LED headlight for 4vac to 16vac unrectified unregulated magneto on older motorcycle


First post.

This bike’s magneto is nominally 6v. I’ve measure 4v to 16v through the rpm range. Max output is 35 to 40watts.

It has a separate lighting coil so lighting draw does not affect ignition/spark but lights run without rectifier or regulator. Headlamp and taillamp only. Not street legal.

6v incandescent lamps are dim and to too often blown. 12v lamps are uselessly dim but last longer.

Rather not add a battery unless it is of significant benefit. If so I was considering four 1.2v aneloop rechargeables in series or a single compact 4v lead acid battery.

From the reading I’ve done so far LEDs are the only way to get significantly improved candlepower out of such a tepid magneto.

This is what I’m contemplating. I hope someone can offer more expert advice on the direction I’m looking at.

common as dirt full wave rectifier for dc output.

Adjustable Regulator so that I can either run led directly or recharge the batteries at the ideal level required of them.

So I’m searching for a maximum brightness single LED SMD that I can experiment with within the existing reflector and pressed glass diffuser lens at different angles, depths, and possibly even mounted on the inside of the glass lens facing rearward for optimum road lighting pattern.

3.X ~ 4.X volts. form factor size is not critical. Something like this unless a larger element is a better candidate.

I’m a complete beginner except for a couple of days of web research.
Would anyone offer some help as to how to set this up?

link to that regulator

What is the bike and is there a wiring diagram available?

It’s usless to use boost/buck driver, since there won’t be enough current at lower RPM, to boost the voltage. I’d use rectifire bridge with some caps to smooth the voltage then the buck driver with desired current. Let’s say you go for a 6V MT-G2 LED and b3flex driver. The driver will start to regulate when the voltage is high enough until then it will be in direct drive, so there’s no harm done to the LED. Once you rectifie the 4V AC you’ll have about 5.6V DC to play with, which should be enogh to make that MT-G2 at least give some glow.

It’s difficult enough at this point in LED development to stay on top of optimum units for specific applications.

I wanted the highest lumen rate per watt available in a single LED in neutral white. Physical dimensions not an issue. After half a dozen forum threads in a couple of forums and a call to DigiKey’s tech support I decided on the XML2 units. Particularly the XMLBWT-00-0000-00L T50E 4 on these datasheets…


While on the phone I asked if he’d be kind enough to suggest a rectifier and regulator for this amount of voltage/wattage on a 49 cc moped. Off the top of my head I’d told him that the output was 35 to 40 watts but it turns out on rereading these diagrams that it is nominally 26.5 watts. Hope it doesn’t matter too greatly.

I ended up ordering the LEDs, an NE12S0A0V06PNFA DC/DC 6amp power module,

and a GBU10A-BPMS-ND 10A 50V Bridge Rectfier.

Schematic to the engine/mag
These are for OEM instructional use. I’ve since wired the mopeds according to this page for simplicity’s sake

OEM Schematics.



35 bucks in parts. You can buy 6v internally rectified led bulbs for this type of use, except for a few reasons. If it fails/blows you’re out 35 bucks. If (when?) It is superseded next month whit 4x the output at even less drain on the magneto and you want to upgrade you’re out 35 bucks. If you just can’t live with the the color temperature or the spread pattern at all you’ve wasted 35 bucks.

But with all the rectification/regulation in place externally with adjustable VDC (regulated via various resisters across two terminals) any low powered 3.x volt to 4.x volt LED can be tried among dozens of candidates or replaced for 5 to 10 bucks each.

If I get no better output than my present unregulated 12v/20w halogen I’ll be pretty disappointed so I’m keeping my expectations low. But most of the reasons for and enjoyment of this in the first place is just to tinker with electronics.

Everything arrived today and I’ve got a few other things to to the scooter to start testing this stuff out.