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Generator for LEDs

Discussion in 'Electronic Basics' started by Ryan, Oct 20, 2005.

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  1. Ryan

    Ryan Guest

    For a few weeks I have been pondering a scheme to rig up a lot of LED
    running lights on my bicycle frame for night time illumination.

    At first I was going to use a 9v and then sets of 3-4 in series with a
    resistor, but as the voltage of the battery drops, the current seems to
    drop so much that I don't know what effect it has on luminosity.

    I have no formal electronics education, it's all been from doing and
    reading.

    I recently purchased a cheap shake-up LED flashlight and I've seen
    advertised the crank kind.

    It seems I should be able to fashion a generator out of the wheel using
    magnets on the spokes, or else pull a small motor out of something and
    use that. As I understand from the archives, I need to find a motor
    with a magnet rather than an electric field. Would a hard drive magnet
    work for the spokes?

    If I cause a DC motor to turn, does it output DC power, or AC power? I
    thought since a magnet moves first toward, and then away from the
    windings (or vise-versa) then this creates first a negative and then
    positive voltage; a sine wave rather. Is this correct?

    Can the magnets pass next to those windings or do they have to pass
    through the middle of it?

    Do those flashlights use a special capacitor? I notice it still has
    power even several days later. I would expect a capacitor to be
    drained?

    So, I envsion a battery-free setup with a dozen or so LEDs fashioned
    onto the frame (very minimal hardware and very low budget) and one or
    more capacitors to keep supplying the current when I have to stop.

    If the magnet idea isn't feasible, then I want to make a generator, but
    not the impossible to pedal tire eaters I remember from the 1980s. I
    used to spin motors from capsela toys and watch them make power.

    If a DC motor consumes X watts at its RPM, does it produce approximately
    that many watts if made to spin at that same RPM? I presume I need to
    determine my current demand which I think will be 20mA times the number
    of LEDs and then find a motor to accomodate this. Is a motor's output
    voltage dependent on its RPM or is it steady? Would a floppy drive,
    CD-ROM, or hard drive have a suitable motor? What about trimming off
    the blades of a computer case fan and using that as a motor, or is there
    something in an old VCR I could use?

    It seems so difficult to design this circuit because voltage can be so
    dynamic and from the LED calculators I've used, this has a dramatic
    effect on the current the LED uses. I don't want to burn them up, but
    I want them to be bright.

    I don't know much about regulation, but if that is the answer, should I
    regulate voltage, or current?

    As I understand, making them flash necessitates a transistor or two,
    which is probably beyond me and probably beyond the scope of this
    project.

    I appreciate any pointers you can offer.

    -Ryan
     
  2. You might well build a simple AC generator that uses a magnet (or
    several) mounted on the wheel, with a pickup cpil mounted on the
    frame. The pickup coil needs to be designed to have an iron core
    (made of laminations from a transformer or ferrite, to keep eddy
    currents low) such that it nearly bridges across the poles of the
    magnet as it goes by. This will drive a big pulse of flux through the
    core and generate a significant voltage across any coil wound around
    that flux. You should be able to connect pairs of LEDs in anti
    parallel, so one lights when the current goes one way, and the other
    lights when the current rebounds the other way. How many of these
    pairs you can connect in series depends on the number of turns on the
    coil, and how strong the magnets are.
     
  3. Ryan

    Ryan Guest

    You might well build a simple AC generator that uses a magnet (or

    Thanks John, you are always very helpful in this group.

    Can this be any old magnet, or does it need to be made of anything in
    particular? How close must it pass to the coil? I presume the
    magnetic intensity influences voltage?

    Do I want to use tiny wire for the coil so that I can make more
    windings? Could I rip thin wire like what is found on the back of a
    speaker cone and make a coil from that? Could I wrap it around an ink
    pen body or what? Does the radius of this coil matter?

    When you say a core of ferrite, is this the stuff you find on the end of
    a monitor data cord? Should I wind around one of those or perhaps
    through one of those?

    If I pull a transformer out of something, does this serve as two pickup
    coils already?




    If I power them with AC current, does the resistor become less important
    since it is effectively pulsed width duty cycle? I presume I could also
    add two capacitors in the same fashion to store energy for each
    "direction" of current?




    I presume more turns equal more voltage, not unlike the rule of a
    transformer?


    Is the magnetic field "falloff" curve for any object the same? For
    example, can one type of metal attract from a farther distance than
    another type of metal given that they have the same measured amount of
    magnetism?


    -Ryan
     
  4. The more powerful the magnet, the larger the output. The (open
    circuit) voltage is proportional to he rate of change of flux passing
    through the coil, and also proportional to the number of turns.
    As I said, the coil should be wound around a good core material, like
    laminations recovered from a small transformer. Wire can also be
    recovered from a transformer (or wall wart power supply or relay). In
    fact, a relay with the coil on a bobbin may be all you need.

    The idea is to have the magnet go past the core so that the core
    almost touches both poles of the magnet, so it intercepts a large
    fraction of the magnets flux. If you have access to old lawn mowers,
    you may be able to adapt the magneto coil that is mounted near the fly
    wheel.
    If you can think of a way to have this core bridge the passing poles
    of the magnets, it would work.
    No. A transformer has the coils wound around the center lag of a
    closed magnetic path.

