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

Hi Mike. Thanks again. 

You are completely right. The set C12 – R22||hix (BJC entry impedance) make up a high-pass filter (in permanent regime) or a differentiator (in transitory regime).  You explain this very well in your graphics (especially in your latest version). 

As I see it:

· The 1.8Ω resistor is a current shunt that allows you to "sense" the (quasi) short-circuit current of the charging coil.

· The circuit keeps inhibited the charge of the CDI capacitor (and therefore the ignition) under three circumstances:

     1. Kill ignition cable to earth

     2. Insufficient  short-circuit current generated by load coil  (0.8v in SCR gate is the threshold @ 25ºC)

     3. Impulsive or non-stable (not flat pulse/wave) current generated by charging coil  

If what the designer intended was to make a "remote kill" with MOSFET as a real kill switch using a simple signal (low voltage and very filtered to avoid EMI), it was NOT NECESSARY a circuit that "measures" the intensity and "shape" of the short-circuit current generated by the charging coil. Therefore, there must be another reason. 

I believe that the motivation of this design is to incorporate an additional safety measure that prevents the accidental firing of the ignition in manual rotation operations of the propeller. It is known that in the Rotax 912 you have to routinely do engine "burp" operation. It is possible that one or both cut-off switches to be inadvertently left alive or the ground connection of the kill cable to be not good enough.

In the Heavy Maintenance Manual there is a section "Cut-in speed of ignition" that says: "Ignition must cut-in between 150 rpm. and max. 220 rpm. of crankshaft speed." In addition, this feature should "To be checked with stroboscope and inductive pliers".

Therefore, to comply with this specification it seems that it is necessary to include additional electronics.

You’re welcome Juan, I’m glad you found that useful. 

The open circuit voltage from the stator charging coil is in the 10’s of volts, not the 100’s of volts needed to charge the ignition capacitor.  The function of the MOSFET is not a simple ignition enable or disable, it’s intent is to drive an inductor similar to that of a DC-DC converter. The primary difference is our inductor is powered magnetically, not from a DC supply. 

Hi Mike.

I really don't understand what you mean by "MOSFET is not an ignition enable or disable, it’s intent is to drive an inductor". I believe that this statement is not correct.

If I am understanding correctly, you are using a grid transformer to simulate the charge coil.  You are not using a real charge coil with a real magnetic rotor, aren’t you?.

That means you are using 60Hz “sinusoidal pulses” equivalent to 60Hz*60sec/5=720RPM. 

The wave forms of a real charge coil will be very different, in both cases short (current) and open (voltage) circuit. 

On the other hand, on your test bench to obtain your graphics, is the capacitor being discharged? I think the sharp peak voltage you get may be because the capacitor doesn't discharge. 

Under operating conditions, the sequence of electrical events is as follows 

1) Each positive cycle of the current generated by the charging coil starts making the MOSFET work in saturation zone and the small initial current is grounded. (pag 3 Mike doc)

2) This current increase and is "measured" by the low-value resistance in source of MOSFET that acts as a "current shunt". 

3) As soon as the current going to ground reaches both conditions:

            1) a stability (flat, plane) that allows to unlock the BJT transistor, and 

            2) a certain level that allows the thyristor to be fired (0,8v in gate) 

the thyristor is effectively fired (and remains in conduction until the current is extinguished) and the MOSFET opens (pag 6 Mike doc)

4) The current stops going to mass and start to circulate to CDI main capacitor. This current progressively increase the charge of the capacitor. I don't think that, in real operative conditions (real charge coil, capacitor discharged), the shape of this current will be a high/narrow peak like showed in pag 8 Mike doc.

5) In this or in another cycle of the wave of the charging coil, the discharge of the capacitor must occur. (pag 2 Mike doc)

Some view the MOSFET in this application as a simple ignition on off switch. The actual function is more nuanced. 

If the MOSFET doesn’t turn off the capacitor won’t charge, 

If the MOSFET never turned on the capacitor would charge to no more then 1/2 peak to peak stator voltage. This would be a fraction of the voltage needed to run. 

The design intent is for the MOSFET to turn off as the wave peaks, the inductive flyback charges the capacitor. 

The testing supply is a simple adjustable bench unit to power the module for the specific purpose of  testing and quantifying the Rotax “no spark at cranking speed” failure mode. By scoping the power wire to the module (orange wire) it’s easy to identify what’s happening inside, and it’s non invasive

what do you think of this schematic which consists in eliminating the 26° signal to obtain a soft start

will you take the risk of installing this assembly on your Rotax 912,

Know that I respect the designer of this schematic but I would like to know the most opinion on this modification to obtain a soft start

Jacques