As far as I can tell, the transformer on my low-voltage kitchen lights has failed*. I ordered what appeared to be a suitable replacement from Screwfix, but the leaflet states that the max cable run is 2m. The current set of lights span 3m.
A couple of questions:
What happens if I use this new transformer?
Can anyone point me to a supplier with a bigger range, (preferably online, and one that states max cable run in the description)?
(AIUI it's not possible to test the output of a LV transformer with a regular voltmeter, correct?)
What sort of "transformer" is it? Big/heavy full of iron or small/light full of electronics? The latter just might object to a longer cable run on it's output, the former won't mind. The cable length limit is probably more due to losses in the cable, heating effects etc so the lights might be a bit dimmer. More information about the number and wattage of lamps fitted, size/tye of cable used required.
If your voltmeter has a range that goes up to around 15v then yes. But even a "mains only" voltmeter would give a rough indication. At the very least a present/absent one.
It'll be fine, they're worried about volt drop on the LV wiring. If you see that the last lamp in the run is noticeably dimmer then beef up the wiring.
No reason to specify a maximum cable run as you could have a run of 1km if the cable was large enough. TLC have a good range at reasonable prices:
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or
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for a direct link if you don't mind unwrapping. Use Premiums & don't bother with the standards, don't know why the have them as they're more expensive.
Sorry haven't tried
One last thought, as these 'transformers' are electronic, the 2m limit may be for stability of the converter. If this is the case then placing just one lamp within 2m will keep it happy. Best practice is mains wiring long, LV wiring short.
The old one is the former, the new one is the latter.
The cable is about 3mm stranded. There are 5x20w lamps at roughly even intervals over 3m.
Based on Fred's response, I installed it, and all seems fine. An unscientific "hold a bit of paper 6 inches from the lamps" test seems to show a bit of a voltage drop, but I can live with that.
As long as the recommended cable run is for aesthetic and not safety reasons, I'm happy!
Ah, OK. The Engrish leaflet with the transformer had a cryptic line about the high-frequency output not working with conventional voltmeters.
The leaflet also says, "By defect situation, do not open the transformer. The case is sticked", and, "exactly in this point tension spikes could be touchly dangerous"... :-/
I'd be very surprised if the cable length was an issue, was the first lamp within the 2m limit? It was 5x20W wasn't it and presumably a 105W transformer? I would normally derate, running perhaps 4x20W off a 105W transformer, but these things are meant to be short circuit and overload protected. You may find that it will start working again once it cools down, but the melting label suggests perhaps not. Transformer wasn't under insulation or anything was it?
My next step would be to get a returns authorisation from Screwfix, then buy the premium unit from TLC (at half the cost :). If asked, don't mention the cable length, but def that they were operated within the load spec
If you're still worried about cable length (as I'm sure you could be), then put the transformer in the middle of the LV run and extend the mains feed to that point - presto 2 spurs of 1.75m.
No, not insulated, and definitely a permanent gonner :-)
Thinking about it, though, it may not have had adequate ventilation all around it, so a combination of factors may have caused the meltdown.
Already ordered one of these. I'm going to shorten the length by a good half a meter for good measure too. I guess it's worth a go returning the old one!
Actually, minimising resistive losses in the LV cabling will measurably
*reduce* lamp life: both brightness and life of incandescent bulbs is
*very* dependant on their supply voltage - from memory, manufacturers' technical info claims brightness is proportional to the 6th power of the supply voltage, and life to a similarly high power (maybe only 4th power) - so a 5% overvoltage would (if these figures are right) give you about 35% extra brightness (noticeable, somewhere between the step from a 60W bulb to a 75W, and the step from 60W to 100W) and a 15% or so loss in life. Because the supply to low-voltage lamps is at 1/20th of the voltage that UK mains runs at, it has to supply 20 times the current for the same power: so a 50W halogen blub is pulling 4A along the wires, while your 5x20W setup is pulling 8A out of the transformer. And since the voltage loss in the wiring is proportional to the current (V=IR, right?) the loss-in-wiring effects are doubly significant - first off, losing 0.5V out of 240V matters much less than 0.5V out of 12V; and secondly - as we've just seen - you need to run higher currents at lower voltages to get the same power shifted. (Yes, this is exactly why the main distribution grid runs at voltages over 100kV...)
But using LV cables which are too thin will have other Bad Consequences: they'll overheat, which in turn may cause poor contact at the cable terminals after a few hundred expansion-contraction cycles, and in the worst case sparky-sparky firey-firey oopsey-oopsey ;-) As Others said already, smartest practice is to have short, thick LV wires and run good old mains cables to transformers/power-supply-units close by, running both mains and LV wiring away from the light fittings (which produce a fair bit of heat, expecially with "dichroic" lamps which dump the heat through the back of the fitting to give cooler running under the lights, and keeping the mains and LV runs separate as far as practicable.
Life is subject to a higher power. (isn't it always) Twelfth power, IIRC, so a 5% overvoltage gives almost half the life. Somewhere around 21% extra reduces life to a tenth.
Well, all 5 of the lights worked before the transformer burned out, which makes me think "no". It seems like a bit of a catch-22 to me - I need a working transformer and circuit before I can test for the problem of the failed transformer.
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