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.

It's because they aren't installing the correct bulbs. Some dimmers work by cutting the 'pulse' that goes to the light early, some of them work by lowering the voltage/current.

When you install a direct LED to one that cuts the pulse, you get flickering. Incandescent bulbs don't do this because they're white-hot and don't change luminosity fast enough for you to notice.

Basically: If they're flickering -- they did it wrong.

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.

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

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.

It could be pulse width modulation?

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.