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LV transformer specs |
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| message from Jeremy Collins on 25 May 2004 |
Hi all,
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:
1. What happens if I use this new transformer?
2. 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?)
Many thanks,
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| Dave Plowman replied to Jeremy Collins on 25 May 2004 |
For a longer run you need thicker cable. IIRC, TLC have a guide on their
site for cable sizes on low voltage lighting.
www.tlc-direct.co.uk
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| Jeremy Collins replied to Dave Plowman on 25 May 2004 |
Nice one! Seems to indicate that my current cabling is
1mm too thin. I'm seeing a bit of a voltage drop,
nothing serious, but if proper cabling means better lamp
life then I guess it's worth it.
The direct link is
http://www.tlc-direct.co.uk/Technical/Lighting/VoltageDrop.html
Cheers,
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| stefek.zaba replied to Jeremy Collins on 25 May 2004 |
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.
Stefek
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| Ian Stirling replied to stefek.zaba on 25 May 2004 |
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.
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| Dave Plowman replied to Jeremy Collins on 25 May 2004 |
You can, but it needs to be under load to have any meaning.
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| Ian Stirling replied to Dave Plowman on 25 May 2004 |
And you will need a true-RMS meter for many 'transformers' (that
are in fact switch-mode power supplies)
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| Dave Liquorice replied to Jeremy Collins on 25 May 2004 |
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.
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| Jeremy Collins replied to Dave Liquorice on 25 May 2004 |
You're 100% correct - it did object. After a couple of
hours it overheated and burned out. Oh well.
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| Jeremy Collins replied to Dave Liquorice on 25 May 2004 |
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"... :-/
Cheers,
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| fred replied to Jeremy Collins on 25 May 2004 |
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:
http://www.tlc-direct.co.uk/ or
http://www.tlc-direct.co.uk/Main_Index/Lighting_Menu_Index/Lighting_Downlig
hts_Index/Lighting_Transformers/index.html 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.
HTH
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| Jeremy Collins replied to fred on 25 May 2004 |
Heh, well it worked, for a couple of hours. The transformer
appears to have overheated (the label on top is slightly melted).
Back to the drawing board (although I'm not sure what to
do next...).
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| Jeremy Collins replied to Jeremy Collins on 25 May 2004 |
Easily within 2m.
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!
Good idea, cheers.
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| Tony Williams replied to Jeremy Collins on 25 May 2004 |
Note that your setup has burnt out the original
(ordinary) transformer and now an electronic type.
So is there a problem there somewhere.... very high
mains voltage, one or more bulbs taking excess
current, transformer in an unventilated position?
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| Jeremy Collins replied to Tony Williams on 25 May 2004 |
Yep, noted. It could be the latter, as the weather has been
getting warmer, but it was working fine for over 2.5 years.
What would cause high mains voltage? How do I test for a bulb
taking excess current? This seems slightly beyond my basic
knowledge of a multimeter...
Cheers,
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| fred replied to fred on 25 May 2004 |
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.
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| Jeremy Collins replied to fred on 25 May 2004 |
Ah, great. It's quite beefy now (stretched tight across a > 3.5m
room), but I've got some bigger stuff too.
Thanks for the link & tip.
Yes it does, thanks.
Cheers,
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