Fredxx wrote:

"The Natural Philosopher" wrote in message
...
Fredxx wrote:

"ppmoore" wrote in message
...

I installed have two halogen light circuits, each consisting of three
12V 35W halogen bulbs. Each circuit is fed by its own 150W 230/12V
transformer. Both circxuits are connected in parallel to a 500W
dimmer.

I put in the lights about five years ago, and all was fine until about
two weeks ago, when both transformers failed about the same time. The
transformers gave off an overheated smell, and a cable that must have
been in contact with one of the units was badly charred.

Strange coincidence, I thought as I replaced the transformers two days
ago. Even weirder, I thought as the lights worked fine for about one
day and again stopped working, with the same symptom: overheating of
the transformers and even melting of one of the transformer plastic
casings. I checked the bulbs and are all rated 35W, so the 105W load
per circuit is confortably within the rating of the transformer.

So my attention turned to the dimmer. Can this be causing the problem?
Do dimmers fail with symptoms like this?

If an old fashioned TRIAC dimmer, I would guess that at part load
there's a significant DC content,

There is not actually. Ther is however a sigificant harmonic component.


which lead directly to heat in your
transformers. A TRIAC triggering is inherently asymmetrical, and I'm
guessing this has deteriorated over time.


Not that I know of it aint.

Perhaps you should look up the various gate sensitivities depending on
the quadrant of operation. I can assure you there will always be some
small difference between any quadrant leading to a small voltage leading
to a disproportionately high DC current flowing through the transformer
primary. It is something the OP could do with a DVM.


Very small by comparison with the actual rated currents of the
transformers. And most dimmers are not 'just' triacs these days. The ons
switching is controlled by a chip or some other method usually.




Whilst there are significant harmonic in any TRIAC dimmer, an inductor
will present a higher impedance to these frequencies, and any flux in
the transformer is inversely proportional to frequency. The first
significant harmonic should be the 3rd.


what about the capacitors uses to snub the switching :-)

And various other stray capacitance effects..

Mind you its true to say that largely toroids present a greater danger
to dimmers, than dimmers to toroids.