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Multiple LED intensity question

Discussion in 'Electronic Design' started by Vlad, Aug 5, 2007.

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  1. Vlad

    Vlad Guest

  2. Guest

    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.
     
  3. Vlad

    Vlad Guest

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

    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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