Magneto-powered headlights

Discussion in 'Motorized Bicycle General Discussion' started by Dougan, Jul 15, 2010.

  1. Dougan

    Dougan New Member

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    I know, I know... this topic has been talked to death. I've seen a lot of threads about this, but none really seem to have a definitive answer. Usually the thread moves toward reasons to use a battery instead. Or to buy a premade kit. Because both would be cheaper, easier, and more robust than anything we'd come up with.

    I don't care about value so much. I just want to learn. I know I'm not the only person here, I mean it's a project bike forum. So I want to try and get a magneto-powered headlight system. I already have experience using LEDs and resistors attached to a 12 volt car battery for car lighting, but that's about where my knowledge ends. So a lot of my analysis here involves some guesswork.

    I'd appreciate it if people stuck to the DIY magneto option as the topic in this thread, as there is a wealth of information regarding the battery option elsewhere.

    Anyway, the system I have in mind would start with the magneto white wire (of course) and end with a CREE LED.

    I tested the magneto with my multimeter to determine that the magneto was producing AC current at 4.5 volts idle, and somewhere around or just over 9v when i revved it high. I would have to assume that the available wattage doubles as well.

    As for the CREE LED, I found this diode here: Cool-White Q5 Cree® XR-E Emitter - LED Supply.com. I figure it's a good starting point for my brainstorming. It seems that they are rated to work effectively with between a 3 and 4 volt drop at 350-700 (even up to 1000) mA. The lower current will provide a dimmer light, but a "dim" CREE LED is still quite bright.

    The LED requires DC power and AC comes from the magneto, so some sort of rectifier would have to be involved, as has been mentioned in other threads on the topic. I'm not closely familiar with them, but I would guess this would work? LED RECTIFIER - AC to DC LED CONVERTER 6V 12V 24V USA - eBay (item 390219097312 end time Aug-14-10 20:08:42 PDT)

    So the big hitch in a lot of attempts is the variable nature of the magneto. Once you rev high enough it blows peoples' setups. But I'm curious here. If that CREE LED has such a wide range of options regarding current... what if I set up a simple circuit: Rectifier, LED, Resistor, Rectifier. I calculated out the resistor's resistance to flow in the maximum current (either 700 or 1000 mA depending on what's used there) at 10 volts DC? I'm almost positive the magneto isn't pushing out anything over 9.5V. So at wide open throttle, the LED woudl be almost as bright as opssible. But at even a mild pace (even 5 mph) the engine's running at least 6 volts, and I would imagine the LED would still be bright.

    It just seems too simple for it not to be on this forum already. What am I missing?
     
  2. Nougat

    Nougat New Member

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    The trouble is that pulling anything over about 3W from the white wire is going to impact CDI performance. This doesn't leave a whole lot of juice to power a bulb.

    Someone on eBay sells an add-on mag coil that fits next to the existing coil, and then you can run all your electrical off of that. I really like that idea, but the setup for sale is prohibitively expensive. If I were an electronics geek, I would try to engineer something like that myself, and get it manufactured cheaply enough to sell at a reasonable price.
     
  3. Dougan

    Dougan New Member

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    Okay. For some reason I thought I read 5 watts.

    Still, though, if the voltage drop is going to be about 3.5 volts and you put 700 mA through it, you should only use about 2.5 watts. Or am I mistaken on how that stuff works?
     
  4. Newbie

    Newbie New Member

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    Try it! Most people here have only tried incandescent setups for headlights.
     
  5. Dogtown Burner

    Dogtown Burner New Member

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    any info on that CREE LED?
     
  6. thegnu

    thegnu New Member

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    I have every intention to make all my lights with led , my only hang up is available power and or any way to stop current from flowing back into the motor from a battery .
    I also can be considered electrically ILLETRATE !
     
  7. Mozenrath

    Mozenrath New Member

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    You can buy separate generator coils on ebay that you install in your HT motor and it will get power from your magneto, but not effect the power going to your CDI.
     
  8. Dougan

    Dougan New Member

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    Haven't tried it yet, but still have plans to. I still think it can be made to work with a rectifier and a rechargeable AA battery/batteries to work as a capacitor. With winter coming on I probably won't get to it until March or April though. Sorry.
     
  9. porch lizard

    porch lizard New Member

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    I made my headlights using the white wire from engine to charge a 6 volt
    12 amp hr battery. The white wire first goes through a diode which turns it into DC current. The diode then goes to an on/off switch to prevent overcharging when off. When on, it charges the battery. The battery then connects to an on/ off /on switch that controls two headlights from the old style wheel generator style systems. One for low beam and both for high beam. The battery then is actually running the headlights, with the engine acting as a trickle charger. I'm estimating about 1-2 hrs of run time on high beam before the battery drops too low. I've swichted out the 3 watt bulbs for 10 watt ones which puts out lots of light, but after 300 miles of riding,
    I have'nt used the lights enough to know how long I can run them.
     

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  10. thegnu

    thegnu New Member

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    o k I think your on to something I my self am using a similar setup only added drains are full time rear only running lights an a single headlamp . heres the full setup dual headlamps combined draw 156 ma/ 70 lumins ,rear running lights 1157 style combined draw 70 ma only when fully lit , an turn signals combined draw 30 ma only when lit , I had planned to use this rectifier at thread http://motorbicycling.com/f50/step-step-diy-rectifier-instructions-32246.html an a diode to stop flow of current back to the motor with a 12v 7.5 amp hr sla battery same as you all I want to do is utilize the engine mag as a trickle charge an built in to my light system the ability to switch off one or both the headlamps, now I like your setup with the ability to shut off the power from the mag .
    maybe between us both we could come up with a solution .
     

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