Why dimmers are different for normal and LED bulbs when both are connected to normal power?


The title 😀

I would understand it for LED strips and so on, but if both bulbs are made for 230V (or whatever in US), why do they need different dimmers? Is there a difference in some dim range or something?


In: 14

LED bulbs do not run at 230V. They have a specific voltage they run at which is created from the original 230v by a driver circuit.

This driver circuit is usually not designed to dim the LED as a response to a voltage decrease or a cut-in-half wave.

To dim incandescent bulb, all you have to do is send less power to it, and it’ll glow less brightly. An old-style incandescent dimmer just adds some resistance so that the bulb gets less bright.

LEDs don’t work that way, unfortunately – if you don’t supply enough power to an LED, it just won’t light at all. In order to “dim” an LED, what you actually have to do is blink it on and off really fast – faster than a human eye can tell – and it’ll look less-bright overall. A dimmer that can dim LEDs is called a PWM dimmer – pulse width modulation. As you adjust the brightness up or down, a PWM dimmer adjusts how long the “on” pulses are compared to the “off” pulses, which adjusts how bright the light looks to us.

To grossly over simplify, very old dimmers adjusted the voltage/current running through the light. Lower voltage = lower current and thus less bright.

Newer dimmers keep the voltage the same but muck with the wave of the alternating current – they actually keep the circuit to the bulb off for part of the A/C wave and switch it on part way through. When/where in that cycle is up to the dimmer, but the end result is your light isn’t dimming because its getting less current run through it, its actually flickering on/off, its just doing this so fast that we can’t see it. Well, most of us, turn a light dimmer down really low and catch the light out of the corner of your eye you might see it flicker.

Anyways, there are various types of the new dimmers and how they adjust this cycle (switch the light on/off – there are leading edge and trailing edge if you want to dig in more) and some of them work with some types of LEDs and others don’t.

LEDs are ultra low power so some of the cheats for dimming an incandescent don’t work

Dimmers for LEDs need to consume less power themselves. Normally they leak some power through the light to keep themselves functional even when its off. On an incandescent this works fine because a few milliwatts won’t heat the filament and make it glow. On a 3 watt LED bulb this leakage power can charge the bulb and make it flash occasionally or glow dimly

The lower current of the LED is also a problem to drive. Most dimmers these days use a triac to chop up the line voltage and only send some of it to the bulb. The triac sits off, voltage hits the right point in the line cycle, triac turns on allowing current to flow, current drops to 0 at the end of the cycle and turns the triac off, then it sits and waits for a bit

Well LED bulbs use low enough current that they won’t hold some triacs on so instead of turning on halfway through the cycle and staying on until the end, the triac will turn on, stay on for a bit, then turn off well before the end of the cycle and cause cause all sorts of flickering in the bulb as it comes on and off.

Controlling a toaster is easy. Controlling electronics is finicky

LED light are not made for 230V, each element run on closer to 2.3V. So an LED bulb needs some way to reduce the voltage for the elements. But the main difference is that while traditional light bulbs have a very linear response to the voltage LED elements are almost purely on or off. The difference between fully on and fully off might be as low as 0.1V.

Traditional dimmers do not actually limit the voltage to the bulbs but rather turn the power off for parts of the cycle. If you dim the light to half strength the voltage is still 230V but the light bulb only get power 50% of the time. On very low settings you might notice a tiny flicker as the fillament cools a tiny bit between each cycle but not much. An LED on the other hand use so little power that even just the stray capacitance and inductance is enough to keep it powered for some time even after the dimmer cuts the power. So you need to turn the dimmer all the way down for it to have an effect at all. And then the LED will not just glow a bit darker but it will turn fully off and fully on for each cycle turning it into a strobe.

There are multiple ways of fixing this. Some LED light have electronic regulators in them and these can detect the precense of a traditional dimmer and regulate the light output to match. This is why you see some LED bulbs labeled as dimmable. The dimmers you see labeled as working with LED lights do not reduce the power by turning the power off for parts of the cycle, or even directly regulate the voltage down. Rather it limits the power in the circuit thorugh a variable capacitor. This allows it to regulate the power down to exactly the voltage where the LED turns off and will keep the voltage right in that narrow band. These do also work on traditional lights although the voltage band will be significantly greater.