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Re: Advice needed on capacitor tests

At 06:31 PM 4/1/99 -0700, you wrote:
>Original Poster: Alex Crow <user-at-alexcrow.clara-dot-net> 
>
>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!

Sounds like you punctured something.  The 50Hz buzzing sound is an arc
inside your cap or transformer.  Testing a cap in this manner is very hard
on components and is not neccessary.  The low loss, high-Q 50Hz circuit can
instantly ruin caps and transformers unless your safety gap is set very
narrow.  I had a similar experience testing a wine bottle capacitor with a
15kv NST.  I used no safety gap, and for a few amazing seconds, the HV
actually jumped AROUND the ceramic output bushings to the case of the
NST--a distance of over two inches.  Then the wine bottle punctured.
Fortunately, the cheap, replacable bottle failed before my transformer was
ruined.  Every wine bottle I tried failed within seconds.  Fortunately I
had an entire case of wine bottles (L2.00 for a whole case at the Banham
boot sale in Norfolk).  When I used them with my Tesla coil, I never
experienced a failure.

The best place to test a Tesla capacitor is in a Tesla coil.  Use a 50W /
1Kohm power resistor on each transformer bushing.  This will reduce the "Q"
of the 50Hz circuit and limit excessive voltage rise.  Also, put your spark
gap in parallel with the transformer so it can do double duty as a
transformer safety gap.  Tune up the system at low power before giving it
full power.  This will allow the Tesla coil to act as a load on the
transformer & capacitor.  Output sparks will carry away the power output of
your transformer so it can't flywheel back and forth through your cap in
runaway resonance.  I assure you your cap/transformer combination will
behave very differently when connected to the load presented by a tuned
Tesla coil.  Testing your cap with only a transformer is almost certain to
ruin the cap or transformer.  From what you've described, it sounds like
you've already punctured something expensive.  Sorry Alex.

Greg


>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
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