It is the magneto coil, though I’d like one of out more knowledgeable contributors to explain the dual coil aspect.

It may be a trigger coil.

In a magneto, as the rotor turns, its magnet moves to set up the opposite polarity of flux in the pole piece with the ignition coil. The pole has both high (spark) and low voltage windings. The low voltage coil is kept short circuited by the points, which resists the magnetic flux reversal until the exact point when the spark is needed. The points open, and the rapid flux reversal generates the spark.

The additional pole may provide an electronic implementation, avoiding the use of contacts.

On 1 June 2024 at 10:06 Macolm Said:

It may be a trigger coil.

In a magneto, as the rotor turns, its magnet moves to set up the opposite polarity of flux in the pole piece with the ignition coil. The pole has both high (spark) and low voltage windings. The low voltage coil is kept short circuited by the points, which resists the magnetic flux reversal until the exact point when the spark is needed. The points open, and the rapid flux reversal generates the spark.

The additional pole may provide an electronic implementation, avoiding the use of contacts.

I like this suggestion very much, and found a circuit!

The advantage is the SCR eliminates the bouncy point contact with a super-fast action electronic switch.  More reliable, maintenance free, and, with the capacitor, the SCR produces an extra powerful spark.

Dave

“I like this suggestion very much, and found a circuit!”

That looks like it. The use of a capacitor circuit (very common in ignition systems I think) has the benefit that there can be a good spark even when hand crank starting. Also, the trigger is the only timing that needs to be right, the capacitor can be charged whenever earlier in the cycle

On 1 June 2024 at 11:42 Maurice Taylor Said:

Hi Dave ,

Never seen a magneto like that before with electronics. What engine is that off ?

Is it off something you have used ?

Maurice

 

I don’t know which engine it’s from.   Following Macolm’s lead I searched for ‘Double Coil Magneto’ and selected Images.   Lot’s of wrong answers, but I noticed that circuit in the pack.   It’s similar to an electronic ignition kit I built for my first car, a trusty Vauxhall Victor, back before the flood!

With a Multimeter, shouldn’t be too hard to debug Dougies circuit.

• Measuring ohms,
  • Ensure the stop switch is open circuit
  • Check both Diodes only conduct one way only.
  • The two coils should both register some resistance, not open or short.
  • The SCR should be open circuit from top (diode D1 end) to ground.
  • The resistor should not be open circuit.  (I don’t know how many ohms it should be!)
• Measuring volts and spinning the crank with the trigger coil disconnected from the SCR
  • a voltage should appear on the capacitor, at least 100V I’d expect so don’t get bitten!
  • a smaller voltage should appear on the trigger coil, perhaps 20V.  It will be a short pulse that swings the needle once per turn, not a steady output
• With the capacitor charged, try shorting it to ground with a fly-lead, effectively replacing the SCR.  If the spark-plug fires, the SCR or trigger is faulty, not the ignition coil.
• Reconnect the trigger coil to the SCR and test for volts arriving at the gate

The ignition coil can be tested with a battery.

I may have got this wrong or there’s an easier way.   Any ignition experts out there?

Dave

PS Ohms values probably higher than I suggested.

What is the capacitor value and voltage? (Unfortunately this website does not support the normal “micro” symbol you can just see on it)