11mo ago

Can someone ELI5 why often lights set up with a dimmer also flicker?

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

26 comments

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.
- But you also need a dimmer that supports LEDs. Some dimmers have a minimum power requirement, which is much higher than almost every LED bulb
- 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.
- 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.
- 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.
  
  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.
  
  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.
LED lights are either all on or all off, the only way to dim a LED, is to make it blink really fast and change the time it’s on vs the time it’s off. Cheap LED lights don’t blink fast enough, so you see them flicker.
- That's not the only way to dim an LED, just the cheapest. Variable current power regulators are the premium option.
  A screw-in LED bulb combines LEDs and power regulating electronics. Some of them handle the variable input voltage a household dimmer provides gracefully, but that's more expensive.
  
  Changing the current can change the hue (color) of the led. In some cases it’s okay in some cases it isn’t. Cinema lights for instance don’t dim with voltage because of that. Instead they have 3 separate drivers synchronized to dim in a canon. One after the other so that there is always the same number of LEDs on at all time regardless of the dimmer level.
Because of the differences in how incandescent bulbs work vs LED and Florescent, and thus how their dimmers have to work.
Incandescent: you just heat up a bit of metal until it's glowing hot. Literally the same effect as leaving something metal in a campfire or furnace until it glows, just super hot so super bright and white.
Florescent: have a tube of gas, and then put a super high frequency voltage across it (thousands of Hz), and enough energy will be imparted to the gas molecules that they will emit photons.
LEDs: apply a constant DC current to a bit of a custom grown semi conductor, and that will give the semi conductor atoms enough energy that they will release photons.
The main thing about those, is that for Florescent and LEDs, they require very specific types of power.
Florescent bulbs require a very high frequency voltage, this is what the electronics in the bottom of a CFL do, convert the 60Hz of AC power from your house into super high frequency voltage.
LEDs on the other hand, can't use AC, and need constant current applied, not constant voltage, so all the electronics in the bottom of those bulbs work on converting your house's 60Hz AC power into like ~24V of DC power.
Incandescents on the other hand, do not care what type of power you put into them since any type of power can make them hot as long as you put enough in. You can feed them high or low frequency AC, you can feed them DC, you can feed them constant current, or constant voltage, they do not care, as long as you put electrical power into them.
Now it comes to dimmers. If you want to reduce the brightness of an incandescent bulb, you just need to reduce the power going to it, which in a constant voltage AC system like a house, means reducing the voltage.
The first dimmers did this by putting a variable resistor in series with the bulb. When it's resistance is zero the bulb is at full brightness, when it's the same as the bulb it's at half, and when it's resistance is way higher than the bulb's then the bulb is super dim.
This is good because it's super cheap and easy and you can precisely lower the voltage while maintaining the exact same waveform, but the problem with this is that you're feeding the same amount of power to the circuit no matter what, the resistor is just burning up the excess and turning it into heat.
So then we landed on how we built the vast majority of classical dimmers you see today: switched dimmers. Since incandescents are just hot metal, and there's a lag between when you heat metal and when it cools, you don't actually need to give it a clean wave form. Modern dimmers just switch on and off really fast to reduce the average amount of power going to the bulb. At 50% brightness, the dimmer might be switching on for 20ms then off for 20ms then on for 20ms then off for 20ms.
It's a great simple solution for incandescents, but when you try and feed the precise electronics of an LED or Florescent, that messy, choppy signal, they can't handle it and often just see it as the power coming on and off or it messes with their internal circuitry.
Maybe not what you needed but here's a demonstration of the dimmer from my favorite ee youtuber electroboom. Starting at around 3:00
- Man, he's great
Dimmer switch limits the power. The light doesn't like that.
Just wanted to let you know that there are a couple of eli5 communities. Just a sec- I'll try to link.
- https://lemmy.world/c/explainlikeimfive
  https://lemmy.world/c/explainlikeiamfive
  
  Thanks! Just joined
Perhaps not the WHY, because I would have thought there'd be a rectifier to convert to DC, but the string lights I've used (costco feit) don't flicker when dimmed.
They usually have a separate wall-wart they run from and often have a wireless remote that dims as well as changimg colors.
I see this as a better way to run LEDs off household ac, as the power supply and controls are designed to work together.

26 comments