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Archive for July 12th, 2012

LED Forward Voltages

Thinking about those batteries in the context of a¬†really big LED tail light for a bike leads to wondering about the variation in LED forward voltages; it’s possible to drive LEDs in parallel if they’re well-matched for forward voltage. A quick-and-dirty test is in order to get some first-pass numbers… and I have bags of nominally identical red and amber LEDs.

Applying a fixed voltage that produces 20 mA through 14 randomly chosen LEDs of each color, then measuring the voltage across each diode:

LED Red V Amber V
1 1.895 1.939
2 1.893 1.921
3 1.903 1.918
4 1.895 1.921
5 1.891 1.918
6 1.935 1.906
7 1.891 1.926
8 1.904 1.930
9 1.901 1.923
10 1.894 1.927
11 1.901 1.914
12 1.894 1.939
13 1.901 1.933
14 1.903 1.925
Minimum 1.891 1.906
Average 1.900 1.924
Maximum 1.935 1.939

Pushing 20 mA through the five lowest voltage red LEDs requires 9.54 V. Applying that voltage to the five highest red LEDs produces 18.2 mA.

Putting those two strings-of-five in parallel with 9.52 V produces 40 mA total: 16.6 mA in the low string and 19.9 mA in the high string, all measured with a fancy Tek Hall effect probe. No, those aren’t reversed and, yes, I did check twice: it makes no sense at all.

Temperature matters¬†a lot in such measurements and I wasn’t controlling for that, plus I didn’t have a constant-current supply. Better numbers await better instrumentation, but I think binning a couple bags of 100 LEDs based on forward current should be straightforward.

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