Magneto voltage limiter

[Pierre95]

Hello,

I recently had some ride on my old moped. After a few miles the front light-bulb burned-out. This is typical issue with those unregulated AC magneto outputs. The bulb is designed for 6V.

Some mopeds are equiped with 2 Zener diodes limiting the output voltage from the magneto.

As both front and rear lights connect directly to the magneto, adding zener diodes would globally limit the AC voltage, which I don't want because the magneto also supplies voltage to indicator lights circuit and horn buzzer and both work better with higher voltages value.

I am thus wondering how to design an AC limiter which takes variable AC voltage with variable frequency as input and produce variable same frequency output but with fixed AC amplitude and of course minimum power loss.

Any idea about it?

Thanks.

[Pierre95]

Re: Magneto voltage limiter

Sorry could a moderator please move the thread to General Electronics Technical Discussion, seems more appropriate.

[petehall347]

Re: Magneto voltage limiter

properly wired in zener diodes should be fine

[stj]

Re: Magneto voltage limiter

convert the lights to led based.
they dont care about the voltage because you regulate the current.

[eccerr0r]

Re: Magneto voltage limiter

back to back zener diodes is an AC limiter and should work as you need but it's an immense power waste.

If you really want no power waste you need to go through a lot of circuitry -
1. Convert AC to DC (easy enough, bridge rectifier and capacitor)
2. Switch mode power supply to get a constant DC voltage (lots of components)
3. convert DC back to AC (lots of components).

This might be more trouble than it's worth. Converting from incandescent to LED lamps with a bridge rectifier and a linear regulator may be cheaper in the long run even if you have to have multiple, one for each bulb.

[redwire]

Re: Magneto voltage limiter

If the magneto is generating 9V, the zener clamps to 6V, wasting the excess as heat. Do you have a battery? It would boil and lose water.

You don't want to use a zener because it shunts excess voltage and just loads the motor down, as well as getting hot.

There are 6V magneto voltage regulators you can buy or make. The Kubota circuit is better than the other popular circuit - that shorts out the magneto. Then people wonder why the coils are burnt.

I would mod the Kubota regulator for 6V. The circuit is from the 1970's though.
edit: the large voltage drop of the bridge rectifier and SCR's (~2V) might cause the lights to be way dimmer at idle or low speeds. Might have to use Schottky diodes and something better than two SCR's.

[sam_sam_sam]

Re: Magneto voltage limiter

Let me ask a stupid question here ——>

why not regulate the the voltage at the light bulb sockets ( or the wiring to the sockets) and let everything else stay the same scene the problem you are having is with the light bulbs burning out

[stj]

Re: Magneto voltage limiter

a lot of regulators wont like voltage dips.

[redwire]

Re: Magneto voltage limiter

Any (series) regulator will lose some voltage (or dropout), so dimmer lighting at idle or low speeds.
Any (parallel) shunt regulator wastes the extra voltage as heat, with the motor loaded down more and it just wastes energy.

You need an AC LDO, connected like this dual LM317 circuit I remember. I'd guess it loses almost 3V though. LT1084 would work but a real LDO (pair) would only lose maybe 0.3V if you picked out a good IC.
This dual Darlington accomplished about the same thing, if you mod it for 6V output.
Not sure how many watts OP's lighting load is.

[petehall347]

Re: Magneto voltage limiter

has it not got separate coils in the mag ? i seem to remember one for lights and other one for battery charging . you could try next highest voltage/wattage bulbs .

[eccerr0r]

Re: Magneto voltage limiter

need OP to reply but usually magneto means no field coil, and without a field coil, the need for a battery is gone... Since everything is AC, it'd make it more difficult to also power with DC from a battery, and thus I'd say engine off = no lights because no battery ...

Phase control dimming/regulation is a solution I didn't think of at first, it might be the only low loss solution without doing the whole conversion process or just using LEDs and eating the loss.

[petehall347]

Re: Magneto voltage limiter

indicators and horn will be dc .

[Pierre95]

Re: Magneto voltage limiter

Thanks to all of you.

