As someone with only a basic understanding of electrical things, please bear with me.

I'm trying to convert the lighting circuit on a vintage motorcycle from 6v AC to 12v DC (so I can use LED headlights etc.)

On this particular model, a magneto rotates around fixed coils. Originally 1 coil was designed to produce 6v and around 40 watts AC.

I have added another identical coil, wired them in parallel and connected them to a 6v to 12v DC regulator/rectifier (specifically designed for 6v to 12v DC motorcycle conversions - I'm sorry I don't have the specs for this unit).

When all wired up and the engine was running I measured around 3v from the regulator. When the RPM was increased this dropped to around 2v (I would have expected it to rise!)

Should I be wiring the coils on series? Or will this make no difference? I'm confused. There is no earth leakage. I understand that there will be some loss from the rectifier.

Also, what does it mean if something runs 'AC/DC simultaneously.'?

Perhaps not your regular type of question on here but any help or advice would be enormously appreciated!

  • \$\begingroup\$ Wiring two coils that produce 6v DC in series will result in a 12v DC source. I don't entirely understand your issue but hope this helps. \$\endgroup\$
    – Peyton B
    Mar 9, 2017 at 23:24
  • \$\begingroup\$ Thanks for the help. Yes, I think I should have them in series rather than parallel! \$\endgroup\$
    – Toastie
    Mar 10, 2017 at 9:04

2 Answers 2


You must make sure the coils are in phase (their windings are at the same position relative to the magneto magnets) and you must plug the 2 coils in series to add their voltage. Plugging them in parallel will only add their current, with no voltage increase.

Your coils will probably output something greater than 6V, as you can only regulate 6V with a higher voltage, because of P-N junction voltage loss.

I don´t think the 6v to 12v will work in this case, since you are producing "12V" in the AC coils. It would be best to use a voltage regulator adapted from a 12V motorbike.

You will also need to use a step-down converter for your CDI (if your bike has one) because was made for 6V, so it will probably get damaged at 12V.

BUT: i think this is not very smart, as leds work much better at lower voltages. 12V led lamps usually carry power-consuming current limiting resistors. Try to find 5V leds, they will work with your 6V motorcycle if you add a diode or 2 in series with it´s power supply, to lower the voltage to something around 5V (no precision needed here).

If you can´t find 5V leds, grab your 12V leds and measure it´s current at 12V, then change the resistor so that you get the same current at 5V.

  • \$\begingroup\$ Thanks for the advice. The coils are in phase -they are identical parts. I will try them in series and see if this helps. The regulator is for 6v or 12v input voltage. The CDI is powered by an isolated third coil and works fine by itself. The main issue is the headlight bulb. I want to use a specific 12v LED \$\endgroup\$
    – Toastie
    Mar 10, 2017 at 9:00
  • \$\begingroup\$ How can a generator accommodate two coils which are in phase? Two coils 90 degrees OUT of phase would make more sense. If you can probe them, see if the voltages add when connected in series. They might not. \$\endgroup\$
    – Whit3rd
    Mar 10, 2017 at 11:22
  • \$\begingroup\$ 90 degrees out of phase will not produce highest possible voltage. The coils at same phase will produce max AC voltage as both coils' voltages will be peak at the same time, which is what author needs. How he did that (or will do that) in hardware is out of my knowledge though. \$\endgroup\$
    – cyberponk
    Mar 11, 2017 at 5:14

You would be better off adding an inexpensive boost DC/DC supply regulator to provide 12V from the 6 volt battery.

  • \$\begingroup\$ Thanks for the advice but unfortunately, with the standard set up the headlight is on a separate circuit to the battery. \$\endgroup\$
    – Toastie
    Mar 10, 2017 at 9:03
  • \$\begingroup\$ I do not understand why that matters. All you do is cut the wire and connect the wire ends to a little circuit board. \$\endgroup\$ Mar 10, 2017 at 14:05

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