RE: Gap Current/Voltage Waveforms

Hi Gary,

At 09:35 AM 8/5/99 -0400, you wrote:

>>	Terry
>Hi Terry:
>What a wealth of information those traces convey!  I have a few questions
>and observations.
>On the gap voltage traces, following firing, the voltage seems to settle
>towards a non-zero level opposite in polarity from prior to the bang.  Is
>this just a DC offset problem with your probe?

I don't "think" it is.  BUT...  It sure likes like an artifact from a DC
blocking filter...  I will have to review the design of the probe and
perhaps do some modeling and some more tests to determine if this is real.
It sure "looks" like a DC probe error...

>Just following the start of gap conduction are at least two modes of
>oscillations on top of the tank resonance, no doubt due to parasitic L's and
>C's in the tank.  What I find surprising is that most of them occur only
>following the initial conduction.  

This appears very real and I think I have found it's nature.  I am working
on a model of this that seems to show this too.  I will post it once it is
ironed out...

>In some previous scope traces that you
>published, major VHF oscillations were re-excited following each zero
>current crossing, but in these new traces, there is relatively minor
>activity at these crossings.  I was about to suggest that the difference may
>be due to single vs. multiple gaps, but the earlier experiments were done
>with a single gap, and your latest traces were done with a 2-gap RSG and
>probably comparable in quenching behavior.  

The heavy current crossing spikes were on my multi gap and static gaps both
of which should be far more likely to quench.  This gap does everything
possible to reduce gap resistance and let the quenching be darned...  The
spikes really should be far more suppressed in this new gap.  It is
interesting to note that the spike are both in the positive direction.
This indicates that they are beyond the frequency response of the
transmitter probe and it is just being turned on and loosing the real
frequency information.  In this probe, that frequency is well beyond 100MHz...

>In both cases, no secondary was
>present.  It would be significant if it were possible to eliminate these
>parasitic oscillations by choice of gap and primary geometry's.  Is this
>what is happening, or is this a measurement artifact?

Something that is REALLY interesting is that the voltage and current are
out of phase a bit.  Also, the voltage across the gap is so high that there
should be a significant loss in the gap.  If I were to put a resistance or
zeners across the gap to simulate the gap voltage, the system would have
high loss and would ring down very fast.  However, if appears that the gap
is highly INDUCTIVE!!.  The voltage is high but the function is an inductor
so little power is being lost.  Quick calculations show the inductance to
be about 20uH.  This inductance, with a bit of capacitance, also explains
the first high frequency oscillations.  I need to play with this more but
it is all falling into place well....

Also note that the voltage across the gap is basically a sine wave...
Also, I think the polarity is reversed in one trace.  When the voltage goes
"up" so should the current.  I just cliped on the leads without ever
checking this...



>Gary Lau
>Waltham, MA USA