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Andrew Gabriel Andrew Gabriel is offline
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Default Using a dimmer with 12V halogen lamps

In article ,
"Fredxx" writes:

"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.


Gate sensitivity doesn't factor because triacs in dimmers are not
driven at levels close to gate sensitivity. However, asymmetry of
a firing diac does factor in for very simple dimmer circuits where
they are used as the firing trigger, but not for higher quality
dimmer circuits where hard firing is used. Also, with cheap dimmer
circuits, symmetry of the triac's holding current will become
significant if the load is at or near the dimmer's minimum load.

It is something the OP could do with a DVM.

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.


A couple of points:
Core losses at higher frequencies are also higher, as the stamping
thickness is optimised for quenching 50/60Hz eddies, so you get more
core heating with higher frequency harmonics. However, I can't see
where OP said that a old style transformer was being used.

Triacs often die in one direction whilst continuing to work in the
other direction, leading to very high DC component (in either case
of a short or an open circuit on the failed side). Connect an ordinary
filament lamp and check the dimmer still goes from nearly nothing to
full brighness. If the triac has partly failed, the light will either
go from 0 to half, or from half to full, but not 0 to full.

--
Andrew Gabriel
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