Using a dimmer with 12V halogen lamps

[ppmoore]

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?

[The Natural Philosopher]

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.

[Dave Plowman (News)]

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

[Skipweasel]

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!

[Fredxx]

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.

[The Natural Philosopher]

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

which lead directly to heat in your

Not that I know of it aint.

[Fredxx]

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.

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.

[Andrew Gabriel]

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.

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.

[The Natural Philosopher]

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.

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.

[The Natural Philosopher]

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.

[Dave Plowman (News)]

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.

[Skipweasel]

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.

[Dave Plowman (News)]

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

[Skipweasel]

More likely to work than one of these...