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Nedlom 09-08-2018 02:16 PM

Anyone know anything about coils / inductors etc?
I've got an old moped (Puch Maxi) that I'm trying to convert from 6v to 12v to run modern lights.

It's a super basic setup - no reg/rec or battery, just a permanent magnet on the flywheel going past a coil/inductor.

My limited understanding of electricity and poor google skills led me to believe that converting to 12v would involve rewinding the coil with double the thickness wire twice as many turns.

Having messed around trying to rewind the 6v coil it was clear that double thickness, double turns wouldn't work (space issues), so bought a 12v "conversion" coil.

Thing is, this 12v coil looks (to me) indistinguishable from the 6v one, same gauge wire (possibly even slightly thinner?) Doesn't seem to be "overwound" or anything so what makes it 12v? Any ideas?


oil pan 4 09-08-2018 02:26 PM

Delta versus wye windings.

freebeard 09-08-2018 02:46 PM

Do you have a picture?

Twice the windings with thinnner wire seems likely. Put an Ohm meter on it and measure the resistance.

M_a_t_t 09-08-2018 02:47 PM

When you have more voltage going through a wire you can use a thinner wire. This is because the wattage (typically) stays the same, so when V goes up Amps go down.

P = V * I

P, in Watts
V, in volts
I, in Amps

Stubby79 09-08-2018 02:53 PM

if they're thinner, and the coil appears to be the same size, then it has more windings and therefore will put out higher voltage but less current.

Chances are the old coil will put out 12v at higher RPM, but there's a limit to how much current it can put out, so as long as the load is high enough, the voltage never gets up that high.

Unregulated systems use the battery and the lights to limit the voltage. Light burns out, battery tends to get over charged and dies prematurely. Battery goes out, lights get too much voltage and blow the bulbs. Half the reason motorcycles, old ones at least, always have the lights running.

I'd slap that new coil on, and throw on a cheap little regulator. If the voltage never gets high enough (above 14v, give or take), the regulator won't ever kick in. But if you don't have enough load on it, it will save your bulbs. A volt meter/multimeter will tell you if you're making roughly the right voltage or not, or if your new bulbs are putting too much of a load on it and you need to use smaller (or bigger) wattage bulbs.

freebeard 09-08-2018 04:04 PM

Why not use 5V LEDs?

Nedlom 09-09-2018 06:29 AM

Here's a link to a pic:

Puch Maxi Magnum Stator Plate 12v

Neither delta nor wye - it's just a single core and winding - real basic stuff.

In the linked picture you can see the black ignition coil and the lighting coil on the opposite side with the black and yellow wire coming out.

Ok, so my understanding was that wire gauge affects voltage and number of turns affected current/power. But actually it's turns = voltage and what power and current are a factor of speed and magnetic force?

So I'll have 12v, less current but the same power output? Ok makes sense.

Stubby that makes sense - I've run the original coil up to full speed, voltage rose, but only from about 6v at idle to 6.8v at wot that threw me a bit thinking the 40year old magnets might be weak, but probably the puny charging system was already at it's limits running the lights?

Yeah, I've got a little regulator too, so I'll see how it goes.

Freebeard, I've thought of that, probably would end up with better lights than the route I'm going, but I need a headlight with a "discernable beam cutoff" for the yearly inspection, and I think led in a reflector bowl would be a fail too. Plus there's no "off the shelf" solution to running led's with unregulated/ unrectified ac (that I know of?)
I just need a "good enough" solution that passes muster with the inspection man's tick boxes.

After that I'll probably throw on a set of high power rechargeable bike lights for the headlight, and leave the stop/tail running off the generator.

Thanks guys.

Piotrsko 09-14-2018 04:35 PM

You have coil theory backwards. Thick wire is more current, thin wire is more voltage.

One way to increase voltage is to spin faster which I guess is impractical. Or twice as many magnets, or stronger magnets. Voltage induced is a result of the rate of flux lines being disturbed.

You could alter the wiring such that the lighting coil and ignition coil are in series then see what you get. I believe the wiring is not empirical because one coil will probably need reversing. Should also be pulsed DC since you are exciting the coils with 1 magnet, if it is AC because your using both sides of the magnet, a simple power bridge diode makes it dc easy peasy.

Nedlom 09-15-2018 09:45 AM

Hey Piotrsko,

Yeah, I had it totally wrong!

Funny enough, spinning faster is probably why tuned mopeds start blowing bulbs. Pushing higher rpm than stock raises the voltage too high. That's just clicked for me - thought it was added vibration or something.

The ignition coil is a sealed unit, so I'm not messing with that - hard to get a quality replacement nowadays, and I'm already in over my head with the electrics! :)

I would guess that it's full ac, rather than pulsed dc, as the magnets are thin/long and follow the curve of the rotor, but I don't have a scope or anything to verify that.

In terms of rectification, would I loose any output power? I'm sticking with regulated ac until I've got it through its final yearly inspection, after that it's considered a historical vehicle, so will be exempt. I'll have a little more freedom then and will probably look at going led.


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