CDI systems info wanted

[Carter Shore]

Arnaud,

There are some schematics of older CDI boxes out
there, that utilize only a couple of transistors, and
no IC's to produce a simple and reliable CDI function.
The simplicity of these systems allows easy analysis
of the the fundamental CDI operation. Attaching a
scope (with suitable HV probe) while operating is very
revealing.

I'm not quite sure what advantage you seek by
eliminating the capacitor. It's an integral part of
the system's operation.

The coil and capacitor form a tuned circuit that 
'rings' when the cap is discharged into the primary of
the coil, usually by an SCR. In fact, if an SCR is
used, there must be a freewheeling diode connected
accross it to allow the coil current to flow in both
directions as the circuit rings. There also must be
some protection from the pulses for the inverter
circuit itself.

The amplitude of the ringing decreases of course with
each cycle as the energy is lost through spark gap
ionization energy, and other parasitic and resistive
losses. At some point, (hopefully after at least one
spark at the plug), there will not be enough voltage
to fire the spark. Recharging and retriggering ( a la
MSD ) can be accomplished at lower RPM.

A single high voltage (400-600 V) power inverter can
easily supply enough energy to run several different
capacitor/SCR/coil sets, one for each spark plug. This
gives the simplicity and economy of a single HV power
stage, with the flexibility and fine control of
individual distributorless triggering.

Does anyone know for sure if the current crop of
coil-on-plug modules implements a true CD circuit, or
are we really talking about an array of tiny Kettering
ignitions?

Carter Shore

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