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When we moved in 3 years ago, we replaced the 12V MR-16 halogen lamps in two recessed fixtures in our ceiling with matching LED lamps.

This evening I smelled electrical burning and wisps of smoke around one of them. Some investigation turned up this in the attic:

enter image description here

The thing at the top of the photo (I'm guessing a transformer?) was 220F according to my thermal camera, and the ceiling below it was 114F. The can is at the bottom of the photo.

The same thing seems to have happened to the other, identical light on the same wall switch -- the transformer from that one is hidden under the plywood floor of my attic, but the ceiling is just as hot in the right spot.

Both lights are connected to Lutron Caseta LED-capable smart dimmer (although I'm not certain it was rated as compatible with 12V lighting).

I have several questions:

  1. Any idea what happened? did something just wear out/go bad? Do I need a different dimmer?
  2. I want to keep fixtures with MR-16 LED lamps, do I have to install another 12V transformer? How do I make sure the same thing doesn't happen again?
  3. We have another pair of the same lights in the other room, should I do the same thing to them (i.e., replace the transformer or whatever)?

Thanks.

MemoryWrangler
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1 Answers1

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Running a standard 'old' transformer from a dimmer (doesn't matter whether the dimmer is LED rated or not) is almost always a bad idea. The way the AC waveform gets 'chopped up' by a dimmer causes 'bad things' to happen in the magnetics in the core of the transformer, resulting in a lot of excess heat.

You have a couple of options to resolve this, but all of them will require the replacement of some piece of the system.

  1. Remove the dimmer and replace it with a plain on/off switch
  2. Swap the light fixtures for similar ones which take GU10 120V bulbs (either halogen or dimmable LEDs) which no not need transformers
  3. Swap the light fixtures for similar ones which accept your 12V MR16 bulbs, but include dimmable electronic "transformers" instead of old 'iron' transformers

It may be possible to modify your existing fixtures to support either option #2 (replace the MR16 bulb sockets with GU10 sockets and bypass the transformer) or #3 (replace the existing transformers on your existing fixtures with dimmable electronic ones, but I don't know whether it's possible to do either of those without violating the safety listing of the fixture...

brhans
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