> Your LED datasheet will
> tell you what duty cycles and currents are acceptable, 
> although if you
> understand what you're doing you can safety exceed the 
> specs.  I've driven
> LEDs directly from 5V using a 1 MHz repetition rate  and a 
> < 1% duty cycle.
> Although the peak power was approaching 10 W the average 
> power disipation
> was <100mW.

And if you don't know what you are doing you can SEEM to be 
doing OK exceeding the specs, and get early mortality. The 
LEDs may die too :-).  Some modern LEDs have far lower 
Absolute max to average ratings than older ones typically 
did. IR LEDs in eg TV remote controls are typically driven 
at very high peak to average currents. Do that with a white 
"phosphor LED" and you may find the lifetime is vanishingly 
short.


        R

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