LEDs

rr

Hello,
·
I use a lot of leds in my work. Usually·I run parallel strings of series groupings. I use 12 volts to power 3 leds in series and the appropriate resister depending on the led color and type of transistor (2n2222, uln2803, and tip120 depending on the needed current). This setup works pretty well for me, but I have been noticing a drop-off in current going to the leds. It seems that as I add more series groupings the leds start to draw less current. Last time I measured, it seemed like it was dropping about .6ma for each addition series grouping I added.
·
My questions are:
Is this supposed to happen and if so is there a way to measure it?



Here is a sample circuit...



Thanks for the help
··

Phil Pilgrim (PhiPi)

Your circuit shows a fixed-voltage supply and a ground. In such a case, you'd be able to parallel as many additional strings as you wanted, and the individual currents would remain undiminished. Real life is different, though. Your power supply will have its own internal resistance, which will be like a small resistor in series with your "+12V" terminal. Also the voltage drop through your transistor driver will increase as you add more load. The net result will be a drop in the current through each string, since the string, as a whole, is excited with less voltage. Assuming you have a robust enough power supply (i.e. not an unregulated wall transformer or string of AA cells), a logic MOSFET with an "on" resitance in the milliohm region will give you the most stable characteristics in the face of additional loading.

-Phil

Desy2820

What are you using to power the LED strings?· It sounds like your power supply may have reached it's limits.....

And as Mike pointed out, how many LEDs is your transistor driving?· Try splitting up the up the LED strings between multiple drivers.

Post Edited (Desy2820) : 11/18/2007 3:11:08 AM GMT