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Transistor controlled ignition coil driver design-need suggestion pls.

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Chriss:
Hi!
A friend of mine asked me to make some sort of transistor controlled ignition coil driver for his moped.
It is a 150ccm engine with one ignition coil and one spark plug but with some sort of mechanical triggered
ignition coil mechanics.

Here is a simple design I come up with, theoretically it should work but I'm not sure practically does it will last and do the job.

A short description of the circuit:

The whole system is working on a Umax=14.7V = battery charging voltage.
S1 is the mechanical circuit breaker which should for now trigger the Q2 transistor and no more the ignition coil.
The LED1 represents the ignition coil in the circuit simulator software for now and it will be replaced with the ign. coil and the R2 will be also removed.
The C1, R3 and D1A are for protecting the Q2 from HV spikes which would maybe produced by the ignition coil when it
is de-energised.

It is actually a PNP transistor in switching mode circuit, but actually that is what we search for actually.
What you think about the parts and the circuit?

Thank you for any suggestion.

Dundarave:
As I recall from my motorcycle repair days ('73 Triumph Trident, among others) the cam actuated "points" were normally closed, opening only at the exact time the spark is called for.  This meant that the coil was actually energised most of the time, through what was known as a "ballast resistor" to limit the current to something reasonable.

There was a small capacitor across the points to reduce the sparking.  The coil's magnetic field collapse was what triggered the spark.

You might want to check the circuit schematic of the moped to see if they still do it that way, as you might need to modify your circuit accordingly.

Assuming the moped uses the old points/coil model that I speak of, I don't understand what exactly you would be changing/eliminating with your circuit.  Back in the day, electronic ignitions were used to eliminate the points entirely, and instead use a circuit triggered by a Hall sensor and magnet mounted on the timing cam.  From what I can tell, if you still use the mechanical contacts, you've not really gained anything.

But perhaps basic moped ignitions have changed drastically in 40 years, in which case, what do I know, lol. :-DD

joeqsmith:
How are you adjusting the timing?  Mechanical?

duak:
i am assuming the ignition circuit on a moped is like that of larger engines.  Some values might be different because there is only one cylinder.  The peak voltage on the primary winding of the coil is quite high, 100 to 200 V, and is very important as it allows the secondary winding to generate the high voltage needed to ignite the mixture.  You will not be able to use a clamp diode to protect the transistor and so require a transistor with a BVCEO of 300 to 400 V.  You will also need a capacitor with a value of 100 to 220 nF in parallel with the transistor that will resonate with the inductance of the coil to generate the correct shape of the primary waveform.

There is a description and waveforms of a simple ignition circuit in this note: https://www.st.com/content/ccc/resource/technical/document/application_note/8d/46/36/a7/b6/f2/45/10/CD00003911.pdf/files/CD00003911.pdf/jcr:content/translations/en.CD00003911.pdf  (I hope this link works - it seems to have repeated info in it)


wilfred:
I remembered this circuit in a magazine project. This link is to a PDF for the instruction from a kit supplier.  Maybe it will give you some info/ideas. It's 7 pages.

http://www.radiomanual.info/schemi/ACC_powersupply/Dick_Smith_K-3301_user.pdf

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