I recently installed new Kasa dimmable light switches (HS220) along with new light fixtures with LED bulbs (E12 2700k Candelabra Light Bulb). The bulbs are dimmable and 99+% of the time they work as expected. But every so often, when on a dimmed settings all the dimmed lights (I have 3 fixtures, 16 bulbs, on 2 different circuits) in unison, start flashing between max brightness and the dimmed setting at maybe 10-40 hz for 5-10 seconds, then go back to their dimmed setting.

Being that they all do it at exactly the same time and frequency across 2 different circuits tells me this is likely an issue upstream, say at the meter, or the incoming power lines, but in any case, my question is how do I get this diagnosed and fixed?

Video of an oscilloscope showing the voltage change when normal and while the strobing is happening, this is pulled from a plug that doesn't have any lights on it: https://youtu.be/UaKK45WQC-o

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    Where on the planet? Time of use device signaling schemes in some areas superimpose signals on the power that sound similar, but only in some places. On the other hand, if only when very dimmed, there may be a need to adjust the dimmers to dim just a bit less (as some bulbs will cut out or flash on some dimmers at the default lowest setting.)
    – Ecnerwal
    Oct 24, 2022 at 0:59
  • @Ruskes I don't see any harsh words? What are you referring to? Oct 24, 2022 at 2:55
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    @Ecnerwal I've got the USA tag, but specifically Texas on Pedernales Electric Cooperative power. If I have the dimmed set to 1-10% it's very noticeable. If it's set to 50% I still notice it, but not as easily and I don't notice it at 100%. It's like they flash from whatever they're set at to 100% very quickly, which is why it's less noticeable at around 50%.
    – Travis
    Oct 24, 2022 at 3:08
  • Has nothing to do with your power company. You can not see the 60 Hz flicker.
    – Traveler
    Oct 24, 2022 at 5:09
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    "You can not see the 60 Hz flicker". I sure as heck could see it in older CRT monitors, @Ruskes! I always had to turn the refresh rate up as high as it would go because it would drive me nuts looking at other people's monitors. I see the flicker in some car's tail lights, too, especially Cadillac's. Don't dismiss people's ability to see flickers that you can't see. Some are just more sensitive to it than others are.
    – FreeMan
    Oct 24, 2022 at 13:58

2 Answers 2


Dimming LEDs via the power line is a complex process, subject to many challenges which make it less reliable. This is why ETC has a dimmer compatibility database, for example. Look at the reports and you will see even within the same brand that a given bulb or fixture may dim well to 2% or poorly to 20%.

Because of this, newer integrated dimming systems are switching to using a 0-10V signal rather than power line dimming. Sending a control signal to the LED driver rather than trying to feed it just the right amount of AC power is much more reliable. There is an entire ecosystem of products available for 0-10V dimming.

What this means for your dimmer switch is likely that you'll need to try a bunch of brands of LED bulbs until you find one with behavior that you like, or perhaps change to a different dimmer switch if you want different dimming behavior.

  • I've also observed what you're describing, and it's very minimal compared to the issue I'm having. I have multiple multi light fixtures on separate circuits, all at the exact same time, start strobing from their set dimmed level to max brightness multiple times per second all synchronized. The effect would be like if you had a single dimmer running all of your lights in your house, then a child grabbed the dimming adjustor and slid it from minimum to maximum at about 10x a second, for 5 seconds, then set it back to the original level and left it alone. Repeat every few hours.
    – Travis
    Oct 24, 2022 at 23:32
  • Are all the lights that are flickering on dimmers, or are some not on any sort of dimmer at all? Oct 25, 2022 at 2:25
  • That doesn't change my basic premise: compatibility between line voltage dimmers and LEDs is hard.
    – KMJ
    Oct 25, 2022 at 4:47
  • And come to think of it, I had a problem where two dimmers in a room would start all the LED bulbs flickering in a pattern. Ultimately I fixed it by changing out the bulbs.
    – KMJ
    Oct 25, 2022 at 14:34
  • It appears that it's a house voltage issue: youtu.be/UaKK45WQC-o
    – Travis
    Oct 27, 2022 at 1:46

Dimming technology, as developed in the 1960s using triacs, was done in a very "hacky" way to keep dimmers cheap. It barely worked with incandescents and was troublesome there.

So this thing departed the station as a trainload of dumpsters which caught fire before the train crashed.

Trying to make it work with LEDs basically amounts to a) having a power supply in the LED which tries to ride through the dimming, and b) having intelligence on the LED reverse-engineer the dimmer signal to discern intent, and c) tell the LED driver to do that.

Dimming is sorely in need of a 21st century reboot, e.g. where is a standard "data over powerline" protocol where the dimmer can simply tell the LEDs what is desired and the LEDs do it.

So the simple answer is "realize how hacked this technology is, and lower expectations". If you want a first-rate dimming experience, go with first-rate technology, such as 0-10V or low voltage LEDs + PWM dimming.


Your oscilloscope data only reinforces my thinking about dimmers and LEDs. The voltage anti-spikes occur on both sides of the waveform, just like a triac dimmer would do. I suspect they are other dimmers also controlling LEDs, and those LEDs have a capacitor in their power supply which is "initially charging", drawing a big surge of current, causing a negative spike.

So it sounds like LEDs behaving badly.

It's possible that this trouble is being amplified by a "lost neutral" at the house's service, which would cause the LED capacitor charge-up to swing voltage much more... but if you can put a scope on an AC waveform, you can look for high/low imbalance between your two phases.

  • It appears that it's a house voltage issue: youtu.be/UaKK45WQC-o
    – Travis
    Oct 27, 2022 at 1:49

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