LED light bulbs and dimmer switches

[AndyC_772]

Hi all,

In my lounge I have 3 light fittings, all controlled by the same switch, and each of which takes two 40W candle bulbs. The switch is an X10-controlled dimmer (no neutral wire).

When one of the bulbs blew last week, I worked out it was time to switch to some low power LED bulbs, so I ordered six of the "warm white, dimmable" version of these:

http://www.ledhut.co.uk/led-bulbs/b22-led-bulbs/new-4-watt-b22-led-candle-shape-bulb.html

It turns out they're not quite as warm in colour as the filament bulbs they replaced, but at 4.3W they do look significantly brighter. In that respect, it does appear that LED bulbs have come of age.

However, if I replace all six bulbs, then the dimmer switch gets awfully upset; it makes a nasty buzz, and the lights all flicker randomly. If, however, I only swap four of the bulbs for LED, and leave two filament bulbs in place, then it all works fine, including dimming.

I guess the problem is down to the nature of the power supply in each LED bulb; a combination of low current to begin with plus a horrible V-I characteristic, which is significantly smoothed by adding a resistive load back into the circuit. But obviously I'd rather have all six bulbs the same if possible.

Is this a known issue with dimmable LED bulbs, or do I just have a crap light switch? I'm happy to give up the X10 control feature, which never worked reliably anyway, but would swapping to a different dimmer switch actually help? Is there such a thing as an 'LED compatible" dimmer?

[Rerouter]

the X10 and the normal dimmers should all be triac controlled, cutting up the waveform, however they have a quirk, to properly work they must have a minimum load maintained during there switched on period,

i am going to guess that the startup surge of one of the led bulbs causes it to turn on for one cycle, then wailt a few more until another is very discharged making them flicker,

you can either find a lower load dimmer, or try replacing the triac for one with the same trigger voltage but lower holding current,

[Neilm]

Start with the obvious - you did double check you we sent the dimmable version didn't you?

[AndyC_772]

With 4 LED bulbs fitted out of 6 they dim quite happily, so I don't think I can really blame the bulbs. The box clearly says "dimmable: yes" on it.

[IanB]

This is the gap between propaganda and real information. Many will tell you that "dimmable" replacement LED bulbs are available; few will tell you the practical limitations of such dimmable technology.

In order to be properly dimmable a bulb has to look somewhat like a resistive load. It is very difficult to make an electronic circuit like a mains step-down LED driver look resistive. Everything about the circuit wants to integrate and smooth the incoming wave form and then feed it to a constant current driver.

This is an area where detailed technical tests of different brands of bulbs could be useful, looking at actual performance in real life situations.

[amyk]

the X10 and the normal dimmers should all be triac controlled, cutting up the waveform, however they have a quirk, to properly work they must have a minimum load maintained during there switched on period,

And I suspect these LED bulbs draw so little power that they don't meet that requirement.

[Odysseus]

Making low power LED bulbs compatible with many different dimmer types that expect several hundred watt loads is a very difficult engineering task which involves a lot of fancy tricks.  I know this since I worked in the LED lighting department for Cirrus Logic, which prides itself on market leading dimmer compatibility. http://cirrus.com/en/applications/app/detail/APP63.html

Unfortunately, I don't know off hand which manufacturers use Cirrus' controller chips.  I do know that they aren't the cheapest.  Many of our competitors claim dimmer compatibility, and they do to an extent, but fall pretty short in comparison.