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12v DYNAMO driving 5v front & 10v rear LED lighting + DIAGRAM

  1. Jan 19, 2008 #1

    I'm wondering how to drive front and rear lights off one dynamo. See diagram for current setup.

    Lights are hooked up to dynamo in parallel.

    The LM7805 and LM317 are voltage regulators which drop the voltage from 12v to 5v and 10v respectively.

    Currently only the headlight works, the rear lights only work if I disconnect the fheadlight.
    Basically it seems as though the headlight is hogging the current, and I assume this is due to some kind of impedance mismatch.

    Is there something I can simply add to partition the current nicely? By 'nicely' I mean that I don't want to unnecessarily waste power.

    I'm using a 5v headlight (1w heatsinked white LED module, from ebay) because it shares the LM7805 with a USB jack that is used for recharging ipods etc. Obviously there's wastage here, dropping from 12v to 5v, but that's just what USB requires.

    Many thanks for any insight,


    Attached Files:

    Last edited: Jan 19, 2008
  2. jcsd
  3. Jan 19, 2008 #2


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    Science Advisor
    Gold Member

    Have you measured the LM317s input voltage when the front light are on? According to the specifications, it needs an input of 3 volts or greater than the output (10v). Thats 13 volts and your dynamo is 12 volts.
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