RE: HV cap flashover, :-0 repost

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <biomed-at-miseri.winnipeg.mb.ca>


Date: Tue, 30 Oct 2001 20:25:51 -0700
From: "Tesla list" <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com
Subject: RE: HV cap flashover, :-0   repost
Message-Id: <4.1.20011030202550.013f1398-at-pop.dnvr.qwest-dot-net>
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Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Gary.Lau-at-compaq-dot-com>


>Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
><biomed-at-miseri.winnipeg.mb.ca>

>Comments added to last reply
>Shaun Epp


>>The fact that your gap fires when your variac is only at 50% proves
that
>>there is some mains resonant-rise occurring.  If there were none, the
>>gap would only fire when your variac was at 100%.  Even though .02uF
is
>>LTR, there is still some resonant rise, though I'm also surprised it
was
>>this bad.  So this leads to the next question.  Just how far apart is
>>your gap?

>My spark gap was set for low voltage,.. 3/16" across 2 gaps, for a
total
>gap of 3/8", as you must have guessed is a static gap, but tungsten.  I
>wanted to test is at lower power settings before running it at the full
>120volts in.  This is what I was going to do just before it died.


>>3/8" is quite wide for a 12kV power supply.  With my 15kV NST, my single
>>gap is just under 1/4".  You may have set the gap with the variac at a
>>low setting, but I suspect that it was done with your cap and primary
>>connected, allowing mains-resonance to boost the voltage.
>>It should be set such that it will only begin to fire when
>>hooked up to just the NST.  That there was arcing over 7/8" means that
>>there is some seriously high voltages preset.  Judging by the
resistance
>>across your caps, it sounds like one or more of them may be toast,
>>although a bad over-volted cap may not necessarily show anything with
an
>>Ohm meter.

>I don't think fixing my existing cap would be worth while, Do you?,  is
it
>possible that by running it at lower power, that I set up additional
>resonances?, I'd have to look at the Mains resonance info again,... I
guess
>it shouldn't make a difference.  Could these caps that I'm using be
doing
>it?  They look like fat disk capacitors  1/2" thick and 1-1/8"
diameter,
>I've paralleled 14 of them, and they servived the external flashovers
:-).

>>The good news is that your NST is still working.

I was going to suggest that loosing just your cap wasn't that bad, since
caps in X-ray equipment are made just for DC filtering applications, not
pulse discharge duty.  But when you said that you were getting some
satisfying 1.5 foot sparks just before the caps toasted, I figured your
experience counts more than my arm chair speculation!  Plus it's
probably not what you wanted to hear.

I don't know if there's something about your ceramic caps that make them
more prone to mains resonance.  I do know that ceramic caps exhibit some
voltage-dependant nonlinearities, but whether this could lower the value
enough to bring them to mains resonance, I just don't know.

That your caps now measure a significant leakage resistance is not at
all a good sign.  Use 'em till they smoke, but don't count on a long
life.  But most importantly, get that gap set correctly - with the caps
disconnected.  Performance will be diminished with a narrower gap, but
your caps and NST will thank you.

Regards, Gary Lau
MA, USA


Thanks for the insites, Gary,

I set the gap from a suggestion from this list service to be inbetween 1/4"
to 1/2" total.  From what your telling me, this was way to wirde.  I'll
repeat my understanding of what you are telling me.   Run the gap across
the transformer without the primary cct connected, and adjust so that it
just starts to arc across the gap at normal (120Vac) voltage into the NST.
Is this the correct way to set up a spark gap??
 Before this I had the gap set as I mentioned in this post, at 3/8" and
only running 60Vac into the NST.   That's serious resonant voltage rise!!!.
Looking back now it was probably not regular 120 flash overs per second.
How does this sound to you?

Shaun Epp