Since last week, every LED bulb and fixture in my household is flickering at a similar rate. Incandescent and CFL are not affected.

Here's what I've had done to try and diagnose the issue:

  • Fuse panel inspected by certified electrician, no issues found and everything tightened
  • City electric provider came on site and tested the meter box including neutral line. No issues found, everything tightened in box
  • Asked neighbors for similar issue. No direct neighbor experienced it, however other people in the same neighborhood on a different street did experience it at the same time but theirs resolved overnight, and mine did not

There are no dimmers on any of the LED switches.

Does anyone have any idea what could cause this or what I could do to fix this (aside from replacing every LED with older incandescent / CFL)? Or any suggestions on how I can diagnose the issue.

I'm in Canada, if that matters.

  • 2
    I would want to put a power monitor on your line or use an oscilloscope and make sure the transformer that feeds your home is ok. If something caused a problem in the windings some strange waveforms can be the result causing problems and harmonics this is rare and a hand held meter that only measures RMS won’t see it.
    – Ed Beal
    Commented Apr 9, 2022 at 17:05
  • 2
    LEDs seem to be more sensitive to problem power than other types of lights. Seeing your neighbours also had a problem would say it is coming from power company equipment. Was there any weather(wind,lighting) storms near you just before it happen?
    – crip659
    Commented Apr 9, 2022 at 17:06
  • LEDs are diodes - in order to handle AC power, whose direction fluctuates, mains LED lights have a capacitor in them in order to ride out the "wrong direction" part. Not an expert on AC electricals though, but maybe you've had just enough of a local frequency drop for the dimmer capacitor-driven part to be more noticeable. Commented Apr 9, 2022 at 17:12
  • Side note: the build quality of the LED bulb matters. Cheaper ones simply have smaller capacitors and are basically much more likely to flicker. Commented Apr 9, 2022 at 17:14
  • 1
    First place I look is a "Lost Neutral", which is a weird type of power outage that won't kill power entirely, but will cause voltage to be highly variable. Check voltages on plug-in circuits all over the house. If some are below 120V while others are above 130V, and turning on a hair dryer or microwave makes the voltage move by 4 volts or more, that's what you have. Good news is, the power company will fix that for free. Commented Apr 9, 2022 at 20:01

1 Answer 1


In North America, most name brand light bulbs are powered by a bridge rectifier, a smoothing capacitor and then a linear regulator in series with ~ 120-150V worth of diodes. Or at least every one I have bought at a Home Depot worked like this. The bridge rectifier and smoothing capacitor output a nominal ~165V peak with some additional volts of ripple. This ripple is dropped across the linear regulator resulting in uniform, ideally flicker free LED power.

This setup is not very efficient, but it is very cheap. The inefficiency comes because the voltage dropped across the linear regulator is directly converted to heat. The larger the voltage across the regulator the more resistant the lights to flicker but the more heat and lower lm/watt. If the ripple on the lines (due to both the 120 Hz from the bridge rectifier, any external noise on the lines, and any voltage droop on the mains) causes the linear regulator voltage to drop to zero, the regulator will enter drop out, and the LED brightness will flicker.

For example, I have a pretty good, flicker resistant LED bulb here that is composed of 7 SMD packages each with an 18V forward voltage (3v*6 junctions). That is 126V. The rectifier outputs 165V with maybe 10V of ripple, so the lowest voltage for nominal 120V input is 155V, while the LEDs will stay lit for any voltage above ~126-127V. That means I have 27V of head room, and any noise/droop/etc on the lines less than this will have no effect on the diode brightness. However, you can also find bulbs with smaller capacitors or less power wasted on the linear regulator that only have 10 or 15V of margin. These will be much easier to make flicker.

My guess is that if you probed your mains power, you would find that it is a few volts below nominal 120Vrms and there is probably some additional noise from a large load somewhere else in your area. The combination of these effects causes occasional cycles in which the RMS voltage dips by more than the voltage drop across the regulator, causing periodic variations in LED power which you see as flicker. This is only a guess, but I have seen it happen in practice.

If this is the problem, your best bet is to simply try a few other brands of light bulbs. A slightly higher voltage drop across the linear regulator or a slightly larger smoothing capacitor will make a large difference if you are riding right up against the regulator dropout voltage.

  • It sounds like the OPs problem just started with LEDs that were working right with no flicker. Unlikely that all of OPs LEDs would start failing at the same time.
    – crip659
    Commented Apr 9, 2022 at 18:38
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    @crip659 They're not failing, they're probably working as (cheaply) designed. Problems like this start when someone else on his local transformer adds a large or noisy load. Commented Apr 9, 2022 at 19:57
  • As others have mentioned, LEDs react almost instantly to voltage variations, larger capacitors may help but won't completely cure the problem. Remember that incandescent bulbs basically emit via a very hot piece of wire (filament) ...not very sensitive to minor voltage variations. Next: There may be a new load installed in your area that causes these symptoms. To diagnose, buy, rent, borrow or steal an oscilloscope, preferably the recording kind and monitor your voltage. Commented Apr 9, 2022 at 21:58
  • @GeorgeAnderson The capacitor cannot remove flicker completely, but the regulator after the capacitor can. If it isn't working, either it's missing (uncommon) in NA or you are driving the regulator into drop out. Does that make sense to you? Commented Apr 9, 2022 at 22:05
  • Thanks for the reply. I have a rack mounted UPS that tracks voltage and it's been consistently 120 - 122v, but it's been like this even before the issues started. The LED lights were working fine for years until this week.
    – automaton
    Commented Apr 10, 2022 at 0:41

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