Magneto thousands of Volts!!! :O


New Member
Local time
5:33 AM
Dec 25, 2023
Dublin Ireland
Lol, the title says it all.

I thought I was clever, that was until I scrambled my arduino to bits.

I added a speed sensor to the bike and processed it into the arduino. (No problem there, worked perfect)

I then added a standard relay (rated at 230vac or so) in series on one of the wires coming from the magneto to the CDI (I think I went wrong here)


So for background, my arduino nano board script basically says, if the 50cc bicycle speed exceeds 15mph open the relay before the CDI to kill the spark plug, essentially turning off the motor. I shop tested this configuration without the motor or magneto going, and it was working fine... when free spinning the wheel by hand with the bike in the air, the relay would start closed as it should and open when the speed reached 15mph. Great I thought! But when field testing on the actual bicycle it was acting erractic. Relay was latching in closed or staying close no matter what or staying open no matter the speed.

SOOO when I went back to the shop to troubleshoot and diagnose, and I saw the arduino was scrambled to bits and I had to reupload the entire script. I've never saw that with an arduino before, I've seen shorts, memory errors, etc, but not a corruption of the uploaded program. I slept on what went wrong and the next day came back for a test (that was today btw). When I woke up I thought... high voltage acts in strange ways on microcontrollers!!! Especially if it was escaping the relay which is only rated for 230vac. I figured maybe the magneto was putting out more voltage than I originally thought. I know spark plugs use 10,000 to 20,000 volts, but I just figured the CDI was what stepped up the voltage (I think this was my error) and the magneto wires before the CDI would be lower voltage. I'm pretty sure these little magnetos with their secondary coils still put out a ton of voltage.

Well sure enough, I idled the motor and took a voltage reading on my meter (between the two wires between the magneto and CDI) and it OL Overloaded the meter even though it was set for Vac and at the max range. :0 [pic attached, turned motor and meter off]


So I'm pretty sure this is what scrambled my arduino microcontroller. Obviously, a basic relay can't switch +10,000v.

Question #1) So bros, does the magneto and it's induced coil actually put out thousands of volts to feed the CDI which then feeds the spark plug? I thought it was 12 or 6v, lol

Question #2) I still need to electrically be able to turn off the spark plug with some type of relay. I know they sell high voltage relays but they cost like $25-80. If this thing is putting out thousands of volts then that would make me believe it steps the voltage up with a primary and secondary coil. It'd be easiest if I could just break into the primary coil. However I'm looking at the wrapped up coil and can't quite figure if the primary is even accessible. Does anyone know which connections are for the primary coil? [Pic attached]


Why do you want to kill the motor at 15mph? To save the Arduino? What is the purpose of this Arduino?
Legalizing the bicycle for my respective country, Ireland. The law requires the motor turn off at a certain speed. Silly I know, but whatever. The Arduino can determine the speed with some sensors I added and ultimately the Arduino can send an on/off control signal to some type of switch.

Anyone know how and where to add a relay to open the circuit to stop it getting to the CDI and spark plug?

Does the main wires from the magneto, while running, actually put out thousands of volts or an I imagining things?
If I had one of those expensive high voltage step down leads, I'd love to know the actual voltage. It definitely scrambled my Arduino and freaked my multimeter.
Would you be happy with a governor to limit your speed to 15mph? Limiting the throttle? Not many electronics are designed for 1000's of volts. Most are 1.5v to 12v dc. Think about the insulation factor needed for small boards, switches, resistors and wires.
Most magneto systems run a few hundred volts from the mag to cdi... The relay could conceivably switch that. However, due to the close spacing of the relay contacts when they're open, an arc can exist between them and that could easily screw things up. A small value capacitor rated for a kilovolt minimum (0.05 uf at 1000vdc, for example) from each of the contacts to circuit ground might solve this, by suppressing the arc. But basically, switching high voltages with a relay is not ideal. If you arrange to have it act as an engine kill switch, it should work. You'll want to wire the og kill switch in series with the relay contacts - actuating either will kill the ignition and shut off the engine.