LED bulbs - dimmable?

[Simon C]

Lidl have some LED bulbs on sale tomo? Are they dimmable? Or is it only certain types?

[Tabby]

of those would soon kill the dropper in the LED bulbs.

NT

[newshound]

What's the principle behind "mains" LEDs? Is there a transformer? a DC power supply? are they back to back in pairs so that half conduct on each half cycle?

[Tabby]

They use a CR dropper. When the supply's triac chopped, the same charge flows per cycle, but the current bursts when the triac switches on mean the resistor dissipates way above its rated power, and fries.

NT

[Jules Richardson]

So, say you have a whole bunch of them in a room - is there a significant inefficiency from each of them having their own "local" control gubbins, when a single box of tricks somewhere and suitable cabling to each of the LED clusters would have been better?

Same question applies to CFLs, too - and makes me wonder if we should be re-thinking house lighting wiring if energy efficiency* is the aim...

• and cost. I hate replacing "whole things" when I could just be replacing the part that's failed :-(

cheers

Jules

[Man at B&Q]

Really? Care to cite your source?

MBQ

[Adam Aglionby]

Most of the multi 5mm MR16 style are CR dropper , big LEDs tend to use a little switching regulator to drive by current.

inefficiency from each of them having their own "local" control gubbins, when a single box of tricks somewhere and suitable cabling to each of the LED clusters would have been better?

This works in some circumstances with current driven LEDs where a constant current loop can work well with significant cabling savings, not a one size fits all solution though.

re-thinking house lighting wiring if energy efficiency* is the aim...

Beena round since before CFL, 2 ioin ntegral , 4 pin external starter, PL format lamps , the gear is in the base or the fitting, commmon in commercial fittings.

• and cost. I hate replacing "whole things" when I could just be replacing the part that's failed :-(

Couldn`t agree more, main failure in CFLs appears to be ballst failure , it just ain`t green.

Cheers Adam

[Mike Tomlinson]

In article , Man at B&Q writes

Bought an 108-LED lighbulb replacement from fleabay. All it has inside is a C and a couple of Rs. There may be a diode too but I didn't spot one.

[Tim Streater]

You mean as in a house having (say) a 12VDC setup for all its lighting circuits? I think we'd need to wait for the situation to settle somewhat first, so there was some vendor consensus about the best voltage. Then what about fridge/cooker lights? It's prolly a good idea, but LED costs need to drop a lot first. We just put in a 4W LED GU10, it's around £20.

[Jules Richardson]

Yes - that or a standard for running lighting wiring back to central point(s) where a common controller can convert AC mains to whatever voltage/waveform the lighting happens to need.

I don't know if it is wasteful (or at least significatly so), hence the question, but it seems like "mains AC and a bunch of control gubbins at each light fitting" might not be the most efficient way to do things, as well as having a bigger impact when something fails.

I think you're right in that the technology's just too volatile right now

- but I'm curious as to whether it's something worth aiming for.

cheers

Jules

[Tim Streater]

Me too, but I don't know enough about small transformers, transformer technology in general, or what might be best for LED lights, to be able to comment.

[Tabby]

CR droppers arent perfect, but they're cheap and simple for low power loads. A 12v 30kHz sine feed would make the LED lights cheaper and more efficient, but I think we're along way from that being the thing to do.

Look at CFLs, 2part and 1 part exist, yet the inherently less efficient 1 parters totally dominate the market. Why? Compatibility.

240v LEDs are the most compatible type currently, hence their domination.

NT

[Tabby]

The best (simplest & 100% efficient) option for dimming LEDs is a capacitor in series with the LED lamp. That and a switch gives a choice fo fixed brightness settings.

NT

[Dave Liquorice]

Beware of volt drops in the cabling... So lets make the assumption you replace 5 ceiling lamps with LEDs at 5W each (so you stand a chance of having the same level of light). Load now 25W or about 2A @

12v. 1mm T&E drops 44mV/A/m so just 11m of cable(*) drops 1v or over 8% of the supply voltage...

[Adam Aglionby]

Hence current loops, most extreme example being airfield landing lights, current loop at a few kV, keeps going for literally miles of cabling and if a lamp fails it arcs across and keeps the circuit complete.

Cheers Adam

[Tabby]

'Series strings'. Trouble is, with LEDs if one dies or isn't fitted properly in its socket, the whole lot go out.

I've updated the wiki LED article, it explains this among other things.

[Dave Liquorice]

How does a series string cope with only wanting one lamp(*) of the string on?

(*) Not "lamp" ie what would be stuffed into a fitting not an individual LED making up an array to form a "lamp".

[The Natural Philosopher]

you short out the ones you DONT want :-)

It should be possible to do this on a house basis. You would use something like a ballast inductor to do the current limiting, and shove all your lights in series. Short out the ones you want off. If a bulb goes, well perhaps have some sort of neon in it that strike because it is now seeing full mains voltage. That tells you which one to replace..

[Tim Streater]

And what about if you want them *all* off?

This series lark strikes me a potentially dangerous if punters think that its safe because any given blub is 12V, but it could in fact under some circs have 240V across it.

Sounds to me like 12V house-lights wiring is a non-starter due to the good point about voltage loss (I should have thought of that). In fact in general higher voltage is better since it means less current (cue discussion about consequent danger of US voltage).