[Prev][Next][Index][Thread]

Re: Unexplained arcing current



I tried a few new things and found some interesting results.

I borrowed a clamp-on AC ampmeter attachment for my DMM, to monitor the
mains current to the NST.  With my gap set to 3/8", powered through my RC
protection network, the meter read 8 Amps, and the R's get hot.

Then I disconnected my 450pF bypass caps.  With the variac all the way
up, the NST could no longer jump the 3/8" gap.  I narrowed the gap just
slightly to the point where it arced.  Now the ampmeter reads only 5
Amps, and the resistors just get warm.  Clearly the bypass caps are key
to this mystery but I would have guessed that their 225pF in-series value
is so distant from the 10.8nF required for mains resonance, that they
wouldn't have mattered.  Yes, I've double checked the bypass cap value.

I'm using my single gap, (http://people.ne.mediaone-dot-net/lau/tesla/onegap.htm)
rather than my 12 segment cylinder gap.  I know I also had resistors
burning up with that one too. 

With the comparatively low energy arcing that is occuring in these
experiments, I can see the nature of the arc geometry much better (safer)
than in normal operation.  One very surprising thing is that rather than
there being the intended ring of arcing coming from the copper half-torus
which represents the minimum arc distance between the electrodes, 95% of
the arc appears to be concentrated in a very thin channel which is
precisely coaxial with the holes (~3/8" dia) through the brass
electrodes.  I can't see where this channel terminates, but it is
continuous and appears to extend well into the electrodes.  I'm now
wondering about the wisdom of sucking air through the arc, as opposed to
blowing.  The sucking may result in a lower air pressure in the channel,
which will result in a lower breakdown voltage per length, and a longer
arc with higher resistance.  Perhaps the arc channel connects to the
electrodes through a broad area inside the electrodes where the air
pressure is at a minimum...

My simulation model exhibits the same current spikes through the
resistors as yours does, but neither model predicts abnormal power
dissipation in the R's.  I've uploaded a model onto my web page.  There's
no link to it but you can find it at:
http://people.ne.mediaone-dot-net/lau/tesla/vgap.sch
It's a very partial model, mostly for the purpose of modeling the static
gap behavior, but with my protection network thrown in too.  BTW, thanks
for pointing out the need for the "extra" resistors in the model.  Those
convergence problems were driving me up the wall!

Regards, Gary Lau
Waltham, MA USA

>Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
>Hi Gary,
>
>	Hmmmmmmm....  I "thought" I had it figured out...
>
>I have suspected that some resistors have poor high frequency
>characteristics that contribute to heating.  But you are using low
>inductance types that I assume have good high frequency performance.
>Probably ceramic cores with a thick film resistive past that is similar to
>those used in water cooled high power RF loads.  You resistors are probably
>still perfectly good at high frequencies...  I doubt if very high frequency
>noise is causing the problem but...
>
>If you have a MicroSim model (the *.sch file) for your circuit I would like
>you to send it to me at terrellf-at-uswest-dot-net so I can check it out.  We
>really need to get to the bottom of this problem.  Many people have
>reported this unusual heating but apparently the cause is still a mystery.
>It does not seem to affect my system but it does others.  Something is
>going on here I don't think we understand...
>
>I will visit your web site tonight to get as much info on your components
>as I can.  I would be especially curious as to what type of gap are you
>using.  Your page mentions two.  I wonder if the heating changes with the
>type of gap used?
>
>Cheers,
>
>	Terry