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12 volt Bicycle Generator : discerning polarities for hooking up loads

Discussion in 'Electronic Basics' started by greenwanderer108, Oct 24, 2005.

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  1. I rigged them on my bike to test the actual output but can't get any rea
    readings (400 milivolts max), let a lone enough power for even th
    smallest loads (i.e. 3volt toy motor). Either I got ripped off or I jus
    am not hooking them up right. I think its the latter.

    At the bike shop, they had two different kinds available. I believ
    they're both made in Taiwan. One of them had only one terminal and tha
    was rated 6 volts. The type that I bought (quantity = two) aparently ar
    rated between 12 - 16 volts and have two terminals. One reads T and th
    other H. The salesguy explained it stands for Taillight and headlight. S
    they're made for lights.

    Anyways, since they are rated atleast 12 volts, I figured it would b
    sufficient to use for other applications such as a charger for the powe
    pack in my electric bicylcle set up.

    I initially understood that generators are just like motors with th
    electromagnetism (two polarities) so how is it that the 6 volt generato
    can have only one terminal.

    And what about these two terminals for head and tail light. Obviously the
    aren't the equivlent of positive and negative leads, cause I can't get an
    power or readings from my digital multimeter (or anologue voltmeter) wit
    the the leads hooked up to positive and negative . I remember the guy a
    the bike shop saying something about the both H and T leads bein
    positive, and and that the negative lead of the light (or other load) mus
    be grounded onto the generator. I tried, but still couldn't get any acurat
    power--or readings with the thing spinning as fast as I could get it.

    How do negative and positive polarities play in with such generators? Wha
    am I missing? If it's rated at 12 volts, why can't I get any more than 40
    milivolts. Do I have to ground the load some how or ??? I am lost.

    If it was like a motor with negative and positive leads, it would be s
    much easier . Can someone please help me out here? What if I want t
    combine the power of both of the generators in series or paralell?

    Thanks in advance

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  2. Peter Howard

    Peter Howard Guest

    Those bicycle generators are AC devices. Try switching your multimeter to AC
    Volts position and see what you see. Unlike a toy DC motor they do not have
    brushes or a commutator on rotating coils. They have fixed coils
    surrounding a rotating magnet so they produce alternating current as first
    the north pole then the south pole sweeps past the coil. And yes, the bike
    frame is one leg of the circuit. May find that clamp that holds the dynamo
    on the fork leg has a set screw that pierces the paintwork to make good
    contact with metal of frame. The H and T terminals on one of your generators
    may go to separate coils of different sizes so head has more power to it
    than tail. Or they may both go to same coil and two terms is just for
    convenience of hooking up. Try measuring resistance from each term to
    generator case using your digital multimeter. Then measure resistance
    between each terminal. If res between terms is just about zero they're
    connected internally to top of same coil. If they're connected to two
    separate coils the res will be the sum of term H to ground plus Term T to

    Here's a link to just one page of some interesting reading.

    However, bicycle gens are low powered devices, rated at about three watts so
    the likelihood of doing any significant battery charging with one is slim.

  3. One terminal is the case of the generator. It must be solidly
    connected to the bike frame so the frame can be used as part of the
    Probably two separate windings, both with one end connected to the
    case as with the single output generator.

    Have you tried the AC setting on your meter? These are almost
    certainly AC generators. Just a magnet spinning inside a stationary coil.
  4. Thanks,

    Switching the DMM to AC made a huge difference. I actually got rea
    readings. Grounding the negative terminal of the light lead to the bottl
    generator case/clip proved effective.

    Now this raises other questions for me.

    Can I wire 3 or four of these generators up in series or paralell t
    increase the power? Let's say I had three of them. If these were DC powe
    supply, I would just hook up negative to negative to negative and positiv
    to positive to positive for a paralell connection or positive to negative
    3 for series connection.

    Since these are AC devices, is series and paralell wirings possible
    effective, or? If yes, then how so?

    Since the H and T terminals are of the same polarity, I would rather us
    them both (all if wired to other generators) for one load (i.e. moto
    charging) instead of two different loads (lights) How must I go about ?

    Thanks again in advance,

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  5. No. Doing that would require that they all be run in precise
    synchronism, and that isn't practical.
    Even if they were DC generators, connecting them in series would be
    problematical, because of the case being one of the terminals. You
    would have to mount at least two of them, isolated from the frame.
    Possible in theory, but not practical.
    How do you know that H and T are the same polarity? Do you get zero
    volts AC between them with the generator running?
  6. How do you know that H and T are the same polarity? Do you get zero
    Actually, I was able to get some voltage between the two, but not quite a
    much between either of them and the casing. I'll have to measure them agai
    and write down the exacts.

