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Low voltage AC ( Schmidt hub ) to constant current ( luxeon star )

Discussion in 'Electronic Design' started by [email protected], Sep 22, 2005.

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  1. Guest

    I have a schmidt hub generator and I would like to use it to power a 3W
    Luxeon Star, if possible two of them.

    The Schmidt hub generator is a AC generator that is built into the
    centre of a front bicycle wheel. As the bike moves power is made. It
    is very efficent and quiet.
    The schmidt hub generator is a constant current source ( approx 500ma )
    but it varies in voltage ( 6v to over 30v ) as speed increases.

    The Luxeon LEDs are high powered versions of LEDs. To operate
    efficently the 3W version requires 700ma-1000ma. It will light with
    less current but it is not as bright. The voltage required is approx 3
    to 4 volts.
    The reasons these LEDs are better than standard halogen light bulbs are
    that slightly more lumens per watt are made and that they have a longer
    life..

    A couple of people I have talked to have directly connected two 3W
    Luxeon Star to either end of the AC supply so that one is on at a time
    switching on and off very fast as the wheel spins. I think the LEDs
    are in series with another diode to protect them from reverse voltage.
    This approach does not give a bright light as the current is too low.

    I have tried connecting a LED dynamics 03021-A-E-700 puck regulator
    across the generator and using this to drive a single 3W Luxeon Star.
    The light was not too bright until I got above 20kph. Then it was a
    good brightness for about 10 seconds. After this the puck regulator
    exploded. Presumably it was getting too much voltage.

    What simple approach do you think would work for converting the
    variable frequency and voltage of the generator to a constant current
    in a suitable range to power the LED?
    Would a puck regulator work, if I capped the voltage with a zener diode
    or two?
     
  2. It doesn't go to 30V unless it's unloaded. Loaded with the designed-for
    6V/3W lightbulbs it only goes to 7 or 8V.


    Jasper
     
  3. Ed

    Ed Guest

    a cheap ordinary diode bridge would double the current (average,
    not peak) improving brightness with little impact to load
    terminal voltage (min speed to light it). It won't be full
    brightness, but much closer.

    Putting two Luxeon LEDs in series at one bridge output should
    light them both as bright as with one, but you'd need more speed
    to light them both fully.

    BTW, data on the page:
    http://www.leddynamics.com/LuxDrive/datasheets/3021-BuckPuck.pdf
    suggests that you were pushing it pretty hard. That 30VAC you
    quote for the generator (vs 24vac rating) is prob about 42V peak.

    Ed
     
  4. Ed

    Ed Guest

    a cheap ordinary diode bridge would double the current (average,
    not peak) improving brightness with little impact to load
    terminal voltage (min speed to light it). It won't be full
    brightness, but much closer.

    Putting two Luxeon LEDs in series at one bridge output should
    light them both as bright as with one, but you'd need more speed
    to light them both fully.

    BTW, data on the page:
    http://www.leddynamics.com/LuxDrive/datasheets/3021-BuckPuck.pdf
    suggests that you were pushing it pretty hard. That 30VAC you
    quote for the generator (vs 24vac rating) is prob about 42V peak.

    Ed
     
  5. <http://www.enhydralutris.de/Fahrrad/LEDWerfer0402/BaronTD.gif>

    Guenther - KISS
     
  6. Jim Adney

    Jim Adney Guest

    I would embed your LED light in the middle of a Schottky diode bridge,
    so that it gets forward biased regardless of which way the alternator
    output is going.
    You say that the Schmidt generator is a constant current device, with
    a current limit less than what your LED lamp requires. If this is
    true, you don't need any kind of regulator, the generator is doing it
    for you.

    I don't have first hand knowledge of these items; I'm just taking your
    word for their characteristics.

    -
     
  7. 22 Sep 2005 03:32:42 -0700, :
    Wrong. 3W-Luxeons work most efficent at about 10-50mA. Efficiency
    decreases significantly with increase of current - especially if cooling
    is less than perfect.
    The difference is not as high as you would expect: Try it out!

