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Old 07-15-2010, 10:30 PM
Dougan Dougan is offline
Motorized Bicycle Senior Member
Join Date: Apr 2010
Location: wisconsin
Posts: 78
Default Magneto-powered headlights

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 CreeŽ XR-E Emitter - LED 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?
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