Schematic for small engine ignition coil

[Averagesupernova]

The 555 ignition coil driver is astable. It will produce a constant arc from the high voltage terminal. Looks to me like someone's driver for a Jacob's ladder. Has no place in an engine ignition. More of the same non-answer crap. EXCEPT for this one:

This one actually eludes to the fact that there is a separate coil that is only for the trigger. That makes sense. It's not difficult to figure an SCR and would be used but this is the only one in this thread that gives a credible attempt at answering the question. Yes I now realize it shows up on the Google link in post #2. But a ways down the list. You would think the other links provided by @jack action would be more informative but I thought they were disappointing. Coming from a supplier of ignition replacement parts I would have expected language that doesn't sound like it's directed towards a kindergartener.

 

[Guineafowl]

Thanks again to Jack for all the searching. I agree with above, it looks like some variant of the two left-hand diagrams, although I’m not sure about the ‘multi-magnet flywheel’, nor how all of that is mounted on one leg of the core, as here:



I doubt that Stihl make their own ignition coils, in fact this one is labelled ‘Walbro’ and as I say, is fitted under various names in so much equipment. Someone knows how it’s made, hence the aftermarket version (which was crap, but I’ve got away with aftermarket coils before).

One thing seems certain - I can test the secondary winding for DC resistance between the HT lead and the core. I also built an inductor ringing tester several years ago, with help from JH and the venerable @Tom.G , which may prove useful, at least for comparison:
https://www.physicsforums.com/threads/testing-windings-tools-gear-for-appliance-repair.915727/

 

[Averagesupernova]

I've thought about the two magnet thing as well. I wonder if they are exactly the same. Is one magnet narrower or positioned slightly different so as to charge the capacitor but NOT activate the trigger?
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BTW, this is certainly not the first time I've heard of Walbro. Common name in small engine parts. Maybe this particular one is exclusive to Stihl though.

 

[Guineafowl]

I've thought about the two magnet thing as well. I wonder if they are exactly the same. Is one magnet narrower or positioned slightly different so as to charge the capacitor but NOT activate the trigger?
-
BTW, this is certainly not the first time I've heard of Walbro. Common name in small engine parts. Maybe this particular one is exclusive to Stihl though.

Yes, Walbro make small engine carbs as well.

As far as I could see, both magnets were exactly the same. I think if we ever get to analyse the circuit, we can consider them as one, as per Jack’s earlier post.

So, the magnet(s) passes the first pole at TDC. The first pole is the one with all the gubbins around it. I assume the trigger event is the magnet passing the second pole, a few degrees ATDC.

So, we have a charging event, and a trigger event. Somewhere involved is a capacitor and SCR, as with most other CDI ignition systems. We have all the parts, but not total certainty about how they’re arranged.

 

[Guineafowl]

Depotting with a heat gun wasn’t ideal - although it made the epoxy crumble, it also melted solder joints and destroyed some components. This may not be very informative, but better than nothing.

Firstly, the transformer part, without the PCB. I’ve guessed the secondary is the right-hand bobbin, from the very thin, numerous windings. The primary appears to be split into several sections, perhaps charge/trigger/?.

Next, the PCB as fitted. You can see the kill terminal, and HT lead connection, with PCB slot:

Finally, some of the components on the other side. Two disappeared, so I’ve marked them in. There’s a TO-92 package marked T, transistor/SCR? And possibly another one above it. I stopped there as I was only doing more damage and making a mess.

Could it be, looking at those diodes, that the charge/trigger events are defined by the direction of flux through the core? At a guess, I’d say the unit is charged as the magnet passes the upper leg, then spark is triggered by the magnet passing the lower leg.