Help with LED DRL set up - complete with pics

[minispeed]

I have some cheapo LED strips that I ordered off of ebay a while back. I used them as DRL on my saab but they quickly burnt out and some started flashing. Then on my miata I tested them all (I had about 12) on a battery for a couple days with a 1a trickle charger, then when I put them on the miata they started to burn out and flicker. So I figured they work fine at 12v but can't handle 14+

So my question is to the electrical experts out there if I was to wire a resistor in series to drop the voltage from 14 to 12 which ohm resistor would I have to use? Would it matter if I use 1 resistor for each strip or could I wire the one resistor in series then split to 4 strips in parallel after the resistor?

I don't want to over do it and cause them to be too dim, they are a good brightness at 12volts already. I had a resistor laying around (all black, no bars to read) and tried it, the lights barely came on at 12v and just a tad more at 14 but it looked really bad.

My other theory is that the previous installs had them inside the headlight housing and the heat from the bulbs (both times HIDs) may have caused them to over heat. So any opinions on if that is more likely? I have wired up a cigar lighter plug to keep one LED strip on all the time for my drive over the next little while so I can tell if it was a heat issue. The install on the kia will not be inside the headlight so it may be better protected from heat.

Thanks.

[orangustang]

Use a potentiometer to measure the (forward) resistance across the strip. Call this resistance R1. The series resistor for one should have a resistance of R2=R1*(14-12)/12 = R1/6. Make sure you use a resistor that can handle the current you'll be drawing, calculated as I=V/R=14/(R1+R2).

To run all four strips on a single resistor, do the same as above, but your value for R1 will be all of the LED strips in parallel. Since they're all (presumably) the same resistance, you can just divide by 4 to get this number for 4 strips. Again, check to make sure the resistor you get can handle the amps you need.

Hope this helps. Good luck!

Also, regarding whether it might be a heat issue, it is possible but not likely. I have some 20W LEDs that I'm modding for headlamps, and they will get hot enough to fail without a proper heatsink, but they have to get REALLY hot for that to happen I think (haven't broken one yet, but I have burned myself a little). The failure you're talking about seems more like an excessive voltage failure. I think if it overheated externally, you would see damage on the plastic before the LEDs themselves would give out.

[oil pan 4]

How many amps do they draw at 12 volts?
And how much do they draw at 14.5?

This is what I did for hot running LEDs used in vehicles.
DIY: Super bright reverse light mod for any 73-91 style tail light - Diesel Place : Chevrolet and GMC Diesel Truck Forums

[oil pan 4]

http://ecomodder.com/forum/showthrea...ent-20498.html

[minispeed]

Originally Posted by orangustang

Use a potentiometer to measure the (forward) resistance across the strip. Call this resistance R1. The series resistor for one should have a resistance of R2=R1*(14-12)/12 = R1/6. Make sure you use a resistor that can handle the current you'll be drawing, calculated as I=V/R=14/(R1+R2).

To run all four strips on a single resistor, do the same as above, but your value for R1 will be all of the LED strips in parallel. Since they're all (presumably) the same resistance, you can just divide by 4 to get this number for 4 strips. Again, check to make sure the resistor you get can handle the amps you need.

Hope this helps. Good luck!

Also, regarding whether it might be a heat issue, it is possible but not likely. I have some 20W LEDs that I'm modding for headlamps, and they will get hot enough to fail without a proper heatsink, but they have to get REALLY hot for that to happen I think (haven't broken one yet, but I have burned myself a little). The failure you're talking about seems more like an excessive voltage failure. I think if it overheated externally, you would see damage on the plastic before the LEDs themselves would give out.

Well I used my meter today and couldn't get a reading. I have two meters, one that goes to 2000k for ohm and one that goes to 20m and they both just read a 1. This doesn't sound right because I doubt they would have that high of a resistance would they? so assuming that the resistance is just over 20mil, at 21000000 it would mean i'd need a resistor that was 1.6th of this and would read 3.5 at the 20m setting. The resistor I tested read 0.03 at 20m (33.5 at 200k) and that one barely let the LEDs light up when I tested them. So I'm obviously doing something wrong, to work around it I went to the electrical supply store and bought a bunch of resistors for tiral and error. Please help.

[orangustang]

You might have to hook it up to a power supply and measure the amp draw instead, then back-calculate using R=V/I. I'm not really sure if LED resistance changes with voltage, so that's the more accurate way to do it anyway.

[minispeed]

So trial and error, none of the resistors works well enough, the lowest that I got was 560ohm 1/4w. The LEDs still light up but they are very dim. It reads 2.5v when I test it after the resistor and testing it causes the LEDs to turn off. Since they are only $1 for a pack I will go back and test some smaller resistors.

I did as above and tested the current, I found when wired up just the LEDs on a 12v source (it's raining out so I didn't want to go test with the car running and had a 12v inside) 131.2mA for 1 LED strip, 6.7mA when the resister is wired in series.

[3dplane]

Start parallelling those 560 ohm resistors. Twist the legs together on each end of two and you should end up with a 280 ohm 1/2 watt resistor.

Do four to get a 140 ohm 1 watt etc.

Or just get a pack that has lower value resistors so you don't have to parallel that many.

I'm guessing you will only need a few ohms to protect them from the slight over voltage in your situation!

Trial and error but keep checking current draw for different scenarios (voltage and resistors) and you will get a good picture of what they are doing!

Barna

[orangustang]

The resistance you're looking for should be about 15 ohms for a single strip, maybe go 16 to be safe. For all of the strips in parallel, it'll be more like 4 ohms.

[oil pan 4]

500+ ohms is way too much.
Normally when resistor droping voltage to run LEDs off 12v power you are looking for something well under 100 ohms.
What is your target volt and amp draw to feed the LEDs?
I can figure out the resistors you need.