Need Help on Small engine obsolete Electronic ignition

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Mic34_

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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

 
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.
 
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.
 
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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.
 
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.
 
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.
 
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.
 
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.
 
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.
 
I know it’s more that a year of activity here but yes, I am very much interested in this circuit as well. I have a few OH160 Tecumseh engines that r built for the Miller Bluestar 2E welders and have an SSI module that u can’t buy anymore. U can buy kits to replace them but a little cost prohibitive. I like building electronics like this myself, as debe has helped me greatly in the passed knows well. I’m going to post photos of what I have in the hopes someone can or have a detailed schematic of a circuit I could build like this one. RPM of the engine can exceed 3000 to attain the correct weld frequency.

 
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Could you use an Automotive Ignition module & coil, with a trigger coil from the 12V battery. The trigger coil would be mounted where the SSI was. Just a thaught. Theres a Bosch ADY17 module thats trigered by a reluctor.
 
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Make something using https://www.onsemi.com/PowerSolutions/product.do?id=NCV1124 as an example.

a) Use the NCV1124 or similar to fire the XR-700 but with a 12 V source.
b) Use the exciter coil to generate 5V for the NCV1124 and use an ATOM module to get the rest of the way. No external power potentially/
c) Disect the sensor and move the coil trigger wires external.

Both methods offload the coil.

That "thing" in your engine has an integrated coil too. Disecting the sensor doesn't look like fun. It's possible that the coil portion is bad,
So, you might be able to disect and move the coil externally. To bad, no cadavers.

You can get an idea of short-circuit current. Exciter voltage(unknown)/ resistance of the exciter (known).

You can use a motor to spin the engine at its RPM and half and full wave rectify and filter the exciter voltage to get a maximun. Debe might have an idea of the voltage. RPM is related to frequency, not amplitude.

maybe something like this https://www.minmax.com.tw/en/product/view/861 can get your control voltage after filtering and rectifying the exciter.

Get a $20 scope. e.g. the DSO150 and a High Voltage probe.
 
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This is how a reluctor system would work. The parts i picked up free from the local scrap yard before they crush the vehicles. The triger coil was from a Toyota , the module was from a Ford. These parts should be cheap. Just fit the triger coil to a piece of metal plate to where the SSI was so the magnet on the fly wheel passes it. If you slot the holes you can then adjust the timing. Will take more pictures of these units later today.
 
Sure appreciate the information here kiss and debe. One thing I c though is that the Tecumseh flywheel doesn’t have an external magnet, it’s has two pins that pass over the built in magnet on the ignition module, I will make sure and confirm that tomorrow . C in the photos above. Also I’ve heard that some of these modules have trouble keeping up with rpms over 2000 or 3000 and I don’t know if that’s true, can’t c how it would be knowing these modules work on high performance engines. U guys know a heck of allot more than I do about this tech. I will look for all these components and try this out. Thanks guys.
 
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This is using the Bosch module & its being run at nearly 10000 RPM. The pickup coil in these pictures is of a wrecked Kipor generator so it will definitely be trigered by a steel peg on the rim of a fly wheel as that is what was on the generator fly wheel. Its one hot spark as its caused the stainless wire to glow. The previous Toyota pickup coil should also work.
 
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