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Re: Advice needed on capacitor tests
Hi Alex,
I haven't been through all the posts of the last five days
yet but I'll comment now for what it's worth (others will too I
suspect):
> 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!
So the gap fired at 1/4" setting? You'd be looking at around 8kV or
so I'm guessing. Sounds like your cap is resonant with the
transformer. At that variac setting, how often did the gap fire?
> 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.
Definitely resonant charging. And probably a blown cap.
> 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
If you back the variac off to zero, reset the gap to 1/4" and bring
the variac back up to 20V i/p, does the gap start firing again?
Malcolm