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LED resistor calculation

Discussion in 'LEDs and Optoelectronics' started by Tezza1, Jun 18, 2014.

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  1. Tezza1


    Dec 9, 2011
    Hi there Guy's:
    I want to run a 3v Bright blue LED from a 3xAA battery pack = (4.5v).
    The vf is 3.2v - 3.8v
    forward current 30MA.
    I know how to do the calculation but am a little unsure about which of the two vf voltages to use in the calculation!.
    Would I use the (3.2v) the (3.8v) or a voltage in between.
    By using the vf (3.2v) the resistor needed would be (47ohm) using the vf (3.8v) the resistor would be (27ohm)? any help would be very much appreciated.

    I don't need another sleepless night HaHa.

    Thanks in Advance
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    I'd use an intermediate Vf. This will balance lifetime vs. brightness. If you want to control the current very tightly, use a current source, not a resistor.
    KrisBlueNZ likes this.
  4. Tezza1


    Dec 9, 2011
    Thank you all:

    A big thank you to everyone how took the time to help me out with my query' I'm quite confident now of how to do the correct calculation.
    Guy's like you make this a great worth while site for us novices.

    (No more sleepless nights)
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