Coils for Ion

[Walter Petermann]

Raymond C Drouillard wrote:
> 
... Instead of looking at the sensor resistance, I decided
to calculate
> the impedance of the spark gap.  Based on your statement that the current
> is around a microamp, and the voltage is 80 or 400, the impedance of the
> gap is in the neighborhood of 80-400 meg ohms.  Of course, it's this
> impedance that we are, in effect, measuring.  While at work (I don't have
> access to the web at home, except for email), I looked at some of the
> URLs that I got off this list and found some secondary inductances quoted
> at 40 Hy or so.  Taking the worst case (80 meg ohm and 50 Hy), I get a
> 3db frequency of around 150 KHz.  That sounds kinda high... I think I'll
> double-check my math later when I'm less tired.
> 
> Anyhow, I wasn't considering the ringing because I figured it would have
> to decay before we can get a good reading, anyhow.  Also, I believe that
> the ringing is due to the primary and the condenser (as in points, plugs,
> condenser) forming a parellel LC circuit.
> 
> Cheers,
> Ray Drouillard, KA8UUU
> 
  ..I get the same, ~150Khz. I have read that the Ion
current measuring circuit
should have a bandwidth of 10Khz (1). Maybe this is a way
around the
problem of 'electrostatic fouling' (2) when high bias
voltages are used between electrodes.

What if question:
If you apply a 400v, 50Khz-100Khz ac signal as the 'bias'.
Would this eliminate the fouling since
the signal is ac?
Then you could recover the Ion signal (like recovering a
radio AM signal) with a 
diode and a 10Khz filter.
The 400v is (supposedly) already available as the charging
voltage for the cdi ignition.

  Walt

(1) Pat 4,359,893
(2) Pat 4,862,093