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[Answered my own question: Advice needed on capacitor tests: update: BIG resonance!]

I answered my own email in the end- sorry! It was resonant charging, I
just didn't expect that much of a rise - I set up a spark gap without
the cap for 5kV, so it would just light, and then attached the cap.
Turned up the power, and bingo! A great raaaaat from the gap and plenty
of nasty fumes and a bit of burnt carpet where one of the Alu leads had
come loose and ingnited....
Looks like my cap is OK for 5kV after all. I was thinking this thing
would be really low Q and then I got the surprise of my life - this is
the first time I've really dealt with resonant stuff of this level -
previously much of my work has involved high voltage DC... Just goes to
show how you can surprise yourself by working things out, even if you
think at first the theory doesn't quite fit.

Sorry to bother you all

Alex (shaking)

Message-ID: <3703A32A.82B494EE-at-alexcrow.clara-dot-net>
Date: Thu, 01 Apr 1999 17:47:38 +0100
From: Alex Crow <user-at-alexcrow.clara-dot-net>
Reply-To: user-at-alexcrow.clara-dot-net
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To: Tesla List <tesla-at-pupman-dot-com>
Subject: Advice needed on capacitor tests: update: BIG resonance!
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Dear all,

I will add to this after another test....

I hooked up my cap across my NST again, but this time put my multimeter on
the output of the NST. I turned up the voltage, and at only 1 or 2 percent
of line voltage I was getting about 600-700 volts on the meter!!  I tried
it without the cap and all I was getting was around 80-100 volts on the
same setting... All I can think of is that the NST windings are resonating
with my cap at 60Hz and causing this voltage rise of about 6 to 8 times!!
Instead of shunts, these things have a small gap in the core which provides
enough leakeage inductance to limit the current.

In fact, I'm just doing the test again right now - with the cap connected I
set the variac for an NST output of 800V. I removed the cap and I'm only
getting about 85V. This must mean that I was actually getting about 10kV
across my cap, no wonder I got the big sparks! this is a rise of nearly
10x!  Help!!! does this mean my xfrmr is useless for Tesla work? I can't
see that my RC filter (desingned for the Tesla frequency) is going to stop
this!

Please respond, I am at my wit's end as to what to do - can I fill the gap
in my core and add limiting on the input to my NSTs? maybe this would
reduce the inductance and stop the ringing. Help!


Alex Crow wrote:

> Dear All,
>
> I finished my first layered cap today, and decided to give it a test
> run. I had oiled all the plates as I built it, and given it a quick duck
> in a bath, but the testing was done 'dry' so to speak. I hooked the
> thing to a variac and gapped the terminations about 1/4 inch apart for
> safety. I turned up the voltage on the variac, and got really loud
> sparks across the termination leads at only 20 volts in!
>
> Then I increased the gap to nearly half an inch, and got huge,,
> deafening bangs at just about 32 volts input to my neon, which I worked
> out was only just over 1300V on the output! I then gapped further, and
> just past 35 volts in the gentle hiss from the cap lowered in pitch very
> suddenly and got a lot louder - and no more sparks on the output.
> I bumped up quickly to 45V in and still no spark, but the hum stayed the
> same.
>
> To put it bluntly, this worries me. I have layered my plates with an
> inch of gap on all sides - I was hoping this will be enough with 2 caps
> in series across my 10kV NST. I have heard that others have been fine at
> 15kV with 1.5 inches of minumum inter-plate distance at the edges. But
> if I can only get up to a fifth of the voltage on the input at most in
> this 'damp' state (admittedly there are air gaps at the edges) can I
> expect them to take 5kV each when they are fully flooded with oil? Is
> this just a case of me being premature and should I wait until I have
> run them in in my deep-fat bath before I get too worried? i know this
> sounds like corona discharge drawing extra current from the transformer
> and thus limiting the output, but will my corona be five times less
> severe when there's no air left in the thing? Maybe it's doing resonant
> charging (0.04uF on a 10kV, 150ma short-circuit current neon) and I'm
> getting a serious voltage rise)?
>
> This cap took me two days to build and I don't want to go through it all
> again!
>
> Any help would be deeply appreciated.
>
> Alex Crow