> I mean using fixed voltage not current.

Fixed current is correct.
LED voltage will vary between LEDs.
Voltage varies non linearly and semi exponentially over a small range
for very large variations in current.
So. increase voltage a little and current may go from OK to well over ratin=
gs.

Controlling the current allows the LEDs to function at a designed
operating point.
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.