Quote:
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.
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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.