Parallel LED strings w/ constant current source

Sorry if this has been covered already; I couldn't find an exact answer in searching...

I would like to drive multiple strings of series connected LEDs (probably 3 LEDs in each string) from a constant current source (LM317). Is it safe to assume that the parallel strings will divide the current equally (i.e. 2 strings, 40 mA source, 20 mA to each string, etc...) between them? Or should I put a small resistor in series w/ each string to account for any manufacturing differences in the LEDs?

Thanks for any info/advice.

Apart from reading the sensible parts of "Why'd my LEDs pop?" under news group "aus.electronics", Id suggest strongly using small resistors, unless the cost is high, In which case you should get detailed data on the batch variations, and specs so modelling can predict chance of failure. If they are cheap, and youre only building one, then just try it without. A CRO might help examine startup currents, therefore risk of future failure. David Merrett

Constant current sources work great if each string gets its own CC source but you'll have some problems driving the two strings with one CC source because LEDs exhibit a negative temperature coefficient (resistance decreases with increasing temperature) like many other semiconductors. You can put a stabilizing resistor in each string (like the stabilizing resistors used for paralleled BJTs) but if you must to do this you might as well dispense with the CC source and calculate a limiting resistor value for each string. Without a switching CC source (efficient) your going to waste the same amount of power either way.

Dorian

"Simone Merrett"

** That means completely ignoring the bizarre crapology posted by the above half wit.

..... Phil