    If you can slide the E and I laminations out of the coil, and then
    reassemble all the E's through the coil from one side, you end up with
    a magnetic structure with 3 poles (a big center one with the coils
    around it, and a smaller pole on each side). This structure can be
    used to intercept the flux from a passing magnet as it slides past the
    three poles.
    The impedance of the coil limits the current. Whether this keeps the
    current low enough to not damage the LEDs depends on the number of
    turns and the strength of the magnets. Some experimentation is called
    for. With a direct connection, you will get a flash each time a
    magnet goes past. if you rectify the pulses and store them in a
    capacitor, then you can dump power continuously through a string of
    LEDs and a series resistor. If you use the DC to charge a small
    rechargeable battery, you can have a glow, even when you stop.
    Yes, same concept. But more voltage implys less current, so you can
    limit the current to a safe value (and drive more LEDs in series) by
    raising the voltage.
    The distance is related to the strength of the field produced by the
    magnet, the distance between the poles, and the permeability of the
    material being attracted.

    Larger magnets (bigger distance between the poles) produce larger flux
    patterns. Really good magnets would be neodymium iron boron types.
    These are sold in great variety on Ebay. But they are powerful enough
    that larger ones will collect scrap metal as you ride.
     
  5. Jasen Betts

    Jasen Betts Guest

    they used to make generators for that purpose that were driven by a friction
    wheel of the tyre.
    DC, kind of noisy DC, but DC.
    yes, but the contacts in the motor reverse that sine wave where it crosses 0V
    through the middle is best. next to will work to a lesser extent.
    possibly a rechargable battery?
    no, at best about 60% of the amount it consumes. probably much less.

    I presume I need to
    it depends on the RPM.
    All those motors have electronic switching instead of the comutator that the
    toy motors you experimented with had. you'd need to remove the switching
    circuit and replace it with a rectifier bridge.
    use the generator to charge the capacitor and use a regulator to control the
    putput.
    ideally you should hook the diodes up in series and reguulate current
    this may require boosting the voltage..
    voltage boosting regulators are more complex still.


    A good motor to use could be the head motor from a 5.25" floppy drive
    they're nice and big (so potentially have a good output) and give 40 pulses
    per rotation. you could probably drive one fast enough to light LEDs with a
    using a cog that engages the spokes near the hub...


    FWIW the turntable motor from a microwave oven can produce hundereds
    of volts when turned by hand... if you could somehow drive one of them
    from an axle you could have CCFL lighting on your bike :)

    Bye.
    Jasen
     
  6. Jasen Betts

    Jasen Betts Guest

    Around, could work through won't

    the idea is to make a path from the north to the south pole of the magnet
    out of metal (or ferrite etc) and have the path pass through the turns of
    copper then move the magnet round...
    If you can cut one of then ends off it it would work, some tranformers come
    apart quite easily.
    no, you can still burn them out with AC if you feed them enough...
    yes.


    Bye.
    Jasen
     
  7. Andy Baxter

    Andy Baxter Guest

    Ryan said:
    A current regulator isn't that hard to build - see my earlier thread on
    luxeon LEDs for various circuits you might be able to adapt.
    For LEDs, you want to regulate current. Using a resistor in series is a
    trick people use when they know they have a stable fixed voltage source.
     
  8. quietguy

    quietguy Guest

    You might like to have a look at Silicon Chip Feb 2004 - they describe a few
    human powered torches using small stepper motors to power led torches - might
    find some useful ideas there

    It would seem not too hard to rig a motor so it is driven off a bike wheel
    instead of being hand cranked

    David
     
  9. Mark Healey

    Mark Healey Guest

    I've been pondering the same thing. That's what brought me to this group.
    I initially planed on using a bunch of hard drive magnets glued around the
    rim but then remembered the brakes. I also decided that it would be best
    to have the rig near the hub so it doesn't interfere with removing the
    wheel as much.
    I took one apart and it's not a stepper, it's just a regular DC motor.
    This is what gave me the idea. I actually plan to cannibalize the
    circuitry of the wind up flashlight and just turn my wheel into a
    motor/generator. The cool thing is that my flashlight has a rectifier
    already in it.
    Actually it is. They make generators that rub against the tire but drag a
    lot. They also make hubs with generators in them but they cost a lot.

    You don't want to drive anything with the spokes, that will bend them
    eventually. There is no room between the hub and the fork ends to attach
    anything.
     
  10. Don Bruder

    Don Bruder Guest

    I've been pondering the same thing. That's what brought me to this group.
    I initially planed on using a bunch of hard drive magnets glued around the
    rim but then remembered the brakes. I also decided that it would be best
    to have the rig near the hub so it doesn't interfere with removing the
    wheel as much.
    I took one apart and it's not a stepper, it's just a regular DC motor.
    This is what gave me the idea. I actually plan to cannibalize the
    circuitry of the wind up flashlight and just turn my wheel into a
    motor/generator. The cool thing is that my flashlight has a rectifier
    already in it.
    Actually it is. They make generators that rub against the tire but drag a
    lot. They also make hubs with generators in them but they cost a lot.

    You don't want to drive anything with the spokes, that will bend them
    eventually. There is no room between the hub and the fork ends to attach
    anything.[/QUOTE]

    Wind a couple "pancake" coils to mount on the forks, then clip magnets
    to the spokes, lined up with the coils. Rectify the coil output. Presto!
    :)
     
  11. Mark Healey

    Mark Healey Guest

    Wind a couple "pancake" coils to mount on the forks, then clip magnets
    to the spokes, lined up with the coils.[/QUOTE]

    I'm looking for something that will have the coils in it that I can
    cannibalize. It's just more satisfying that way.
    Why? The flashlight circuitry already has a rectifier.
     
  12. Don Bruder

    Don Bruder Guest

    My bad - didn't notice that part.
     
  13. Mark Healey

    Mark Healey Guest

    Actually I was hoping someone would know what I could cannibalize the coil
    from.
     
  14. Don Bruder

    Don Bruder Guest


    Old AM radio loopsticks with the ferrites removed, perhaps? Then again,
    those might be too fragile to be useful for any significant current
    without cooking themselves.
     
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