Auto/Motor Schematic for small engine ignition coil

[jack action]

To be fair we had to wait until post #27 to know that we were dealing with a Stihl HS45. Stating as much information as possible in the first post helps everybody give quality answers quickly.

I'm not an expert on ignition systems and never really worked with small engines, but what baffled me was the two magnets thing. I don't see what the second magnet does. Once I knew the company name, a new search brought me the following images, the first one should be your flywheel where it is not clear that there are 2 magnets, and the second one comes from another machine of the same company that shows that a single-magnet flywheel - along with its ignition module - is replaced by a 2-magnet flywheel. The magnets are 180° apart and the poles are not facing the same direction. The only purpose seems to be giving a higher spark energy.

I also found this:

The use of two magnets in Stihl saw engines offers several advantages. Firstly, having two magnets ensures a more consistent and reliable spark. If a single magnet were used, there could be variations in the strength of the spark generated, leading to inconsistent combustion and potential engine performance issues. Having two magnets helps to eliminate this problem by providing a more balanced and robust spark.

Secondly, the use of two magnets also provides redundancy. If one magnet were to fail or become damaged, the other magnet would still be able to generate a spark. This redundancy is crucial in ensuring that the engine keeps running smoothly, even in the event of a magnet failure.

Additionally, the arrangement of the magnets on the flywheel is carefully designed to optimize their performance. The magnets are strategically placed to ensure maximum magnetic flux and spark generation. This design consideration helps to further enhance the efficiency and reliability of the ignition system in Stihl saw engines.

This seems to mean both magnets are near TDC. Otherwise, how else could one magnet replace the other one in case of failure? Below, they even specify "two or more magnets" which makes me even more baffled; If they are not all near TDC, where are those magnets?

Different models of Stihl chainsaws utilize varying numbers of magnets in their ignition systems. The number of magnets can impact the efficiency and power output of the chainsaw. Chainsaws with more magnets generally produce stronger, more reliable sparks, resulting in improved fuel combustion and overall engine performance.

For instance, Stihl's higher-end professional chainsaw models, such as the MS 661 or the MS 880, typically feature systems with two or more magnets. These chainsaws are designed to handle heavy-duty tasks and demand a high level of power and performance. The additional magnets in their ignition systems ensure a more consistent and powerful spark, enabling them to cut through large logs or tough materials with ease.

Here, they sell a conversion kit from mechanical to electronic trigger module and specify:

The ignition trigger replaces points and the condenser on most chain saws having just two magnets [north and south]. If a flywheel has more than two magnets, then the ignition chip trigger will not work.

I understand that they are talking about a single magnet with two poles when specifying "two magnets [north and south]". But it doesn't tell me how models with multiple magnets work.

 

[Lnewqban]

Yes, the links were well-meaning but didn’t capture this type of coil (strictly: ignition module). It’s certainly not a plain transformer, from its function and from the potted form, which looks like it contains a PCB, or at least other components.

I have noticed a certain level of grumpiness on here, not just directed at me. Presumably related to these turbulent times (as pointed out by another member), crackpots, non-Googlers and ‘do my homework for me’ types.

I believe that your perception on “a certain level of grumpiness on here” is inaccurate.
This is an international forum, in which people of different cultures and carrers come together, on a voluntary basis, to discuss and learn from others, and help if possible.

You don’t know the answer to your original question, but you are certain that all the above “well-meaning” efforts to help where incorrect or misleading.

I hope that, having the opportunity now, you open the useless coil, dissect it, and you understand its functioning parts.
As a token of gratitude to the ones that selflessly, and at the risk of sound grumpy, you could later come back to this thread and explain your findings.

That would be helpful also to the persons facing similar problems, who will read this in future moths and years.

 

[Averagesupernova]

I lost faith in believing anything one of the links provided when I read this:

These engines operate on a two-stroke principle, which means that they require both fuel and air to power the combustion process.

-
Uh, no kidding? So a four stroke engine requires fuel, air and two other things for the combustion process? How about if I were to invent a single stroke engine. Could it run on just air without the fuel? Give me a break. Same low quality answers as anywhere else we've found. We are looking for the type of information we'd get from Jim Hardy. Details that actually tell us something.
-

You don’t know the answer to your original question, but you are certain that all the above “well-meaning” efforts to help where incorrect or misleading.

If I had taken every answer I've gotten to my questions throughout life at face value when I am pretty sure they were actually non-answers I wouldn't have gotten very far. Just because one doesn't know the answer to a question they are asking does not mean they are unable to spot a bogus answer to said question.

 

[Guineafowl]

The Stihl HS45 happened to be on my bench at the time, and triggered the question, but as I say above, this system is ubiquitous in my experience. Even on four-stroke mowers, where it must be a waste-spark system. The exact machine didn’t seem relevant, only the description given in my first two posts. I will concede it would have made a starting point for research, though.

There’s a grumpy response to my comment on grumpiness! Built up over a few years of seeing it directed at many posters. Not to be taken personally. Please don’t let us argue about that.

From Jack’s post, I see that the two magnets might be for redundancy, so we could analyse the circuit using only one, for simplicity. That photo looks much like my flywheel - I’ve tried to emphasise the two (doodling on pics is a very Jim Hardy thing to do!)

So it looks like one way to find out the answer is to depot the old coil. It’s a hard, black resin, presumably epoxy. Some say methylene chloride to dissolve, some say heat, some say don’t bother. What’s the best way to do it?

 

[jack action]

To me, this is a single magnet, one with two poles, north and south. Here is another representation where the single magnet is more obvious:

Of course, the ignition module contains electronics that replace the contact breaker and capacitor.

Remember when in post #2, I gave you a link to a Google search? Well, here is what I found out from that link that is relevant to your questions:

From https://www.electroschematics.com/555-ignition-coil-driver/ :

From https://forum.arduino.cc/t/need-help-to-drive-an-ignition-coil-with-a-vb921/350723 :

From https://www.rmcybernetics.com/science/diy-devices/diy-ignition-coil-driver :

From https://makingcircuits.com/blog/universal-motorcycle-cdi-circuit-capacitor-discharge-ignition/ :

These all represent the type of electronics that might be present in your ignition module, which also incorporates the coil as well. They, of course, work with steady 12V power, but your circuit must work with the induced power by the magnet, which may also be the positioning signal at the same time. In post #31, there is clearly an electronic module that exists to replace a breaker/capacitor set-up for any mechanical setup. Stihl just incorporated the module with the coil. By adding the term "batteryless" to the Google query, I could find:

From http://gpzweb.s3-website-us-east-1....AndCoilWiring/IgnitionTypesAndCoilWiring.html :

From https://www.electro-tech-online.com/threads/pointless-magneto-internal-schematic.144306/ :

This circuit can be constructed to replace the ign points in a magneto ignition system. Ive fitted it to my Victa 2stroke mower. You just remove the points & condencer, & fit this circuit in its place on the ign coil. Depending on the polarity of the pulse from the coil, the unit may have to be connected the opposite polarity. The circuit is actualy a copy on an Atom Ign module that's no longer produced.​

These are very similar to the circuits I suggested to you in post #8.

But you will never find the exact diagram online as Stihl surely keeps its industrial secrets.

This is why I still think you had the best answer you could ever get in post #2.

 

[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?
-
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.