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?

ppmoore wrote:
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?

you should NOT use dimmers with toroidal transformers. Use electronic
ones designed to work with dimmers.

However this begs the question of why its worked OK for 5 years.

In article ,
The Natural Philosopher wrote:
you should NOT use dimmers with toroidal transformers. Use electronic
ones designed to work with dimmers.

That was the only way you could dim them before electronic PS became
available. You need a dimmer designed for an inductive load, though.

--
*Ham and Eggs: Just a day's work for a chicken, but a lifetime commitment

Dave Plowman London SW
To e-mail, change noise into sound.

In article 6cada57a-ef1e-42a1-bedf-
, says...
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?


Two problems here.

Firstly, transformers do not regulate their output voltage accurately.
Their specified voltage is usally at their rated power - so you only get
12V at 150W in your case - running at lower load may well cause the
output voltage to rise - overdriving your lamps somewhat.

Secondly, unless you've the right sort of dimmer, then you're asking for
trouble dimming an indcutive load. Dimmers work by only turning on the
device for part of the AC waveform. Doing that cheaply involves a very
sharp rise-time in the voltage - something which inductive (and
capacitative) loads won't be happy with. The result is a sort of
electical "BOINGGGG" 100 times a second, with voltage spikes rather
higher than the equipment may have been designed for.


Of course, if you're using a dimmer/transformer combination which are
designed to cope with this, then you can ignore all that!

--
Skipweasel - never knowingly understood.

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

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.

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
[email address is not usable -- followup in the newsgroup]

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.

Andrew Gabriel wrote:
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.


All my dimers that have failed, have failed completely. Either full on,
or the board burned out, or both in fact as I discovered when I repaired
a track.

In article ,
The Natural Philosopher wrote:
All my dimers that have failed, have failed completely. Either full on,
or the board burned out, or both in fact as I discovered when I repaired
a track.

I've got some 75 watt halogen mains spots (a bit like a small PAR38) on a
dimmer, and when one failed it not only blew the dimmer to bits but
tripped the 10 amp MCB.

--
*Husbands should come with instructions

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
says...
I've got some 75 watt halogen mains spots (a bit like a small PAR38) on a
dimmer, and when one failed it not only blew the dimmer to bits but
tripped the 10 amp MCB.


Ah, that'll be shorting along the plasma from the fail-point. When
thefilament fails it creates a happy little ball of plasma which has a
very low resistance. If you're unlucky this can reach from feed-wire to
feed-wire and, in effect, present a dead short.

Bulbs have fuses in them lately, but they still don't seem to actually
work, hence the blown bits.

--
Skipweasel - never knowingly understood.

In article ,
Skipweasel wrote:
Bulbs have fuses in them lately, but they still don't seem to actually
work, hence the blown bits.

The purpose of the triac in the dimmer is to protect the fuse in the
lamp.;-)

--
* I like you. You remind me of when I was young and stupid

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
says...
Bulbs have fuses in them lately, but they still don't seem to actually
work, hence the blown bits.

The purpose of the triac in the dimmer is to protect the fuse in the
lamp.;-)

More likely to work than one of these...

http://www.voltimum.co.uk/news/9428/...ing-Alert.html

--
Skipweasel - never knowingly understood.