Low Voltage Lighting Problem

[Bluestu]

Hello, I am almost finished on a kitchen re-fit project and have a problem with some low-voltage lights. Now, I have undertaken none of the work myself, having used a Part P qualified electrician (as required for Building Regs), however, as he is coming back next week and most of the tradesman to date have played on my ignorance in their field of expertise and generally taken the P**s I would like to at least gain a small understanding of what the issue might be.

So here goes:-

I have a lighting circuit with 6 x 20Watt Halogen Bulbs. These are controlled by a dimmer at one end and a normal on/off at the other so I can control from either end of the kitchen. 2 of the lights have their own transformer and the remaining 4 share a transformer designed for more than 1 light.

Initially, the first transformer on the circuit blew when the light was switched on from the on/off switch. Then yesterday (after it had been replaced) it blew again. Today, the next transformer on the circuit blew (the 1st having not been replaced) after being switched on from the dimmer.

In all instances the transformers work for a period of time before they seem to blow and survive being switched on/off from either switch.

So that's 3 transformers blown. Yesterday, I had a full electrical installation report carried out and it found nothing wrong (it was not done because of this problem I hasten to add).

So my question is:-

1. Is the problem likely to be the transformers?
2. Is the problem likely to be the normal switch type on/off switch?
3. Is the problem likely to be the dimmer switch?
4.Is it possible to buy a transformer that doesn't just blow and have to be replaced, but has some sort of trip mechanism?
5.Is it possible that some makes of transformers and dimmers just are not compatible?
6.Am I clutching at straws???

Any help, advice or tips very much appreciated

Stu

 

[Steve]

1. yes
2. no
3. yes
4. not that im aware of
5. some dimmers and transformers are not compatible - dimmer must say "suitable for use with transformers" and transformer must say "suitable for dimming". also, dimmer rating should be doubled (i think) when used with halogens or transformers
6. no. somewhere along the line your spark has chosen incompatible equipment. He should fix this at his own cost. I dont know if it is to do with having 3 transformers on one dimmer in this particular case.

 

[Bluestu]

Many thanks Crafty - that at least gives me an understanding of what I should be doing/saying.

Thanks again and have a good weekend - Stu

 

[breezer]

dimmer rating should be HALVED not doubled, best thing is 1 transformer / lamp and make sure no space insulation (fibre glass stuff) is around transformer as this can cause them to over heat

 

[Bluestu]

There should be no issue with heat as the transformers are sitting on top of a wall mounted unit and nothing surrounding them - however, one did feel quire warm before it blew a day or so later....the transformers have a thermal cut-out which allows them to work again when they've cooled down, but these never work so I guess they're gone.

It's interesting what you say about 1 transformer per lamp - this I can do quite easily in terms of accessability.

 

[Steve]

dimmer rating should be HALVED not doubled

thats what i meant . . .

 

[breezer]

dimmer rating should be HALVED not doubled

thats what i meant . . .

i knew that, don't tell me you actually typed halved, but we coud all see it as doubled

 

[AEM]

Take a look at these transformers:

http://www.tlc-direct.co.uk/Products/LTYT70.html

"Thermal and Overload protection. Both of these features are fully reversible so as not to damage the transformer or any equipment on the circuit."

 

[Steve]

dimmer rating should be HALVED not doubled

thats what i meant . . .

i knew that, don't tell me you actually typed halved, but we coud all see it as doubled

it meant to read as "you should buy a dimmer with double the rating of the lamps" of course.

 

[Bluestu]

Well I've been quite busy with this problem for the last week or so and after losing a couple more transformers I now know the following:-

1. I have a Varilight 1Gang, 2 Way 1 x 400W/VA Dimmer Switch. This is confirmed as suitable for dimming low voltage lights.

2. I am running 6 x 20W Bulbs.

3. I have 3 Transformers (225 VA). These transformers are suitable for dimming.

4. The power supply to each transformer is separately fused.

The sparky has confirmed his wiring is all ok and this is backed up by an installation report.
I have removed the dimmer and put in a normal on/off switch and for the past 5 days there have been no more problems.....the transformers also don't get as hot as they used to with the dimmer before they blew.

I have no idea what to try next other than keeping the normal switch and putting in lower wattage bulbs. This would be a shame as all the other lighting in the kitchen can be dimmed.

I've spoken with the dimmer manufacturer who tells me the problem is probably with the transformers (but then he would). He said I should ensure that the transformers are actually dimmable (even though it is printed on the side that they are) and if they require leading edge/trailing edge capability. His final piece of wisdom was to suggest I buy Varilight transformers rather than the Brackenheath ones I was sold!

Any more ideas would be gratefully received as I feel I am now being pushed pillar to post.

 

[Qedelec]

What is the VA rating of the transformers.
It is possible they are drawing more than the dimmer can handle but why they should blow and not the dimmer is a mystery

 

[Bluestu]

2 of the transformers have a VA rating of 60VA and the other has 95VA (225VA Total).
The dimmer has a rating of 400W/VA with a minimum of 50VA

Varilight have assured me it is not possible to overload the transformers.

The lights have now been running on the normal on/off for around 5 days now and the transformer is cool to touch, previously they lasted less than 1/2 a day on the dimmer and were very hot to touch. Also, despite the fact they had a thermal cut out which is supposed to reset once the unit has cooled they never came back on line which suggested to me it was something more serious.

Do you know what is meant by leading or trailing edge?

As i've been writing this I've just had a call from my supplier who has told me he has spoken with the transformer manufacturer who has stated they are compatible with Varilight and that as a last resort he will swop to Varilight transofrmers as recommended by Varilight.

 

[ban-all-sheds]

I've spoken with the dimmer manufacturer who tells me the problem is probably with the transformers (but then he would). He said I should ensure that the transformers are actually dimmable (even though it is printed on the side that they are) and if they require leading edge/trailing edge capability.

This is a little known compatibility issue with dimming ELV lighting "transformers" (in quotes because they aren't actually transformers, which is why the problem arises)

Dimmers do not work by lowering the peak voltage, they reduce the RMS voltage by simply chopping out part of the waveform - they either turn on part way through each cycle (leading edge) or turn off part way through (trailing edge)

So you might have a "transformer" that is labelled as suitable for dimming and you might have a dimmer switch that is suitable for use on ELV power supplies but you still need to match the leading/trailing edge characteristics of each.

Click.

 

[Bluestu]

Thanks ban-all-sheds - That's what the guy from Varilight was eluding to and much better described by you than I could ever have attempted to!

Your response reassures me as I have tried everything suggested other than installing Varilight transformers (which arrive tomorrow) and I though that would just be a case of going through the motions. Perhaps this will be the fix.

Thanks again for your time - Stu

 

[smokeyJoe]

This has got me wondering what the mode of failure is for the transformers.
Perhaps using a 'trailing edge' type switch with a 'leading edge' transformer causes high voltage ringing in the primary, causing it to breakdown?
I can't think why there would be a problem using a 'leading edge' switch with a 'trailing edge' transformer. Any ideas?