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  1. #1
    Cries on hills supton's Avatar
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    Bottle Generators

    I'm tempted to try one out. Maybe add a rectifier to a small battery (or large capacitor), use a more modern blinking LED rear light, and perhaps swap the headlight bulb to a replacement LED. I like the generator idea (no batteries), and I probably wouldn't be going faster than 15mph at night anyhow.

    I assume the overvoltage protection on these systems is built into the generator, right? Is it easy enough to defeat? I'd rather modify the wiring at the bulb and/or insert appriopriate circuitry, so that in over-voltage situations, I could charge a battery or run a second light (maybe the second light comes on above a certain speed). I have an electronics background, btw, just not a background in bikes or bike generators. [I like the idea of the hub generators but a) don't like the price and b) don't like the idea of building wheels just yet.]
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  2. #2
    Cries on hills supton's Avatar
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    I was in Target today, and I picked up the Schwinn generator set. I used my drill to spin the generator, unloaded, and found at first maybe 3Vrms--not fast enough. Found a sanding disc wheel, and tried that to get the speed up a bit. Found 14Vrms, no clipping as seen on the scope. I tried to take apart the generator, but it appears non-servicable. I'll hook it up on the bike and see if I can carry the voltmeter for a short jaunt, and see if the voltage is unregulated on this model. It could be that the light regulates the output, or that any load causes it to regulate properly (or close enough). I didn't see any sort of regulator in the light either.
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    afaik, these generators are unregulated. They are meant for filament bulbs, which are pretty tolerant of voltage variations. That said, if you ride fast, you will blow the bulbs. I think they pick a bulb/generator combo so that the bulb is normally run under-voltage at about 10mph. If you regularly ride faster than 15mph, you'll blow bulbs a lot.

    One way round this problem is to increase the diameter of the generator wheel. Rubber bands wrapped round it can work. Of course you then have too dim a light a low speeds.

    I'm not much of an electronics guy, but couldn't the unregulated output be fed to a capacitor? If it's actually an ac output, then a simple bridge rectifier should work, shouldn't it?

    The capacitor output could go to a current source, such as a buckpuck, maybe with a voltage-limiting resistor.

    How does that sound to you?

  4. #4
    Cries on hills supton's Avatar
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    Not far off. I was thinking of running the output into a bridge rectifier, cap and then I'm not so sure just yet. Either a buck/boost convertor and generate 13.8V; or buck/boost to 6V and a largish capacitor on in/out, for running just the 3W generator light.

    I'm still playing with this idea in my head; if I have to carry around the SLA then odds are, it will be large enough for the whole ride w/o issue. The bottle generator would be for backup instead; so maybe a 6V buck/boost system aimed at lowering losses extending bulb life. Maybe do 12V so I could run a 2 or 3 Watt halogen for an upgrade in light output over cheap 6V bubs.

    Hey, maybe find a couple of those 1W LED lights and run a couple of those ... Now that's an idea! I think those lights are pretty simple inside--probably just applying the 6V alkaline batteries through a lossy resistor to run the LED's--so perhaps I wouldn't need a buck/boost convertor as much as a decent white LED driver. Rectify the generator, largish cap, then white LED driver. Shouldn't require very good voltage regulation, have to go look at those IC's.

    More options to toss around...
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  5. #5
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    Most standard bottle generators are current limited using the inductance of the generator. Cheaper models generally are not as well designed as the more expensive bottle generators, or the hub models, and so they aren't as well regulated, and will tend to blow incandescant bulbs. When fed through a bridge rectifier to some modern high powered LED's this stops becoming a problem, as the LED's have sufficient current rating to take virtually anything the generator can throw at it, as it should always be less than 1 amp.
    Another interesting thing to note is that you probably don't need a driver board, just drive the LEDs directly. As noted there is no need for current limiting, and also the generators' output voltage will rise if you have more LED's in series, automatically. But -, the more you have in series, the higher the speed before you have a good amount of current flow to get good light. A single high powered LED with a Vf of 3.3v or so should light up at walking pace. Two in series will have a speed/light output characteristic fairly similar to the 6v incandescant light. If you have 3 or 4 in series you may not get much light output below 15kmh.
    If you do use a driver board, theres a possibility of overvoltaging the input to the board if you aren't using all of the generators' output, because the output voltage will rise in this situation, possibly above the maximum input voltage of the driver board. You could limit the input voltage with a zener diode perhaps, but IMO it's better to just go direct drive.
    Another thing you could try is to feed the AC output of the generator through a bipolar capacitor before the rectifier. This will have the effect of increasing the output current available at higher speeds, maybe up to an amp. If doing this on a bottle generator you may run into problems with slippage of the contact wheel.
    If you run a system which has batteries and a generator (through a rectifier) feeding the lights, then the generator output voltage will match the battery voltage, within reason i.e. from 0 to 15 volts. In this instance you can safely use a driver board - as long as the battery is part of the circuit the voltage will stay reasonably constant and any unused energy from the generator will charge the battery. If the lights are taking more current than the generator is supplying then the battery will slowly discharge.

