LED Lighting - the technology

[Chris B]

Could someone be good enough to explain (or point me to a web page that will do the job) the differences in LED lights designed to replace halogen bulbs.

I was just perusing a recent CPC catalogue and there is a whole host of

12V LED described as "replacement for halogen lamps". A small number of these (2 out of the 10) "must be used with a 12VDC constant voltage LED driver", although there doesn't seem to be any consistency from type number.

Only 2 of them are described as "non dimmable" and one of these needs the constant voltage driver and the other doesn't.

So any further help would be appreciated.

I have two ceiling lights, presently running on 12V halogens. They essentially consist of a pair of spiral rails with the 12V halogens bridging the rails. The ceiling rose is very bulky so I assume that this contains the transformer, but I have no reason to suspect that it is a constant voltage LED driver (the light fittings pre-date LEDs by a long way.)

Thanks

[Dave Plowman (News)]

12v halogens run from a 12v AC supply. Might be a simple transformer or a SMPS.

If it uses the fairly common MR16 bulbs, replacement MR16 LED will probably work ok, as they contain electronics to cope with the AC supply. However, some halogen SMPS may not work at the lower load of LEDs.

A bare LED (with no internal electronics) is more usually run at constant current, rather than constant voltage.

[Martin Brown]

It is a slightly tricky proposition since the original halogen bulbs run very hot and they have to be insulated to protect the ceiling.

LEDs don't run that hot but are very intolerant of overheating >100C.

The ones intended for a nominal 12v DC "constant voltage" LED driver will have three LEDs in series along each chain and a small series resistor to limit the current (maybe a constant current source).

More common are switched mode constant current drivers with output voltages which can vary depending on the number of LEDs being driven.

Non dimmable LED assemblies typically use constant current drive and attempts to dim them by external means merely results in the controller maintaining light output by drawing ever more current from the available part of the main cycle until something breaks.

Not sure how the newer dimmable ones are being done.

Old transformers will be AC 12 ish volts (potentially a fair bit higher when unloaded or driving low power 7W LEDs instead of 50W halogens).

Bare LEDs do not react at all well to being driven at constant voltage and will be ruined by ac so there is invariably some mechanism to rectify and limit the current in all but the crudest of LED devices.

[whisky-dave]

Most dimmers for LEDs are trailing edge rather than leading edge.

I think pound shop oines and the like rely on the internal resistance of the batterey.

[Brian-Gaff]

That is right, its the current that kills them. The dimming ones I believe do it by duty cycle switching, though it is possible to dim leds by reducing the voltage, its very non linear and can cause a kind of cyclic brightness wobble effect. I tried to get leds to handle analogue audio back when I could see, and it was not as easy as one might think to get a nice sound at the other end!

Brian