LED Lighting - the technology

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.

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.

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.

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