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Can someone ELI5 why often lights set up with a dimmer also flicker? (mander.xyz)

submitted 3 months ago by Mothra@mander.xyz to c/asklemmy@lemmy.ml

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Seen this in many houses, people upgrade their lighting setup and install a dimmer. Which works. But usually it also makes the lights flicker unintentionally, which is super annoying IMO.

Now, my understanding of electrical engineering is pretty rudimentary so I'd appreciate more something that explains the concept in a way that Cavewoman Mothra can understand rather than something technically accurate.

Thanks

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[–] slazer2au@lemmy.world 66 points 3 months ago (4 children)

Power coming into the house is AC which means 50-60 times a second the power goes from +110/240V to -110/240v.

LED lights run off DC power, so to change the power type a capacitor is somewhere that holds enough charge to keep the item working until the AC power is back to a usable positive value.

Dimmers limit the power going to the light, so the capacitor doesn't charge enough to keep the light and circuitry on for the full negative swing of AC power.

This is ungodly rudimentary, and corrections are welcome. There is also many nuances I am missing.

[–] jjagaimo@lemmy.ca 19 points 3 months ago* (last edited 3 months ago)

Dimmers will typically use a triac which cuts up the sinusoidal waveform. It doesnt actually lower the amplitude per se, but it limits the fraction of the time the waveform is on. Kinda like this. This means that a lot of the time the led isnt gettingas much or any power. The average power will be lower, and if the LED driving circuitry isnt designed to compensate for this, the LED will flicker.

Clarification on triacs: they get turned on a certain fraction of the way into the cycle. Triacs will stay on until the voltage across them is 0. Conveniently the zero-crossing of the AC wave (when the wall voltage crosses zero to start foing negative or from negative to positive) does just that.

[–] Wootz@lemmy.world 11 points 3 months ago* (last edited 3 months ago)

Well written.

I think an important concept to introduce is Pulse Width Modulation, or PWM for short.

Normal AC Power coming out of wall looks like a sine wave, in that it smoothly cycles between +110/240V and -110/240V. This means that 50% of the time the voltage is positive and 50% of the time the voltage is negative.

PWM usually deals with signals which are either entirely on or off, with no transiton between them. This way, you can vary the amount of "power" delivered by varying how much of the time the signal is on and how much of the time it's off.

Dimmers usually modify the sine way in a way that tries to accomplish the same thing, by chopping up the signal to make the effective "on" time be shorter than 50%.

With non-dimmable LEDs, this messes with the AC to DC circuitry in the lamp in the way slazer2au says, because the lamp doesn't retain enough power between two on-cycles to stay on.

[–] Mothra@mander.xyz 7 points 3 months ago (2 children)

Thanks, some of this makes sense. But why is it then not constantly flickering? They usually flicker for, say, five seconds then they stop flickering for 20 then they flicker again and so on. Or they flicker for like a minute then they're fine for a couple more minutes, then back again flickering. The timings vary a lot from house to house.

[–] Admetus@sopuli.xyz 8 points 3 months ago

Building on their comment, perhaps the capacitor is building up energy and dissipates it every 20 seconds. Like beats in resonance when you hear a pulsing in the volume when a guitar plays a single note or chord.

[–] JoMiran@lemmy.ml 3 points 3 months ago* (last edited 3 months ago) (1 children)

In my experience some brands of "dimmable" LED lights flicker and some do not. If you problems with flickering lights, try a different brand on that socket as an experiment. It might be the quality or type of components used.

[–] wewbull@feddit.uk 1 points 3 months ago (1 children)

No LED bulbs will work properly with triac based dimmers. "Dimmable LEDs" are horrible hacks that just about cling on for dear life, and many just won't work at all. Those dimmers are for incandescent bulbs.

The right way to dim an LED is pulse width modulation of the DC power, not chopping up the AC wave. That's what smart bulbs do because they have the dimming logic after the power supply is convertered from AC to DC in the bulb enclosure.

[–] JoMiran@lemmy.ml 1 points 3 months ago (1 children)

Well, this is anecdotal, but I bought an older home (circa 1990's) with dimmers already built in. We went through a number of LED bulbs and most flickered and some did not.

[–] wewbull@feddit.uk 2 points 3 months ago (1 children)

Those that worked worked more by luck than by design though. That's what I'm trying to say. Different dimmer, and you'll probably get different ones working.

[–] JoMiran@lemmy.ml 1 points 3 months ago

Can you give me a few examples (links)/to what I should look for if I decide to go through the effort of swapping the old ones out?