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I am building a wall cabinet with downlights in the shelves, for which I would like to use MR16 LEDs. However, my first attempt was obviously deficient, since two of the LEDs blew. I'd appreciate some help to figure out a) what went wrong, and b) what I need to install them properly.

enter image description here

As you can hopefully see in the image above, I have a single transformer, which has the following specifications:

Electronic Transformer for LV-Halogen Lamps
PRI: 220-240V ~50Hz 0.27A
SEC: 11.5V~ 10-60W λ=0.99

I have four 12V, 4W MR16 LEDs wired in parallel to the transformer. When I connected them all up to the power, they seemed to burn fine, without any noticeable flicker; after about 30 min, one of the lights went out, and I thought it might have been my wiring. After another 10 min or so, another light blew with an audible pop. At that point I realised there was something wrong with my setup and switched it off.

When all four LEDs were burning, and when only three were working, I took some voltage readings, which were around 4.5-4.6V (nowhere near the expected 12V).

I have considered 4 possible issues here:

1) The LEDs are expecting DC input, and are getting fried by the AC current. When I bought the transformer (from the same store that sold me the LEDs), I specifically asked if I needed a DC transformer, and the assistant replied that they didn't have any other kind of transformer, and that this was what they used in their in-store demonstration arrays. The LED packaging makes no mention of whether they need AC or DC current.

2) The LEDs are wired in parallel, when they need to be wired in series to give the necessary resistance for the transformer to work.

3) The LEDs are too far from the transformer, resulting in voltage drop (the maximum distance is about 1.5-2m).

4) The transformer is faulty.

I don't have the knowledge to diagnose which of these issues (or combination thereof) is at fault here, so any advice will be gratefully received.

Edit: I am in South Africa, with 220V AC mains electrical supply.

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I suspect if you use a RMS meter you would see that the actual reading was not 4v –  Steven Dec 30 '12 at 2:11
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1 Answer

Electronic transformers designed for Halogen lamps will kill LEDs.

Electronic transformers are very simple switched mode power supplies. They "chop" the input 230 V/125 V sine wave in a way that will yield an average output voltage of 12 V. However, the peak voltage will be much higher and will eventually kill LEDs (that's no problem with Halogen lamps because they react more slowly). In addition, the chopped waveform may cause radio interference if the cables to the lamps are too long. The waveform may also be the reason why your voltage readings are off.

Instead of an electronic transformer designed for Halogen lamps, you should either use:

  • a conventional AC transformer (the heavy kind); or
  • an electronic DC transformer designed for LEDs.
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Just a comment on this. I think it's still a valid and correct answer, but switching to a conventional transformer did not solve the problem for me. Eventually I had an electrician come in, and he replaced my thick solid-core wiring with much thinner wire that he called silicone wire. That seemed to do the trick, although I also switched back to halogen at the same time, so it's hard to be sure what made the difference. –  rudivonstaden Oct 23 '13 at 14:11
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