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Re: Advice needed on capacitor tests
Reinhard
Actually it appears not to have failed. The hissing and buzzing I think was
just the movement of the plates and dieletrics, as I have not clamped the
thing tightly, and also corona from the edges of the entended-foil plates
where they hang out of either side. I have tested it and I still get
infinite resistance, so hopefully all is well. However I will be using the
oil bath from now on!
There was one bang from the edge of the bottom plate to its neighbour,
which left a small black mark, but I easily wiped that off with my finger.
Taught me to be a bit more careful though!
BTW, I didn't ask about ceramic caps - I replied to someone else and duly
commented that they would probably be dire!
Thanks,
Alex
Tesla List wrote:
> Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
>
> 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