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    If you are looking for an electronics project go for it, but if you want more watts for less effort a hub generator is probably the way to go.

    FWIW I was thinking of rigging a plastic gear to my spokes and connecting that via chain drive to an electric model airplane engine running it as a generator. I figure a model airplane engine must be extremely efficient and outdoor friendly. Maybe a model boat engine. Never did it. Mounting is the biggest obstacle. You might try to upgrade a cheapy bottle generator with a better engine. Gearing is also a problem as to get the volts you need 1000s of RPMs, check specs before waiting your time.
    Last edited by geo8rge; 08-03-07 at 07:13 AM.
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  7. #7
    Healthy and active twobikes's Avatar
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    I just got back from two weeks in Germany. A previous post on this forum indicated bottle generators are difficult to find in the USA. They were readily available in German stores for 6 Euros (about $8 US). A rubber cup designed to increase the diameter of the spindle slightly was also readily available.

    A Zener diode provides good regulation, but can be damaged by too much current. Also consider an LM317 variable voltage regulator. A few components will be needed connected to it to achieve the desired voltage. But, it will take an input voltage from 1.2 to 32 volts DC and give you any voltage you want so long as the source voltage is 1.2 volts higher. The LM317 is available at Radio Shack and other shops.

    Batteries also function as voltage regulators, as noted by tspoon. Drop some rechargeable batteries into the circuit in parallel and they should have the effect of stabilizing the voltage at the desired level. I have not tried it, but what I have read indicates it should work.

    I used a bottle generator for years on a ten speed bike. I always found I had to drop down a gear to power the generator. I bought a couple of generators in Germany and want to try one again.
    Last edited by twobikes; 08-04-07 at 01:09 PM.
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  8. #8
    Senior Member wahoonc's Avatar
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    It is my understanding that the better quality generators are regulated. Typically 6v3W output. How they do it I have no clue. I do know that the old dynohubs were not and you could blow bulbs on a good downhill Take a look at the generators and info on Peter White's page. He does not and will not answer electrical questions but he has some good info on output and types of generators available.

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  9. #9
    Cries on hills supton's Avatar
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    Interesting stuff. As for zener/LM317 regulation, I wanted to avoid that: they regulate by wasting power. They take that extra voltage and burn off wattage as heat. I go slow enough as it is. Might not be a problem on some rides, but I have a somewhat hilly ride that I take, just recently I hit 40mph on it. [I go more like 10 on the ride back up that hill, maybe ...] All stuff to play around with in due time, the battery/halogen setup I have right now is doing the trick well enough.
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  10. #10
    Thawing Member Aloyzius's Avatar
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    Aw gee. I'm an electrician and I don't even know what the hell you're talking about.

  11. #11
    Healthy and active twobikes's Avatar
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    Quote Originally Posted by Aloyzius View Post
    Aw gee. I'm an electrician and I don't even know what the hell you're talking about.
    My dad was an electrician and I was his go-fer, so I learned a bit about that and can do quite a few things safely and correctly. I was always interested in radio circuits and have tried to read several books on electronics. My bookshelf is far more impressive than my actual knowledge, although I have had a couple of successes in the midst of my several failures. But, electronics stuff brings many considerations to the table that are seldom if ever part of the equation in residential wiring.
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  12. #12
    Cries on hills supton's Avatar
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    Quote Originally Posted by geo8rge View Post
    If you are looking for an electronics project go for it, but if you want more watts for less effort a hub generator is probably the way to go.

    FWIW I was thinking of rigging a plastic gear to my spokes and connecting that via chain drive to an electric model airplane engine running it as a generator. I figure a model airplane engine must be extremely efficient and outdoor friendly. Maybe a model boat engine. Never did it. Mounting is the biggest obstacle. You might try to upgrade a cheapy bottle generator with a better engine. Gearing is also a problem as to get the volts you need 1000s of RPMs, check specs before waiting your time.
    Hmm, I'm at work, but I think that bottle generator wheel is about 2cm diameter. I'd guess it would sit about 1cm down on the tire; or at about the 33.3cm mark on my 27" tire (as from the center). This would give it a step up of about 33.3:1.

    6mph is about 10km/hr; I get about 77.5rpm. That translates into about 2,600rpm on the bottle generator. Give or take a bit, given that the numbers are fudged.
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