Firstly, there is no battery at all, only the unregulated magneto output. As the stock version is missing indicators, I already designed a regulator circuit driving an astable multivibrator tuned to approximately 1Hz for the blinking to take effect.

Indicator lights are LED, which helps saving power for front/rear lights and the horn.

Each element (Front light, rear light, indicators, horn) takes the power from one wire, the other being moped's frame. This prevents usage of full wave rectifiers, as they create another ground reference separated from the chassis.
Other constraint: the lamp holders I bought for the indicators connect such a way it is the anode of the LEDs that receives the frame which leads to creating a negative DC voltage, in order to drive the cathode.

You don't want to use a zener because it shunts excess voltage and just loads the motor down, as well as getting hot

Yes, that's precisely what I don't want: shorting the Magneto output through Zeners as it will have lot of negative effects.

Thanks for the schematics redwire, I will have deeper look

has it not got separate coils in the mag

It actually has: one is for ignition, the other for lights and horn.

indicators and horn will be dc

Sorry, the horn is not a buzzer. It requires AC current in order to make the membrane vibrate.

Both front and rear lamps are 6W.

why not regulate the the voltage at the light bulb sockets ( or the wiring to the sockets) and let everything else stay the same scene

This is exactly what I would like to achieve. I am not against creating another DC voltage from Magneto's AC output. However, don't know how to implement full wave rectification which keeps the same ground reference (moped's frame). This is why I was thinking of AC to AC regulation.

[stj]

Re: Magneto voltage limiter

if you use leds - this may interest you.
you can use them in parallel for higher currents too.

[eccerr0r]

Re: Magneto voltage limiter

Localized DC regulation is probably your best bet. The grounding/return wire you'll have to isolate it from your own "ground."

LEDs are good because they inherently use less power, so you can waste a bit and still have original brightness. Trick is to get it as small as possible.

You could still use the back to back zeners but put the zeners in parallel with the LED chain and have the dropping resistor pass current to the LEDs also pass through the zeners. That way the magneto won't see the shunting, it will still see the series resistor. You could put raw LEDs in the fixture with the zeners and ensure you have the dropping resistor outside the lamp fixture.

Because of the localized grounding your options are limited as you have to stick stuff inside the lamp housing, unless you're willing to drill some holes or something to set up your own isolated wiring system.

[Pierre95]

Re: Magneto voltage limiter

After a few weeks of involvement on other tasks, I tried designing my own AC limiter.

Top wave form shows an amplitude modulated sine wave which is representative of the variable output AC voltage from the magneto. The signal changes from 0 to almost 50V peak to peak. The output signal (bottom) never goes above ON Zener + ON diode voltage (6,8V+0,7V). The zener and diodes maintain the voltage on the base so that when the voltage on emitter goes above the limit the transistor starts acting as a resistor which forms a voltage divider with R2 (=the lamps)

This is only a "working principle" schematics as enough powerfull transistors will have to be selected, to dissipate the extra power.

This has the advantage of only doubling the dissipated power each time input voltage doubles.

[eccerr0r]

Re: Magneto voltage limiter

Thanks for following up, it looks like you got a solution that may well work (no guarantees, simulation doesn't always equal real life!). Go for it, though I still prefer LED retrofits

You still will need to deal with dropout voltage, though I don't know what your typical voltage is (which depends on RPM of course and that changes...) and couple that with your minimum required brightness.

The only thing I'm a bit concerned about is the reverse breakdown and long term reliability. But that's another issue...

[stj]

Re: Magneto voltage limiter

another option, rectify the ac, run it into a bank of caps and then feed it through a regulator into led lamps.
the caps will smooth any dips or flickering.

[eccerr0r]

Re: Magneto voltage limiter

Would need a huge capacitor if he's insisting on incandescent which seems to be the case... but yeah another benefit for retrofitting LED's, you can do stuff like this...or heck even have a switching converter and have perfect fuel efficiency running headlamps and still have constant brightness regardless of engine speed...

[stj]

Re: Magneto voltage limiter

and add a digital speedo!