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Multiple LED intensity question

Which one of these three solutions is the best for the best light intensity?

http://img329.imageshack.us/img329/5950/solucijenw2.jpg

More info:
source voltage - 12V
diode forward voltage - 3.7V
diode forward current - 20mA
number of LEDs - 23 (blue, 470nm)

Provided that the current through the LEDs is the same, all three
solutions will give the same light intensity.

The first one wastes most power - 2/3 of the power is dissipated in
the series resistor, and only 1/3 in the LEDs.

The third solution probably won't give a particularly equal
distribution of current through the LEDs - three nominally 3.7V LEDs
in series gives a total nominal voltage drop of 11.1V.

Since I don't know which blue LED you have in mind, I don't know the
tolerance on the nominally 3.7V forward voltage drop or the
temperature coefficient of the voltage drop, but I do know that
current through the different strings of three LEDs won't be
particulary close to 20mA, and the total light output will probably be
lower than you will get from the second and third solutions.

Hope this helps.
 
V

Vlad

Yes, thank you. I also talked to some people and find that second solution
should be the best. Third one is a bit sensitive.
 
Yes, thank you. I also talked to some people and find that second solution
should be the best. Third one is a bit sensitive.

Have you looked at getting one of the many chips designed for driving
LED's? I know TI, National, and Maxim make several varaieties, and you
can usually get them from Digikey for about the cost of that pile of
resistors. They have much higher efficiency (>80%) and even give
options like PWM dimming. Also, since they are current controlled,
they can give you the greatest brightness without having to worry
about toasting your LEDs

--Adam Shea
 
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