knock ?

[garfield at pilgrimhouse.com]

On Mon, 16 Mar 1998 22:48:42 EST, JCs DOOR <JCsDOOR at aol.com> wrote:

>On SAAB cars  ,80 volts  is put on the tip of spark  with a high voltage diode
>( 70000 v. or better. when plug fires  or cycl.dets.  80 volt dropes to
>ground
>as gas conducts. sorry its not any harder then that.

Not contradicting Jim, but my guess is the 80 volts is dictated by what
is necessary to forward bias a diode stack capable of 70K+ breakdown,
more than anything else. Here's why.

Several vendors (Fagor & Fuji, that I know of) make HV diodes for
industrial & medical usage (microwave & xray HV supplies), that are 12KV
each and have a forward bias V of between 8-12V per diode (these diodes
themselves are actually several diodes stacked in series). Stack them up
to be able to stand up to spark HV, and you're looking at around 80v
just to forward bias the diode stack. There is of course no such thing
as a single junction diode of "70000 v. or better", just alot of diode
junctions in series, but that wasn't Jim's point, anyway.

These same diodes are used to make the diode "coil joiners" (us digital
guys just call them HV "OR"s, heh) that allow redundant ignition coils
to fire one plug in eXperimental aviation IGN systems (thas how I know
bout them). In this case they're conducting the spark current in one
direction (NOT microAmps by any means), and blocking an opposing HV
spike from the other side, so the HV diodes in the "ionization
measurement" scheme needn't be as beefy as these, but I still suspect
the measurement potential for checking to see if there are conductive
ions in the aftermath of an explosion, is dictated more by the forward
bias necessities of the diode stack, than anything else. Thas me wee
observation, maties.

Thanks, Jim, for providing the practical hint as to how one goes about
measuring microamp currents, just after an electrical holocost has
occured. With the concept of a diode stack forward biased with say
60-100V, but opposing the HV spike, the only remaining practical
question I have is, how long of a period do you wait from the HV ringing
of the coil going ape-shit, before you can measure the forward current
in this diode stack to see if you have HAD combustion or a dud. Seems
like if the mixture *didn't* blow, the ringing could stretch out into
the very time period you needed to measure to find out IF you had
ignition or no. Curious stuff, this. But fun, no question.

Garfield

P.S. Yes, I DID look at the waveforms on the page pointed to, but I
didn't see any scope shots of what happens, and how long the coil
secondary rings, if the charge *DOESN'T* explode. Did I miss sumpin?
Member, my eyes ain't so good anymore, specially with all this Geritol I
been taking.