    Andreas
     
  8. I suppose that you would need these four things series connected:

    1) The hub generator, which is essentially an AC voltage source.

    2) A switch to turn the light ON/OFF

    3) A choke. I suppose that an iron powder would be ok.
    I can't give the exact design parameters, because I don't
    know all the components, especially the generator.
    The choke (an inductance) is needed to limit current
    at high speeds (higher frequencies) so that you don't
    accidentally destroy the LED.

    4) Two (2) parallel LEDs connected opposing directions.

    You could build the circuit and test it first with low speeds.
    Use a multimeter to measure current while on the test ride.
    If the inductance is not big enough, increase the number of
    winding turns. Some iterations may be needed, but as you see,
    active components are not absolutely necessary.

    Disclaimer: I haven't tried this circuit by myself, so I don't
    know how well (or badly) it works.

    Nikolas Ojala
     
  9. No, it's an AC current source. As he says.
    Bike generators, including the SON, usually have a built-in choke, they
    saturate the core. That way the voltage runs from about 4 to 7 volt going
    from 7 mph to essentially infinity.


    Jasper
     
  10. Guest

    Assuming, that is, it's got a load with the proper resistance (or
    impedance).

    Standard load is a lamp (or pair of lamps) designed to have 12 ohms
    resistance when carrying the standard current of 0.5 amp. That's your
    6 volts, 3 watts. If you present the generator with, say, 24 ohms
    resistance, and spin it fast enough (say, 14 mph?) it still puts out
    very close to 0.5 amp, but generates 12 volts to do so. This is how I
    run the second headlight on my generator set, in series with the first.

    If you give it essentially infinite resistance (an open circuit) a
    generator can produce high voltages indeed! How much, exactly, depends
    on the design of the generator.

    - Frank Krygowski
     
  11. dvt

    dvt Guest

    Let me see if I have this straight. You have two headlights connected in
    series. Neither headlight gets bright until somewhere around 14 mph. Is
    that right? Or do you have something more complex so the first light
    gets bright at slow speeds, while the second kicks in at higher speeds?

    I know the 14 mph number is just a wild guess, but I used it to simplify
    my question.
     
  12. Guest

    I've got a simple on-off (SPST) switch in parallel with the second
    headlight. When closed, it shorts out that headlight to take it out of
    the circuit. So the generator sees only one bulb at low speed, and
    lights it up perfectly.

    I leave the switch closed almost all the time. Opening the switch
    allows current to flow to the second light, a sort of "high beam" that
    I use mostly on descents - and, frankly, just because I like playing
    with it. ;-)

    BTW, I plan to replace the switch with a three position, "center off"
    selector switch. Each side will be in parallel with one of the
    headlights. This will allow me to select the left headlight, the right
    one, or (at high enough speed) both lights. It will also give me a
    quick solution if I should blow one bulb.

    The setup I use now works well with my Soubitez roller (or bottom
    bracket) generator. It would also work with a hub generator, and (I
    believe) with the FER 2002 spoke driven unit. From what I hear, it
    wouldn't work with most bottle generators. Their smaller roller
    diameters require more tangential force to turn, and they'd slip under
    the increased load of the two lights.

    - Frank Krygowski
     
  13. dvt

    dvt Guest

    OK, I got it now. I know you're a bit of a simpler is better kinda guy,
    and I didn't imagine you had some electronic circuit switching on the
    second headlight when the generator was up to speed.
     
  14. Guest

    Not that I haven't thought about it! But it was hard to see how that
    would beat a switch.

    - Frank Krygowski
     
  15. The normal arrangement is that the secondary headlight has a switch on it
    that makes a short across its terminals; dynamo headlights sold as
    secondary have a fitted switch with this effect. For example I've got for
    our tandem a 2.4W Lumotec Oval Plus as a primary and a Schmidt E6-Z as a
    secondary; the Oval's wider beam is slightly more useful for low speed
    maneuvering and the E6's tight beam more useful at high speeds.

    In practice the two-light speed is around 10-12mph such that you only need
    to switch the secondary off to climb steep hills.
     
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