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LED from dynamo

Discussion in 'Electronic Basics' started by lance house, Sep 7, 2004.

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  1. lance house

    lance house Guest

    Hi,

    Ive just got a front-hub dynamo for my bike. I want to hook up some
    white leds on the front and some reds on the back, preverably with a
    battery as well which gets charged from the dynamo output as well as
    protecting the leds when the voltage gets to high.


    I read an article on a bike website
    http://www.sheldonbrown.com/dynohubs.html

    which suggests a bridge rectifier to make it DC and then charge a
    battery from there.


    At general cruising speed it outputs 8-12V DC (via rectifier) and I
    reckon down a big hill I could easily top 20V. My multimeter only goes
    to 200mA and it overloads it going pretty slow. I blew a standard bike
    front led flasher hoing pretty slowly but the dynamo only gets a faint
    glimmer out of a 2.5v0.5A halogen bulb.

    So what do I need to do to be able to protect the LEDS from blowing
    while still putting out a reasonable amount of light at lower speeds?

    cheers

    Lance
     
  2. Gene

    Gene Guest

    First of all, bicycles do not use dynamos but alternators. There are many
    ways to solve the problem once the AC from the alternator has been converted
    into a filtered DC. These are the most simple ones:

    1. Put in series to the series combination "resistor - LED" a voltage
    limiter. It can be build using a simple Zener diode, resistor and a NPN BJT
    power transistor.
    2. LED's are actually current devices; as such they "like" to operate in
    constant current mode, therefore a current source could be good solution.
    You can google and find how to build a current source.
    3. Use a voltage regulator

    It is evident that the "surplus" power generated when you are going
    downhill, for example, will be "wasted" in the devices that are limiting the
    voltage, therefore they must be able to dissipate the heat with a suitable
    hitsink. To avoid this issue, more sophisticated circuitry is required.

    Gene
     
  3. You are dealing with an 6 V AC source and a 2 V DC consumer. The easiest
    way to deal with that situation is to use 6 superbright red LED as
    follows:

    6 V ~ o----->|--->|--->|--------o Gnd
    | |
    ---|<---|<---|<----


    This simple circuit served me well for several winters. Because each LED
    carries current only during half the cycle, the slight overvoltage does
    not cause any problems, and no current limiting is necessary. The light
    is much brighter than a standard light bulb, and lasts much, much longer
    (I lost mine after several years of operation due to theft of the bike).

    For the front light I still prefere halogen bulbs, but in principle a
    similar circuit with 2 instead of 3 LED in each branch should also work.

    Note however the legal situation in your place: You may be required to
    carry an approved light source on your bike and to operate it while
    riding in the dark. Thus such homebrew construction, although better
    than anything you can buy, may be technically illegal.
     
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