Why Rotax-Ducati have implemented such a complex kill magneto circuit in CDI modules?

Hi Kevin.

I agree with you. Carmo make a god job in repairing modules and it is reasonable priced, but …

I must admit that one thing a do not like of Ignitech modules is that is “programable logic”. If kill function is a mere digital entry signal to the microprocessor that must be periodically polled by the program, UHMMMMM. Probably it will be good enaught for a motorcycle but it is not good for our flying machines. I really hope that implementation of the Ignitech ignition inhibit circuit will not be like you comment.

Hi Mike. Thanks for share your awesome work.

Well, I do believe that the circuit is certainly complex. Of course, I'm not saying there's nothing wrong with it. To illustrate my point of view I add a PDF comparison, based in your diagram,  between two circuits that inhibit ignition. One is employed by Ducati-Rotax. The other is, basically, the one used by the rest of the CDI systems and traditional aviation magnetos. There must be some reason for Ducati-Rotax did this design.

Since I saw the first circuits, the only thing that is clear to me is that the real kill switch is the MOSFET and that the Thiristor acts as a signal inverter of order that makes MOSFET go to the saturation zone. But... what does the BJT do? I think you, Mike, understand this very well.

I need a little time to assimilate all the explanations you have put in your document.

By the way, I think in your circuit there is an error in the connection of D21 and C7 (they must be connected between gate and source of the MOSFET).

That will work but four issues come to mind immediately that may need to be addressed. 

You will need an ignition  switch rated for 200+ VDC 

there is no EMI filter protection on the kill wire, it may generate substantial noise.

You will slightly increases the the load on the stator charging  coils. The MOSFT shunts into a load resistor. 

That rapid discharge rate may exceed the design limits for the cap

Thank you for your comments, Mike. 

I give you my opinion. 

1. That the conductors and the kill switch work at a relatively high voltage (but without much energy) does not seem to me a serious inconvenience. It is true that it is preferable to work with low-voltage "signals", but the rest of the ignition systems of aircraft engines work like this. 

2. Honestly, I don't think directly deriving the current from the charging coil to mass is a problem. The existence of the 1.8Ω resistor seems to have more the "current shunt" function to generate a voltage (V1) than a protection function.

3. I also don't see how it can affect the discharge capacitor. The D4 diode prevents it from being discharged through the ignition kill switch.

4. You are completely right, EMI/RFI is sure to be a problem but, as they do in other aircraft ignition systems, the shielding of the conductor/switch assembly must help minimize this interference.

On the other hand, I'm still "digesting" the explanations you put in the document. 

I have trouble understanding the function of the BJC (you call it peak detector). I am killed by the presence of the serial capacitor C12 that configures a sort of "high-pass filter" to the signal that drives the transistor. Certainly, I need more study. 

C12 functions as a differentiator.