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Powering x-opto white LEDs

Discussion in 'Electronic Design' started by Alex Bird, Oct 28, 2004.

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  1. Alex Bird

    Alex Bird Guest

    I'm making some LED lights for illumination as part of some costumes,
    in fact I have to put these things together tomorrow!

    In my prototype I used some ultra-bright reds I had lying around, and
    my series resistance calculations were fine. The device is simply a
    D-type flip-flop, reed switch and npn transistor, enabling the light
    to be switched on and off by waving it near a magnet.

    For the real things I ordered these:

    Made by X-opto. Who they?? I cannot find any online presence.

    Lens Material V F max V R max Chromaticity [email protected] View angle
    water clear GaN 4.2V 5V X=0.31, Y=0.32 3000 15°

    As the intensity is quoted for 20mA, I factored in the forward drop of
    0.7V per LED and calculated the resistance. Nothing. I Checked all
    my cables, measured voltages.
    Eventually I tried upping the current, and finally with 5V connected
    directly across them they are putting out what looks like 3000mcd to

    Am I right in thinking that I can do this because VFmax is 4.2V, and
    4.2 + 0.7 = about 5? Am I wrong to assume a forward drop of 0.7V?
    Does anyone know anything about these LEDs?

    Thanks for any help,
  2. Tim Wescott

    Tim Wescott Guest

    LEDs have a different forward drop from a silicon diode -- red ones can
    range from 1.5V on up. In your case I think you need to use the 4.2V
    instead of 0.7V in your calculations.
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