Generator help

[Dougie SwanParticipant @dougieswan43463]

<p style=”text-align: left;”>Hi</p>
Apologies for posting this here but can anyone help with this

It appears to be some kind of coil that is fitted to a small boschmann generator I have

It is mounted so that the magnet on the flywheel passes it bit I have never seen anything like it. It feeds a coil to produce a spark and I think it bust

What is it called

Thanks

Dougie 

[BazyleParticipant @bazyle]

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

[noel shelleyParticipant @noelshelley55608]

It could be an exciter coil. Early belt drive Hondas had one to feed rectified ac to the field coil of the alternator. No excitation and you will have NO output. Noel

[MacolmParticipant @macolm]

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.

[mgnbukParticipant @mgnbuk]

Do the wires from the coils all go into the ignition coil module ?

If so it could be a self-generating CDI ignition, with the larger coil providing the voltage to charge the circuit & the smaller being the trigger coil that times the spark. This type of ignition system seems to have replaced the older magneto set ups in small engines & motorcycles.

Do you have continuity on both coils ? I would expect both to have a reasonable resistance (maybe a couple of hundred Ohms – not a really low value like an ignition coil primary winding, for instance). If both coils show continuity it could be the module that is at fault, but many of the modules stop the engine by shorting a wire from the module to ground, so it may be worth looking at any kill switch & disconnecting that from the module if there is one.

Nigel B.

[Macolm]

On 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

[Maurice TaylorParticipant @mauricetaylor82093]

Hi Dave ,

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

Is it off something you have used ?

Maurice

 

[MacolmParticipant @macolm]

“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

[HopperParticipant @hopper]

In motorcycle world, known as a flywheel magneto. Standard issue on budget bikes from the old BSA Bantam and its Villiers-powered contemporaries through to many of today’s ubiquitous Asian step-throughs and scooters.

The second, smaller trigger coil is a later development eliminating the old points. Would be cheaper to manufacture and  need no maintenance.

[Dougie SwanParticipant @dougieswan43463]

Thank you for all the replies

This is from a little generator I have

There are 4 wires coming from the unit, one goes to the spark coil three others go to a sealed black box with one coming out and going to the coil

I tried a new coil but no joy

Here are some more pics

[mgnbukParticipant @mgnbuk]

That rather looks like a generic Chinese clone of a Yamaha two stroke generator – usually rated around 650W. I have a B & Q branded variant & I see that Machine Mart still carry a SiP branded variant.

The complete ignition assembly can be had from AliExpress

I have not been successful at finding a UK source of parts for these yet, though SiP or Machine Mart may be able to help – most like for a lot more than the complete setup from Aliexpress, though.

Nigel B.

[Maurice Taylor]

On 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

[MacolmParticipant @macolm]

The circuit diagram fits well with the pictures of the parts. Though a simple circuit, just about any of the components could be faulty, but most may be able be checked to a reasonable extent with a multi meter. Unfortunately, the module probably contains the electronic items, which spoils things a little but it could be much worse.

Check both coils are continuous, excitor a few ohms, and the trigger likely tens of ohms (but neither zero ohms). Being subject to direct vibration and heat, a fault here is quite likely, with a broken connection quite possible.

If there is a diode test on the meter, a diode should conduct one way only, not conduct the other (and not be short circuit), and the SCR would show up as two diodes provided you understood it sufficiently. The capacitor should not test as continuous, but will look like that when the probes are first applied as it charges up. It will behave the same again if the probe connections are reversed, but if left connected to the meter long enough, it should end up a high resistance.

 

Good luck.

[MacolmParticipant @macolm]

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)

[V8EngParticipant @v8eng]

This is a useful generator parts (etc) supplier with a nice finder & help system. I’ve not used them for some time now but found them to be good when I did.

linky thing here:-

https://generatorguru.com/

 

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