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Dimmable Mains LED lamps? Answered

I have changed all my internal lamps to 240 volt LEDs - I notice some LEDs are dimmable and some are not any one know why?

As a supplemental  question anyone know how they step down the 240 volts for the LEDs? I have yet to take one apart to see.


It depends on the technology used for the switcher in the bulb. If the bulb outputs a current as a fraction of the input volts - can we call it fixed transconductance, then it can be dimmed using a standard dimmer, if, on the other hand the bulb is designed to control the current at a fixed level INDEPENDENT of input volts, it can't be dimmed conventionally.

In addition, the dimming WAVEFORM can affect the suitability of a dimmer for an LED, depending on whether its "leading" or "trailing" edge triggered.

If you want the low-down use "leading edge vs trailing edge dimmers" LED for a search term.

Those bulbs just 'sip' the 220 volts with a special AC>DC>AC>DC converter. This technology is called Switch Mode Power Supply, and it's the same miniaturization that makes your phone charger a tiny brick instead of the old transformers from forever ago.
The trick is they only pull from the 220 for a very short time domain to take the power they need. At very high frequency they sample the incoming voltage and use just small portions of it, with active feedback to make sure it's a nice smooth output (that the LEDS want, constant current).
The trouble comes in where dimmer switches introduce choppy waves that the bulb could care less about. It's only taking slices of the power for a short time (many thousand times per second) so the choppy AC that is 'dimmed' is ignored, 220 'dimmed' still has plenty enough voltage to make smooth xyz power output.
The dimmable bulbs themselves have to have some cleverness in them to both supply 'constant' current, and detect when there is a dimmer switch in the circuit causing choppy input, and reduce the current accordingly.

I should read Rick's bio before giving him the electronics for 5 year olds...

"28 years as Computer systems engineer Trained as Electronics engineer in the Royal Air Force"

It's OK :-) - I read - and thought that most of the cheaper LEDs used the reactance of a capacitor to drop the voltage - But some up markets types now use a switched mode Power supply.

Complex little devils aren't they.

The LED bulbs that dim have extra circuits that adjust the output voltage to the LEDs based on the power coming in through the socket.

Like any high power LEDs the light bulbs have a built in power supply or driver that takes the 240VAC and converts it to the 12/24 VDC needed to power the LEDs and gives them the constant current they need.