Rally 200 with Ducati CDI

This page describes my effort to replace the original Femsatronic CDI with a standard P model CDI (Ducati) for use in Rally 200.

I know this issue has been discussed and explained elseware, but even though my Femsatronic ignition seemed to function I wanted to replace it with something more easily available in case of a malfunction.

The original Femsatronic CDI has the spark plug cable molded in the housing, so in case the cable broke at that point it would be impossible to fix.

The Ducati CDI seems more sensitive so it starts producing sparks at a lower rpm, which in theory should improve starting.

Differences between Femsatronic and Ducati

There are no easily available sources for the detailed electronic circuits in the two CDI's. Their function is the same: a diode rectifies the AC voltage from the stator coil to about 200Volts. This charges an internal capacitor and at the time of ignition a silicon controlled rectifier (or Thyristor) conducts in such a way that the capacitor is discharged into the high tension transformer which raised the voltage to more than 10 Kilovolt.

The capacitor is charged with the positive cycles only, the negative cycles are not used. But the Femsatronic designer (for some reason) has found it neccesary to load the negative cycles with a 250 ohm resistor. This resistor is positioned in the stator back plate at the same place where you normally find the capacitor for the points ignition. It also has the very same appearence so most people think this is a capacitor. It is NOT.

This resistor is not used by the Ducati CDI, therefore the blue wire is not connected in the new setup.

A more important difference lies in the trigger circuits for the SCR. These are not well shown or explained anywere, but someone has by experimentation found that the trigger output from the Femsatronic should be loaded with a resistor and a capacitor in order to trigger the Ducati SCR properly. Without these components the ignition becomes erratic or out of timing.

My engineering nature forced me to do my own research on this problem.

The following table shows the results of the ignition timing from various resistances and capacitances.
Measurements done at idle speed (first number) and higher rev. (second number) with stroboscopic timing light.
Values in paranthesis are erratic.
Resistor Capacitor
  None 1F 2F 5F
None 27 - (30) 25 - (23) 24 - (27) 21 - 16
1Kohm 27 - (32) 25 -   23 23 -   21 22 - 18

Specifications call for 24 degrees before tdc

Conclusion: 25 - 23 degrees is best, so 1K resistor in parallel with 1F capacitor. A higher value capacitor might also be used.

Drawing shows the Femsatronic diagram. Resistor "R" loads negative pulses through diode D2, while D1 charges capacitor C1 to 200 volts. "P" is the coil to trigger SCR through diode D3.
Ducati CDI is designed for a different trigger coil

This drawing shows how the stator wires are connected to the Femsatronic and to the Ducati CDI. Note the position of the resistor and capacitor when using the modern type CDI.

IMPORTANT: use a ceramic or foil capacitor not electrolytic or tantalum. This is due to the nature of pick-up coil output which is both positive and negative pulses.

Recommended capacitors from Digi-key:

through hole:  445-2857-ND
Surface mount: 490-4795-1-ND

Note that the blue wire and internal resistor is not used.

 

Pictures show the setup on my 1974 Rally 200. A 1Kohm resistor and a 1F ceramic capacitor is mounted on a circuit board glued to the Ducati CDI.
If leaded components are preferred they may be soldered directly to the spade connectors.

Jan Haugsted
jha@tpradio.dk

www.vespa-klub.dk

 


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