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Need Help on Small engine obsolete Electronic ignition

Thread starter #1
Hello .... I am new to this site but I have read a lot answers from very knowledgeable people who have helped many on this site. I have an older 11 hp small, single piston gasolene engine on a generator with a bad and obsolete SSI ignition (BM5104) . The ignition system consists of an exciter coil, trigger coil and ignition coil. I would like to try and build my own SSi unit that matches my old one. I replaced my exciter coil because I thought the output of 14 to 25 volts ac was too low even though the resistance checks of the exciter coil = .95 ohms and the trigger coil = 69 ohms which are good according to the specs. The ignition coil also is good.Sorry guys but I have already discarded the old SSI module so I don't have it to take apart. I am attaching a diagram of the transistor/SCR crontrolled induction ignition circuit from the engine manual. I really need someone to take a look and give advice on component values if that is possible. I would like to stay with the ac Exciter coil supply system that is already there. Any help is appreciated

WI-390 electronic ignition.PNG


Active Member
What makes it hard to tell for sure is that not knowing the free running voltage of your exciter coil, nor do I know the peak current that through the bjt or scr.

I'll assuming there is some standard values that the car manufacturers have used.


Well-Known Member
Most Helpful Member
Your picture shows a magneto system, similar to what I have in my aircraft. Energy is generated by a moving magnet in the flywheel and the "exciter coil". That establishes an initial current in the "exciter" coil through the NPN to ground. When the "pulser" coil is tickled by the flywheel magnet, it fires the SCR, which simultaneously turns off the NPN by shorting its base, causing the current that was formerly flowing through the NPN to flow instead into the primary of the ignition coil. The primary pulse voltage is stepped up in the secondary of the ignition coil to fire the plug.

So are you looking for someone to select a suitable NPN transistor and SCR?

Since you didn't put in a country when you registered for the web site, we have not a clue as what to suggest.

I suggest you use a much simpler "Kettering" ignition system, where the spark energy comes from a battery, rather than from a magnet moving past a coil. You could always use the existing "exciter" coil as an alternator to recharge the battery, like a car.
Last edited:
Thread starter #4
Thank you Grossel for your prompt reply to my problem . I did attempt to build a circuit using components that are used in a capacitor discharge ignition that was supplied from an exciter coil that produced an AC output of around 120-180 volts to the capacitor charging circuit. I used a C106DG thyristor for the SCR, a BC547B for the transistor,
1N4007 for both diodes, and 1K ohm resistors for both resistors(which was just a wild guess in the dark on the resistors). My results were NO SPARK .... But bear in mind that with my brand new exciter coil produces around 15 to 25 Volts AC at cranking speed using the starter.( probably because my old circuit is an induction ignition and not a capacitor discharge high voltage system). That's just my guess. So thats where I'm at.


Well-Known Member
Most Helpful Member
Unfortunately, your "schematic" is not likely complete. It may be enough to show the theory of operation, but not necessarily all of the parts used in the implementation.
Thread starter #6
Thank you MikeMI for your reply ..... Yes sir ..... This does sound like the same circuit ...... Yes I am from Florida, USA. I am looking for suggestions for suitable components
such as NPN transistor, SCR and Resistor values. The circuit seems fairly simple but I have just a basic knowledge with electronic circuits and would appreciate any suggestions . I did see on line one BM5104 unit for sale but the price was very high.
Thread starter #7
Thank you KEEPITSIMPLESTUPID for your reply and this may very well be the case and I do understand that but thats all I got. I am prabably reaching at straws.


Active Member
I can to a certain degree agree that the schematic is missing something. One thing I noticed is that spark is in theory (as stated in figure) should happen when voltage turns on. It takes a lot more to make a spark from a rising edge, than by a sudden cut off feeding circuit.
At least that is my thaught bout that.


Well-Known Member
there's a technology that was used in the past called CDI (capacitive discharge ignition) where a capacitor gets dumped through the ignition coil primary. it used a magnetic sensor in a similar way as in the circuit you show here. most used SCRs, and some used BJTs. these days you could probably use MOSFETs and get good performance and fast risetimes because of the switching speed and low on resistance.

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