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Re: Advice needed on capacitor tests
"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."
You can't determine the secondary voltage of an NST loaded with a
capacitor by just measuring the primary voltage. The breakdown length
of the spark gap is a much better indicator of relative voltage. I have
observed effects similar to yours, in which I was monitoring the
secondary voltage with a volt meter, and increasing the primary voltage
with a variac. By coincidence (since the effect depends on the
capacitive loading), at about 20 volts input the output voltage jumped
up to quite near the transformer rating of 9 kV. I attribute this to
change in the leakage reactance with flux density in the core, and think
it is normal behavior for such iron-cored devices. Bottom line might be
that you should never run without a safety gap, particularly when using
so-called "matched capacitance", which is the correct value for series
resonance and can lead to insulation failure if the core doesn't
saturate first.
Ed