# Wire Gauges - What to use for LED application?



## reviewum (Jul 14, 2004)

Hi Friends,

I asked my buddy Doug Owen this question, but thought I'd post it here as well:

I need to re-do my project here:
http://www.nifty-stuff.com/gt-lights-seats.php

I'm going to use the LM317 to run my LEDs in my car. I want to use the smallest gauge wire I can. How do I determine what the smallest gauge wire I can use? It isn't like an LED is pulling too much current right?

So, masters of the electron, what gauge(s) should I use, and where should I get them?

Thanks in advance!


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## evan9162 (Jul 14, 2004)

If it's just 5mm LEDs, any wire will work, even thin 24/26 gague speaker wire, phone cable, etc. - You're just passing 20-30mA.


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## jayflash (Jul 14, 2004)

It looks like, stranded, 24ga would suffice if the insulation is good enough to withstand abrasion and vibration.

A larger wire size affords additional strength which is why I'm wary of going smaller than #24 wire. Others may be aware of a high quality, smaller gauge, wire that could be recommended.


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## evan9162 (Jul 14, 2004)

Good point on the durability issue - one thing to keep in mind as far as the insulation rubbing against sharp corners/etc.


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## reviewum (Jul 14, 2004)

Thanks for the points! I'm not too concerned with durability at this point since the wireing will not be in the headliner where it would be rubbing around. I'm still deciding the placement of the LEDs in the car and like to be able to move them around (so does the wife... she loves them for reading to the baby in the backseat and the ability to shift their positions).


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## idleprocess (Jul 14, 2004)

There are companies out there pushing the limits of power-over-ethernet with 50-watt loads 100m away from the power source - that's 5 amps @ 12V.

No 5mm LED is going to take that much power - so use whatever wire works best for you.


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## pbarrette (Jul 14, 2004)

Hi reviewum,

Just don't forget that when you start to use the smaller wire guages that you will have to figure in the resistance of the wire itself. This is true especially at the lengths that you are using.

Personally, I would try some really small 30-32 guage enameled magnet wire and twist the lengths into pairs. If you are using small enough wires and you get the right color of enamel coating, they could be virtually invisible in your setup.

A common option for car mods like this without cutting into the headboard is to run the wiring along the edge of the headboard. This works very well if your car has a plastic trim piece that slightly overlaps the edge of the headboard since there is almost always enough room to gently stuff the wire into the plastic trim. Then you would only have wires showing where they need to come out towards the center of the headboard.

Alternately, you could place your lights on either side of the rear passenger cabin in the upper back corners near the rear window. Then just position the lights so they are angled for good in-cabin viewing. This would make the wire-stuffing trick even more useful since you wouldn't have to have any wires showing.

So, I strayed a bit off topic with the layout ideas, but I hope it helps some.

pb


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## reviewum (Jul 14, 2004)

I was just going to comment on idleprocess's post and ask about magnet wire (the REALLY tiny stuff) when I read pbarrette's post. It would be great to find a tiny magnet wire that matched the inside of my car!

I'm using the weather stripping on the sides of the car heavily in this project. I'm still stuck with what to do about the windshield / front as there isn't really a good place to hide anything. There is a bit of a gap, but when I put my finger up there to feel the distance I almost burnt my finger... I couldn't believe how hot it was, so I'm going to avoid that area!

Well, I'm going to play around a bit more. This design with the LM317 is WAY better than I had before!

I'm running it at a constant current of 40 mA. This way I can stick two LEDs in parallel and get 20 mA each, or just one LED and overdrive it!

Actually... this brings up a question I had: With a constant current circuit, do I still have to worry about the dreaded THERMAL RUNAWAY? It seems that the LED may still get a bit toasty, but it won't get more current so it won't start to go exponential on me. Is this correct?

Thanks for the posts!


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## jtr1962 (Jul 14, 2004)

Since you're running off 12V why not run the LEDs in series and drop the constant current to 20mA? There's more than enough voltage overhead. Assuming each LED drops 3.5V, and the LM317 needs about 3.5V to start regulating, you'll be fine at an input of 10.5V or above. With a series circuit you don't have to worry about matching the forward voltages of the LEDs. I don't even run LEDs in parallel any more for that reason-it's a real PITA.

No, you don't have to worry about thermal runaway with a constant current circuit. That's the point of using one. As the LEDs heat up the forward voltage will drop but the LM317 will see to it that the current stays the same.


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## reviewum (Jul 14, 2004)

Two reasons I didn't do these in series:

1) I mistakingly thought that the LM317 required 6 volts of headroom, so I thought two 3.3 volt LEDs plus 6 volts was too high. Whoops.

2) I actually use to have these setup in series, but realized that if I wanted to remove (turn off) one side of the lights the whole thing was off. 

Does this make sense?


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## reviewum (Jul 14, 2004)

woah.. what was the original topic anyway???? oh yeah, wire! So, no worries with the REALLY thin / tiny magnet wire?


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## evan9162 (Jul 14, 2004)

Personally, I wouldn't use magnet wire because it's typically more stiff than a stranded wire of equivalent size, which makes it harder to work with, and also more prone to breaking when bent multiple times. Also, the epoxy insulation is very thin, so slight rubbing or even sharp bending may expose the bare copper.


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