Archive for August 13th, 2013

Series-Parallel LED Current

The collection of LEDs that I’ve been abusing with 100 mA pulses at 20% duty cycle got lined up in parallel, with three LEDs in series, driven from a bench power supply set to limit the current to about 180 mA:

Series-parallel LED test fixture

Series-parallel LED test fixture

I sweetened the mix by adding a few other LEDs that had served their time in hell, then took some data by clipping the Tek Hall effect current probe around each of the white wires in turn:

Color LED1 LED2 LED3 Divisions Current – mA Total voltage
Red 1.98 1.98 1.94 4.7 23.5 5.90
Red 1.95 1.95 2.00 3.4 17.0 5.90
Red 1.97 1.97 1.97 5.1 25.5 5.91
Yellow 1.97 1.97 1.96 4.5 22.5 5.90
Yellow 1.97 1.96 1.97 4.6 23.0 5.90
Red 1.98 1.98 1.94 4.6 23.0 5.90
Red 1.95 1.95 2.00 3.3 16.5 5.90
Red (new!) 1.99 1.96 1.94 6.4 32.0 5.89

Despite the decimals, don’t trust anything beyond the first two digits.

The LEDs started out with 6.03 V across them at that current, then settled down to 5.92 V after a few minutes of warming up.

The “new” Red string replaces a trio of old LEDs incinerated by a DVM probe fumble. They have a much higher current at the same voltage; the older LEDs have been abused enough to pass a lower current.

As an experiment, I swapped the LED with the 2.00 V drop in the string with the lowest current (line 7) and the LED with the 1.94 V drop in a string with higher current (line 6), only to find that the current followed the LEDs. Evidently, those LEDs were the limiting factor, even though their forward drops weren’t the same in their new strings.

So it seems binning based on forward drop doesn’t help much. Perhaps just line up a bunch of three-LED strings (forcing all of them to see the same forward drop), measure their current, and reject the highest and lowest strings to get a decent match among the remainder?


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