Changing Transformers for 12V Tension Wire Spotlights Ask questions and find answers to many subjects relating to electrics and electrical work

[normandybrit]

I've fitted some 12v tension wire spotlights and not wishing to use the huge transformer that came with the kit I sent off for a couple of Osram HTM 105 Halotronic transformers. Nice disreet units. However, on trying both of these units I'm seeing mains voltage going in but no readings of anything coming out. Am I missing a trick, being stupid, or have I ended up with two duff units?

[Mr White]

You said a "couple" of transformers why a "couple" you can not use them on the same tension wires, also most electronic transformers will not work without any load. So if you are trying to check the voltage with nothing connected they will not work.

[ericmark]

The Osram HTM 105 Halotronic transformers is not a transformer in the old way it is a switched mode power supply, these have a minimum output of 35 watt and maximum 105 watt either under or over it will close down.

With the switch mode supply the output frequency is much higher in the 43 kHz range, so the output cables have to be kept short it states in the data sheets no longer than 2 meters, also switching between electronic transformer and lamp not permitted so can't use them with tension wires as you can remove and refit lamps while switched on.

For the tension wires type you need a wire wound transformer.

[normandybrit]

May thanks for the replies.
Mr White. I wasn't explaining myself clearly. I have 2 sets of tension wires, so one transformer for each.
Ericmark. You are correct! I've just jury rigged up a test and Hey Presto with a 50w load they work! I tried extending the cables to the maximum I'm likely to need and there is a definate (but useable) drop off in brightness at 3.7 metres. I presume the brightness goes into decline after the 2 metre mark. Do you think reliability will suffer if I continue with the Osram unit? I'm hooking up 5 x 8w LEDs on each setup.

[ericmark]

With DC you can get lamps designed for boats and caravans with an input voltage in the range of 10 to 30 volt, using these lamps there would not be a problem with volt drop or distance.

The problem I find with 12 volt spot lights is there are so many power supplies, AC, DC smoothed and rough outputs that it's hard to advise. Theory a DC power supply should have a smooth ripple free output, however with most LED lamps they do not really mind if there is a ripple or not so many power supplies designed for LED lighting are rather basic, what I don't know is how the lights designed to run off batteries will work when connected to a poor quality power supply.