that will interact poorly with a PWM power source?

Yes, quite a bit of both.

For more on this subject see my answers to

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  • \$\begingroup\$ So to summarize (and confirm I have the right idea): the best I could possibly do with a solid state relay and PWM is exactly what a dimmer does. So if a normal dimmer with non-dimmable LED lamps causes problems, so will trying to control one with PWM. \$\endgroup\$
    – Azendale
    Commented Jun 13, 2017 at 23:57
  • \$\begingroup\$ Yes. The phase-angle control is a kind of PWM but locked to the mains frequency. The LED lamp is likely to have some kind of switched mode power supply internally. Dimmable LED lamps have a dimmer symbol on them - in Europe at least. \$\endgroup\$
    – Transistor
    Commented Jun 14, 2017 at 6:30
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If you're talking about an AC (110V/220V) LED lamp the answer is NO.

Most LED bulbs have as you say a "special circuitry" called a driver. This driver produces constant current or voltage and feeds the LEDs. If you try to pass PWM on its input it will behave strange and may fail, cause this is not supposed to happen in normal operation. A dimmable LED driver (fitted into a bulb) is designed to be dimmed with a triac dimmer (AC phase control) and is more complicated than the non-dimmable.

There are some small DC bulbs (like 12V, 1-2W or less) that contain only groups of 3 series LEDs, a series resistor, eventualy a bridge rectifier or single diode and no other electronics. These can be successfuly dimmed (like a LED stripe) with PWM.

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