Low Voltage Strange Dimming result !

11 Jan 2006
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United Kingdom
I think I'm being a bit thick here....but maybe one of you guys could help me with something I am obviously not getting...!

I have a room with 3x12v low voltage spotlights (20w each)
They were running off one transformer with a special LV dimmer switch.

When the dimmer is used, 80% of it works well, but there are "spots" on the dimming range that cause the spots to flicker slightly.

I thought maybe the transformer was the problem, so I bought two more transformers and re-wired one transformer for each spotlight.
This had no effect, and in fact now, when the dimmer is turned up to "full on" - the lights go off - almost as if the surge/protection circuit has killed the current.

The transformers state they are of the dimming variety, and as I said, the dimmer switch is a special circuit based dimmer switch specially for "lv" lighting.

I'm a bit stumped...!!

Any ideas?

My next thought was maybe 20w was too low but before I go out and buy 3x 50x spots to terst a theory I thought I would pick your brains...!!

Many thanks for the help!
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yeah...tried that. Didn't make any difference.

Could I have a fault/earthing of the supply do you think?

I thought that either 240v is either good, or it isn't!!

I'm baffled!
The dimmer and the electronic "transformer" both work by turning the power on and off. The more ON time compared to OFF time then the brighter the light.

But they don't do it at the same speed. The dimmer does it at 100 times a second and the transformer often at a higher frequency that varies with brightness. At some settings the two frequencies interact and produce a beat note and this causes flicker.
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Ok...problem fixed for anybody that's interested/has a similar problem...!

Having investigated up in the loft and having moved a few bits around, I noticed that the low voltage cable had got very close/crossing over the 240v line and must have created an electromagnetic field between the two cables that interfered with the frequencies the transformer was producing.

Moving the 240v cable well away from the 12v cable solved the problem.


You learn something every day !!!

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