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Re: Advice needed on capacitor tests
Hello Alex,
>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.
If I understood your setup correctly, you just paralleled the cap across
the NST and cranked up the voltage (meaning no TC connected) via
variac, right?
I hate to say this, but it sounds like you blew a hole through the poly
dielectric... (:o((. The hissing sound you described, sounds exactly
like a miniature "flame" going from one plate to the other (through
the dielectrica). Iīm afraid you will have to investigate this further.
First, disconnect all equipment from your cap. Then take an DMM
(20Mohm range) and measure from one plate to the other. (To be
safe, short the cap first). You should get an "infinity" reading. Any
kind of resistance reading means the cap IS bad. But even if you
get an infinity reading, you will still have to seperate the layers
and take a closer look. More than likely, you will find a hole
and/or carbon tracking.
Another, but less likely, possibility is that you blew your transformer
(due to voltage rise). However, this is easy to check. Just remove
the cap from the xformer. Place the spark gap (1/4" apart) parallel
to the xformer. If your xformer is center tapped, use a dual gap
(i.e. from one hot leg to ct to the other hot leg) Slowly crank up
the voltage. Your gap should fire easily at 50% of the variac setting.
PE has a very BIG disadvantage using it as dielectrica.
Temperatures above 60-70°C are very "dangerous" in terms
of PE-longliveity. PE starts melting (it is a paraffinic compound)
above 120°C. The smallest of air inside a (more or less) dry
fired cap WILL produce corona. Not only does this corona cause
the PE to decompose in a chemical fashion (see my partial
discharge mail and have a look at Gary Lauīs website), but
also, the corona heats and weakens the PE. I tried to get
around the oil in rolled PE caps, but none of my caps lasted
very long. Every dry fired PE cap I took apart, had a yellowish
discoloring around the plates. This is part of the paraffinic
componds actually being boiled out of the plastic.
BTW: The reason you were getting "loud" bangs instead of the
steady state (that is: w/o a cap installed) sizzle, is because you
were charging the cap. At a certain voltage level, your gap
conducted and the cap dumped all the energy into the gap. This
discharge is much more powerful (in terms of Watt-seconds)
than the xformer alone. As the cap needs recharge time, you
donīt get a steady discharge, but rather a short bang,
followed by a short silence and then it starts over again.
If your cap HAS died, donīt become discouraged. Every coiler
has fried caps, transformers, etc. Itīs just part of the game and
should be viewed as a learning experience (only). You might
want to consider the route that Richard Hull, Terry Fritz have
tried, and Gary Lau and I have gone: Build yourself a MMC
(series / parallel arrangement of many small caps). There
are several advantages to this, one of the best being that
even if you DO fry some of the caps, you will always have
some left, that you can reuse. Any kind of homebrew poly
cap that fails, is unrepairable 99% of the time. I would like
to dispell a myth here: The MMC is NOT more expensive
and NOT more work to build than a rolled (or other) PE
cap. It is a different kind of work, true, but it is not more
difficult or more expensive.
I believe you asked about using ceramic caps. If you mean the
small disc type caps, forget it. They are way too lossy. They
will heat up and crack in a matter of minutes (sometimes
seconds). They do have one possible usage in a TC, tho. Use
them as bypass caps in your RCR filter (a la TF). Go for a 4-5
times voltage rating on them. For your 10kV xformer this would
mean 20-25kV per half (if the xformer is center tapped). 7.5kV
ceramic units are easy to get (try RS components in the U.K.)
and are nF/$ or B.P, in this case, wise the best buy. They
come in bags of ten, so one bag will suffice. Use four in a row
(times 2) for a total of 8 in your RCR filter.
Coiler greets from germany,
Reinhard