The criteria I had when laying out my circuit was that the lights would
operate down to 10V and still not exceed the max current limit at 15V.
I'm working from memory here, but I believe the SuperBright LEDs I used
had a forward voltage of 3.5V.

Putting 2 in series with a 220ohm, 1/4W resistor maximized the
efficiency while maintaining the range I wanted.


Okay, thats not too bad because your using a resistor to cover HALF the voltage
drop....of course your losing half your power in the resistor rather than
having it produce light in the LED...

My previous post was more concerned with folks both running the LED at absolute
max power ratings,,,,and trying to use the LED voltage drop to cover something
like 80 percent or more voltage range...thats when you have to be a little more
careful...

Your 1/4 watt resistor is NOT high powered enough....youll be running it close
to or past its maximum power rating for normal voltages....

Get something more like 1/2 watt or 1 watt resistors....or use 2,3,or or even 4
of the 1/4 watt resistors with appropriate resistance values in series to
spread the power load....resistors are cheap and there AINT no harm in using
ones that can handle significantly more power than expected....while the risk
to benefit ratio of trying to push their limits seems rather high....

Also make sure the resistors can cool effectively.....their power ratings dont
mean diddly if they are well insulated or packed into a tight space and can
warm up...

take care

Blll