Multiple LED intensity question

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

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

Reply to
Vlad
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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.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

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

Reply to
Vlad

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

Reply to
adshea

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