How to measure, how much power my moped generates

Hello guys.
I know the topic title might be a little bad, but I really didnt know what to call this thread. Please let me know if you have better suggestions

I own an old Puch Maxi moped, but the generator is a little funky, it outputs waaay too high voltage than what I would expect to see.
So I want to rectify it, as I need DC voltage in the end, and measure the voltage when drawing a specific current, to see what I have to work with. Like when I draw 1A or 2A or something else.

When I measure the voltage with no load, I get up to around 100VAC free wheeling. (But I figure this would fall quite a bit when I put a load on it.)

My initial thought was to make a constant current regulator with a load, and then see what the input voltage on the regulator would be, when I change the drawing current. But I am unsure how to do this, so it can handle potentially 140VDC (100VAC rectified).

How would I go about doing this?
Any suggestions would be great. And please let me know if I need to elaborate on some things

Puch Maxi ! - you just lost your street credibility :grin:

The voltage will drop down to much less than 100V when a load is placed on it. My suspicion is the voltage you are measuring is partly spikes from the magneto ignition interfering with the meter reading. I would suggest doing it a much simpler way, the results may not be as scientific but the circuitry is much easier: use one, two, three or more lamps and measure the voltage as you connect more across it. Measure the current if you can, otherwise use the lamp Watts rating to give an approximation of the current (I = W/V).

You will probably find that one side of the generator coil is connected to chassis so it may not be possible to use a full-wave rectifier to get DC. Even if you did, you would find it very difficult to get 'clean' DC from such a simple generator/magneto circuit which is really intended just to power a lamp.

Brian.

The older dirt bikes I had, were either unregulated or
had a big finned zener clamp of some sort. To get one
bike to run, after hanging a head and tail light on it, I
had to add a big electrolytic cap on the IGN line. The
ignition system itself is pretty tolerant of voltage
variation, if it's points and condenser type.

If you put a 'big finned' heat sink on a Puch Maxi it wouldn't be able to carry the weight!. They were fairly common in Europe but probably not further afield. Think of a pedal cycle with a small 2-stroke 49cc engine fitted to it. Some models didn't even have a speedometer, with a top speed of 30MPH they didn't need one! Lightng, when fitted was a glow worm in a glass jar mounted on the handle bars. :lol:

Brian.

LOL Brian, no way! My Puch Maxi is awesome, and they are actually getting kinda popular again.
I am doing this experiment to see what kind of LED configurations I can use

I guess using light bulbs would do the trick. But then I could also just use power resistors? I was wondering if I could roughly use the same calculations as I would with DC?

There are so many variables it doesn't make sense to do it very scientifically. With such a small engine and other potential loads on it (like riding!) and of course that it shares the electrical generator with the magneto ignition, the power level will vary wildly. You can certainly use LEDs, but in such an unpredictable system it would be a waste of time to work out the power availability exactly. Give a pro power supply designer a brief like "constant 12V @ 3A output with 50Hz - 5KHz input frequency and voltage varying between 10V and 100V with current limiting at between 0.1A and 2A" and they will laugh at you.

There are two ways to approach this:
1. if you are using a battery, design a battery charger and a light controller as individual systems. Bear in mind that a battery will bear some abuse so you can risk overcharging it for short periods (while riding at high speeds :grin: with the lights off). The LEDs can then be driven by a simple switch mode regulator circuit for highest efficiency.

2. Without a battery, rectify the generator output and simply limit the maximum current to the LEDs with a resistor.

Method 2 is by far the easiest but will just be a more efficient version of the present incandescent system and the light level will vary with your engine speed. Method 1 is more complicated but also needs a battery (guess: 6V 10AH) which may be difficult to fit to such a small vehicle.

Brian.

Oh darn it, the mail notifying me about your reply went into spam :/
Anyway, I see your point.
My former regulator died, so now I have a 12V/12A battery mounted, and it fits nicely, so I think I will look into the option of majing a battery charger like you suggest. It makes alot of sense.
Thank you for your thoughts and help