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LED electrical problem - help please

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Post Sun Oct 21, 2012 8:55 pm

LED electrical problem - help please

I could do with some help with this probelm please.

We are trying to repair a set of internal, built up acrylic lettering which are illuminated by about 50 LED modules which run from a 12 volt 100w driver. When we removed the acrylic letter faces, we found that the wiring is discolouring and also at the wiring entry point through the aluminium composite letter back plate is discolouring too. The LED's modules have also burnt out and I'm not sure what's causing the problem.

Just an idea, but usually when we maintanin these signs, these LED modules are mounted on to foam board with the wiring passing through the plastic, so could there be an issue because the wiring is passing through the aluminium composite, and maybe there's electro magnetic field in the wiring is reacting to the aluminium?

Any opinions would be much appreciated
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Post Sun Oct 21, 2012 9:39 pm

How long have the LEDs been installed, and what make are they?
Tbh the ACM should if anything help the LEDs as it will act as a heatsink to keep them cool , it's recommended for LEDs to be mounted on aluminium etc and not plastics for this reason and not onto plastics directly.
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Post Mon Oct 22, 2012 8:19 am

The LED modules are around four years old, although in my experiance, it's usuallly an individual LED modules that would fail rather than the whole lot of them, and I'm more concerend though about the heat marks where the 12 volt LED leads have passed through the aluminum composite board - here's a photo to show the problem
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LED on ACM back plate.jpg
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Post Mon Oct 22, 2012 10:13 am

Neil
i would check grade of wire & wattage of transformer is correct because it seems to be generating a lot of heat and when it is rewired give the wire room to breath, drill something like a 6mm hole. The tight hole is not allowing the heat in the wire to dissipate hence the problems at the entry point.
Keep us posted
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Post Mon Oct 22, 2012 11:04 am

Hi Kevin,

Thanks for your input.
The pwoer supply is 100 watt and the wiring is suitable for 12 volt so that's not a problem.

I'm concerned that the fault may be because the wiring is passing through the aluminium skin on the ACM sheet

Has any one else had an issue with wiring through metal sheet before ?
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Post Mon Oct 22, 2012 11:51 am

I can't see the acp causing any issue.

Just throwing some things in the pot:

Is the insulation reacting somehow with the foam, has the insulation actually gone to the extent of the 2 wires shorting?

50 modules, is the power supply up to supply enough wattage for all of them?

I think the foam is the issue not the acp, I think there needs to be more breathing space if you must use it where the cable passes through.
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Post Mon Oct 22, 2012 11:57 am

I have asked Katie at our head office to look at this with Gareth our electrician as soon as possible, will advise when we have a response for you.
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Post Mon Oct 22, 2012 12:13 pm

Just another thought, have the modules failed or caused a higher load on the cables generating the excess heat.

This increased heat has caused the foam burn.

I don't do led's but are fuses fitted the 12v side as a safety measure?
Looks a great potential fire risk if there was more plastic.
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Post Mon Oct 22, 2012 2:20 pm

Thanks for your replies.

I'm doing some research and I'm being told that it's easy to fall into the trap of thinking that just because it's a 12 volt lighting system that you can use any low voltage cable.

LED signage installers beware !

I'm wondering if the red and black cable that this sign has been wired with is not thick enough to take the electrical current and maybe the first run of power to the lights should be through 2.5amp flex.
Any opinions on this would be welcome.
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LED on ACM back plate.jpg
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Post Mon Oct 22, 2012 2:48 pm

It's the amps not the volts you need to calculate.
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Post Mon Oct 22, 2012 4:09 pm

Message that I hope helps:

So he has a 100 Watt 12 Volt power supply, depending on the power supply that has been used they usually have circuit protection inbuilt such as overload and short circuit protection. So this means we could do with more information on the power supply such as make and model.

We also need to find out the load of the LEDs, Watts per module.

Is the cable all brittle leading up to the module or just around the entry point?

As a first guess I would say the hole is either too small or sharp edge, causing damage to the cable which has cause a short circuit to the dibond. You get bigger electrical arcs the bigger the power supply as they produce a larger current, this may have happened a few times causing the sign to go off then come back on.

If the charring of cable is only around the entry point I would definitely say it is down to this. Maybe a larger hole and grommit or a sheathed cable instead.
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Post Mon Oct 22, 2012 9:57 pm

Leds

100 watt power supply at 12V draws 8 amps at max loading that is many many leds, (how many depends on the power output of the the led).

What current rating was the wire.

I think a short circuit due to worn or cut insulation is more likely with positive shorting to dibond

Typiccaly a 3 led module draws less than 0.5 watt so there would have to be 200 in line to draw 8 amps
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Post Mon Oct 22, 2012 10:34 pm

Thanks to Sam and Gareth from Applelec, and also to Adrian

All really helpful and detailed information, and from the help you've all given, I'm certain that there's been a break in the cable insullation around the cable entry point. I'm confident to go back to site now to resolve the problem.

Thanks for everyone's input and reading the post

What we do without this valubale resource of the ukboards :D

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