    Generally for AC bottle generators (alternator to be PC)are the H and
    the same or opposite terminals?

    Maybe I'll just have to open one of them up and see which one connects t
    the casing.


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  7. Peter Howard

    Peter Howard Guest

    Don't forget that theres no such thing as free power.There must be some form
    of input energy to convert to another form of energy, and with some losses
    inherent in the conversion process. One of those bottle generators is a
    significant drag on a bicycle wheel, four of them is four times the drag.
    Not so bad if it's the wind or falling water spinning the wheel, a different
    matter if it's YOUR bursting lungs and burning leg muscles that's providing
    the input. One thing is sure, you won't get out in electricity what you put
    in as muscular power. You'll get even less efficiency if you store the
    electricity in a battery and then use it to drive an electric assist motor.
    In other words, if you pedal twenty miles with a bottle generator adding to
    the wind resistance, rolling resistance and transmission losses, you won't
    get another twenty miles of blissful and silent progress on electric motor
    power. If you're interested in electric assist, it will be much easier on
    your body to charge your battery at home from the wall socket. I'd even say
    that's also the way to go for night-time lighting.
    However, if you love to experiment, I'll say that when I was a kid I was
    never very impressed by the design or construction of those bottle
    generators. Why not look into using stepper motors salvaged from dead
    computer printers/photocopiers etc? Here's a link to get you started.
    That link was in the first three I found by Googling "stepper motor
    Just for fun, I walked into my workshop and picked up a little old printer
    stepper that's been on the workbench for two years. It took me 30 seconds
    with a multimeter to establish that its five wires consist of one common and
    four coil wires. (read the article) After that, an AC voltmeter conected
    from common to one coil wire read 6volts with just a flick of the motor
    shaft with my fingers. I then started to look with more interest at three
    BIG steppers marked 24volts that are part of a junked 1980's DEC dot matrix
    printer. That will have to wait till later.
    If you can get hold of a junked printer or two, I daresay you'll find a
    supply of steppers that are far superior in tolerances and construction to
    any Taiwanese bicycle generator.
    Again, when I was a kid, a pal of mine had a truly horrible little gasoline
    bicycle motor made by Solex in France. Its crankhaft had a grooved unglazed
    porcelain roller directly attached to it (unglazed and grooved for grip)
    This motor was mounted over the front wheel and was held off the tyre by a
    lever arrangement. You got the bicycle up to speed, flicked the lever and
    the roller dropped onto the wheel. Iif you were lucky the motor started and
    off you went in a cloud of blue fumes and noise.From my memory of the roller
    size, the reduction from roller to wheel was around 15:1. Of, course, in wet
    weather there was considerable slippage.
    Anyhow, this could be your answer to driving a stepper as a generator. Mount
    it above the wheel, about where a centre-pull brake goes, with some form of
    friction-ful roller attached to the shaft and contacting the tyre. As you
    don't have the weight of a gasoline motor holding the roller in contact, use
    s big spring or bungee cord or tie half a brick to it. All of this supposes
    that you have a few hand tools and bits and pieces of scrap metal to make
    brackets and such like.

    In the meantime, you need to establish if that bottle generator has two
    windings or just two terminals attached to one winding. Use the ohm-meter.
    If there are two windings, if you're lucky they will be opposite in phase.
    In which case you can connect the anode end of a diode to each terminal,
    twist the cathode ends of the diodes together (the ends with the ring
    printed on the diode body) and get full-wave rectified DC at the commoned up
    cathodes. 1N4004 diodes for 10c each will do for the expereiment. If there's
    only one winding in there, you will be stuck with one diode and half-wave
    rectification. That means 10VAC turns into about 4.5VDC. Not very efficient
    and only good if you have power to waste. (You only get to use half the
    waveform and you lose about 0.6V across the diode).

  8. If you are trying to make a DC supply from these generators, there is
    an easy way to make DC with two rectifiers (preferably Schottkys, for
    low forward drop) and two electrolytic capacitors. Connect a cathode
    to the output terminal, and the anode to the negative side of the
    capacitor. The positive side goes to ground (frame). Connect the
    anode of the other rectifier to the output terminal and the cathode to
    the positive side of the second capacitor. Connect the negative side
    of that capacitor to frame. This gives you two half wave rectified DC
    outputs in series for twice the voltage you would get from a bridge.

    Parallel the outputs of another generator (or another terminal from
    this one) with another two similarly connected diodes feeding the same
    two capacitors.

    Connect your loads across the ungrounded ends of the two capacitors.
  9. Jasen Betts

    Jasen Betts Guest

    If you rectify the output they can be run in parallel.
    prbably seriies too... but one of them is hard enough to
    put a layer of polythene cut from a soft-drink bottle between the
    clamp and the frame, and then rectify....

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