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I have 16 ceiling spots, each with its own transformer on one dimmer. After a few minutes one of the spots will start to pulsate (flash on and off at varying speeds depending on dimmer level). It's always the same spot. I have tried changing the lamp, transformer and wiring between the spot and transformer - no joy.

Any suggestions???

Are the spots deigned to have a dimmer connected? Some are switch mode are can't be used with dimmers. I know many of the transforms I have used have a minimum as well as maximum wattage marked on them and these can't be used with dimmers.

Thanks for the prompt reply and suggestion.

All spots and transformers are identical. The problem is with one single spot and only starts after about 10 minutes When the dimmer is turned right up (bright light) the spot doesn't pulsate. Equally, when the dimmer is turned right down (dim light) there are no problems. The spot only pulsates when the dimmer is turned to half setting. I am really puzzled.

is it possible to swop entire fitting and lamp with another position to see if problem moves as well.
If it does' it maybe the lamp connection breaking down.

at last resort swap fitting lamp and TRAN with another one that works ok and see what happens then.

Is there space around tran for cooling, try running with lamp hanging from ceiling, to reduce heat so tran is cool , maybe the thermal cut out in the tran operating.

You have me wondering now. I expected switch mode transformers although you have not said if there is a minimum wattage or amps marked on the transformers. I dimmer switch normally works by switching the power off for part of the cycle using resistance would produce too much heat. A simple transformer will just change the volts and amps in to volts and amps out at same wattage so dimming switches should work. but the better transformers turn the power in to DC then store in in a capacitor then turn it back to AC at very high frequency because it is so high the transformer is very small and then the electronic switch the power on for a proportion of the time and so control to output to exactly what is required so any input power variations will not effect to output unless it gets well out of parameters. These switch mode transformers are very much lighter than there non switch mode equivalents. And they normally have a wide input voltage i.e. 100 to 240 volts non switch mode will be 220 to 234 if you have a lap top that will have switch mode also some mobile phone chargers. Since you have checked the wiring and it all comes from same dimmer switch I will assume that the switching frequency of the one transformer just happens to line up with chop frequency of dimmer switch. It is the only way I can see it happening to just one transformer. Either that or there is some special circuit in the switch mode transformers to monitor the pulse chopping of the dimmer switch and it has gone wrong in one odd one. So what does it say on the transformers. You really have me wondering now. Are all bulbs the same wattage. Please do tell me what is written on them I am really intrigued now.

Thanks guys,

I think I may be getting somewhere. It must be the transformer. I swapped around the bulbs, wiring, transformers etc and the only time the bulb pulsated was when I was using one particular trasformer. I checked this transformer against the others and the spec seems to be the same but I believe it to be a different batch from the originals.

Details of original 15 transformers :

ASPECT ALT 60D/T
For LV Halogen Lamps - Dimmable
PRI: 240V - 50Hz 60VA
COS + 0.99
SEC11.6V - eff Max 4.9A
Ta 50 degrees Tc 85 degrees
20 - 80 Watts

Details of single new transformer (pulsates) :

ASPECT ALT 60D/T
For LV Halogen Lamps - Fully Dimmable
PRI: 240V - 50Hz 60VA
COS + 0.99
SEC11.6V - eff Max 4.9A
Ta 50 degrees Tc 85 degrees
20 - 80 Watts

Now, the only difference between the original 15 transformers and the single new transformer is that the originals are slightly darker and the single new transformer says it is fully dimmable rather than just dimmable. Thinking it was just a single faulty transformer I went to the store and changed it for another identical transformer (lighter and fully dimmable) To my surprise this replacement transformer caused similar pulsating.

I will return to store in next couple of days to get another transformer (either different make or at least different batch) and will keep you posted.

As you can see, this is becoming an obsession!

You have, as I expected, got switch mode transformers, and yes they are dimmable but according to one dimmer manufacture only using a “Low Voltageâ€

Although tran says on it dimmable, always check instructions .

Sometimes says in instructions dimmable ONLY with compatible dimmers, sometimes leading edge only or sometimes trailing edge only have to be used.

Best ones I found are the mode et 55 range dimmable with virtually any dimmer and I have had no problems with them.
The mode ones do what it says on it perfectly.

Sorted! I bought a different make of dimmer as advised and the problem is solved.

Thanks again guys - great Forum.