No EE background. Suggest a smoothing cap?

Discussion in 'The Projects Forum' started by frascati, Aug 17, 2013.

  1. frascati

    Thread Starter Member

    Jun 1, 2010
    28
    0
    Nominal 6v magneto trail bike system. Measured 4vac ~ 16vac. 40w max output. Unrectified, unregulated.

    Considering these LEDs for headlamp duty
    http://www.google.com/url?sa=t&rct=...S39zYl3UX0vqC1HNg&sig2=L5P81vaIAZnRDZXBDFlmxQ
    XMLBWT-00-0000-000HT20E7
    XMLBWT-00-0000-000LT40e4
    XMLBWT-00-0000-000LT50e4
    XMLBWT-00-0000-000HT30E7

    This full wave rectifier
    http://www.digikey.com/product-sear...ng=en&site=us&keywords=GBU10A-BPMS-ND&x=0&y=0

    This DC/DC regulator
    http://www.digikey.com/product-sear...g=en&site=us&keywords=NE12S0A0V06PNFA&x=0&y=0

    I've tried to decipher smoothing cap formulae on line but it's over my head. Please suggest an appropriate DigiKey part number?

    Thanks so much.

    Frascati
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,420
    3,355
    Why do you think you need a smoothing cap?
     
  3. RichardO

    Well-Known Member

    May 4, 2013
    1,230
    382
    You can't power an LED directly from a generator. You have to have something to limit the current to the LED. In low power LED circuits this is normally just a resistor.

    In high power applications, you need a switching regulator to get any efficiency. If this is a bicycle light then you have to remember that any energy lost is supplied by your legs.

    Other requirements that you must consider are:
    Higher currents do not produce proportionately higher brightness.
    The LED brightness decreases with temperature.
    The LED will need lots of heat sinking to keep it from getting hot.
    Optics are important to get the light where it is needed.
    The optics are not 100% efficient.

    In practice, I needed about 15 watts of input power to get 450 lumens which is about the same as as 30 watt incandescent light bulb.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Don't try to change from an incandescent to an LED; your electrical system isn't set up for that. It works fine as it came from the manufacturer; keep it that way.
     
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