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Powering x-opto white leds

Discussion in 'Beginner Electronics' started by Alex Bird, Oct 27, 2004.

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  1. Alex Bird

    Alex Bird Guest

    Hi,
    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:

    http://www.rapidelectronics.co.uk/r...CAT_CODE=30319&STK_PROD_CODE=M61376&XPAGENO=2

    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
    me!

    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,
    Alex
     
  2. You are wrong to assume a forward drop of 0.7V. Different types of LEDs
    have a different forward drop. Red LEDs usually have a 1.4V drop,
    whereas white LEDs have a 3.6V drop. The current goes up 10 times for
    each 60mV the forward voltage goes up just like normal diodes.

    The typical forward voltage for GaN white LEDs is about 3.6V. Taking
    them up above that will cause them to draw more current. It sounds like
    you have a multimeter, so measure the current through the thing as you
    vary the voltage. It should be about 20mA at 3.6V. If you take it up
    higher than that, you risk destroying the LEDs. They will get brighter,
    but their life will decrease.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
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