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Re: rolled caps




From: 	Phil Chalk[SMTP:philoc-at-ozemail-dot-com.au]
Sent: 	Saturday, September 06, 1997 9:28 PM
To: 	Tesla List
Subject: 	Re: rolled caps

Tesla List wrote:
> 
> From:   Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
> Sent:   Saturday, September 06, 1997 10:05 AM
> To:     Tesla List
> Subject:        Re: rolled caps
> 
> >
> >
> > From:         Mad Coiler[SMTP:tesla_coiler-at-hotmail-dot-com]
> > Sent:         Friday, September 05, 1997 4:16 PM
> > To:   tesla-at-pupman-dot-com
> > Subject:      rolled caps
> >
> > To all that have made rolled caps,
> >
> > I have just recieved my order of 30 mill LDPE sheets. I plan on
> > making the dielectric three sheets thick. This would be 90 > > mills, and I hoped this would be enough for 12000V.
> >  Question (1): Should you put aluminum flashing between each 
> > sheet of the dielectric or just set three sheets together 
> > without anything inbetween.

I am just about to head off to the garage to complete my second
poly/foil stacked plate cap, and face the same question.  In this one
I'm compromising a bit on dielectric thickness to fit the capacitance I
want into the volume available.  I've settled on 72mil poly (last one
was 80) as 9 sheets of 8mil.  I will be putting an al sheet between
every 3 sheets of dielectric, and taking connections out every ninth. 
(If I were using 10x8mil sheets, I'd do it every two.  I don't like the
idea of single sheets of _any_ thickness.) This should result in a
higher-Q cap than simply having 9 sheets of PE between Al plates. Of
course its easier to do in a stacked cap than a rolled one !  I use
'catering foil' just thick al foil & believe its just as good, & easier
to work with than flashing.  I am using many plates in parallel, though,
so currents are well distributed.  In a rolled cap with connections at
one end of the plate only, flashing may well work better.
Further to what Fr. Mc Gahee has said: For a given plate area &
dielectric thickness, you'll have say X capacitance & Y dielectric
strength (voltage rating).  If you split your cap into (my case) 3, by
interposing 2 additional plates between each set of 'main plates', you
now effectively have 3 caps in series, each of 3X capacitance & Y/3
voltage rating - the series connection gives the same cap & voltage
rating as if they were not there. (neglecting the nonlinearity in PE
voltage rating with thickness) The Q however is improved due to the
thinner dielectric 'per cap'.  This works however many times you split
it, but it is strongly recommended to use at least two sheets of PE
between each set of plates, for reasons of imperfections etc already
discussed. I can't see much difference between doing this & physically
building 3 separate caps, except for the coefficent of sweat.  My two
bob's worth.


> >  Question (3): Will three layers of 30 mill also work for 15kV  > > or do i need more?

Can't say for that config. but if it helps at all, my first plate cap is
0.0085uF & has 20x4mil PE between plates for 80 mil total. Now used at
15kV 60ma & still going strong.
> >
> > Mad Coiler
> >
> Dear Mad Coiler,
> If you want to get the absolute MOST out of the 30 mil polyethelene,
> then make THREE caps using 30 mil sheets and then connect the
> capacitors in series to get the best voltage rating. As has been
> stated on this list a number of times in different ways by various
> individuals, the voltage rating PER MIL of dielectric insulation
> INCREASES as the thickness of the dielectric DECREASES. Note that I
> said voltage rating PER MIL. I also want to say that the Q or Quality
> Factor of a capacitor also increases with decreasing dielectric
> thickness. Thus, three 30 mil caps in series will be MUCH better than
> a single 90 mil cap. The trade-off is you need more flashing or
> aluminum foil and more containers and oil. For the slight increase in
> cost, you get a MUCH more ROBUST TC capacitor, and one that is more
> versatile and higher quality. (Better discharge characteristics).
> 
> Something that we have to keep in mind is that the destruction of
> capacitors in Tesla coil use is very much related to breakdown due to
> HV corona. In GENERAL, you should not attempt to make the dielectric
> breakdown rating of a single capacitor section greater than 12KV when
> it is to be used in Tesla coil service.
> 
> Above 12KV, the corona gets so wild that it will destroy the cap,
> especially around the edges. Avoid sharp edges. Curve corners.
> 
> 30 mil thickness is a really good thickness to use. By sticking to
> just a single 30 mil thickness PER SECTION, you will make the job of
> rolling the capacitor much easier. (Remember that you use two
> dielectric sheets in a rolled cap, so you will be rolling a total of
> 60 mils thickness...)
> 
> Roll just the two uncleaned sheets of plastic up around a 1" diameter
> PVC pipe to get an idea of the diameter of the completed roll. Add a
> bit for the flashing, and then get a PVC pipe whose INSIDE diameter
> is large enough to accomodate your cap.
> 
> Another advantage of building several lower voltage and higher
> capacity caps is that you have greater freedom to put them in series
> and parallel combinations. For example, four .04 MFD at 6KV
> capacitors could be connected singly and in combinations to make:
> one  .01   mfd at 24KV (four in series)
> one  .0133 mfd at 18KV (three in series)
> one  .016  mfd at 15KV (two in parallel with two in series)
> two  .02   mfd at 12KV (two in series)
> one  .0266 mfd at 9KV (two in parallel with one in series)
> one  .03   mfd at 8KV (three in parallel with one in series)
> four .04   mfd at 6KV (single)
> one  .04   mfd at 12KV (two series groups connected in parallel)
> one  .06   mfd at 6KV (two in series with one in parallel)
> one  .08   mfd at 6KV (two in parallel)
> one  .12   mfd at 6KV (three in parallel)
> one  .16   mfd at 6KV (four in parallel)
> 
> As regards your question about cleaning, you can use alcohol to
> clean. This has the advantages of being fast-drying and
> non-contaminating. You can use regular isopropal rubbing alcohol
> available at any pharmacy or drug store. I would not want soap
> contaminating my LDPE, although I am sure that there are coilers out
> there that have used soap and water successfully. But why even take
> the chance? Besides, alcohol is something every coiler should have
> around as a general purpose cleaning agent. Use stuff that is as
> lint-free as possible in the cleaning process. Also, wash your hands
> thoroughly and dry them before beginning the final assembly.
> 
> As you clean the LDPE it will usually develop a hefty static electric
> charge. Not too dangerous, but it can suprise you if you are not
> ready for it!! The staic electricity will make the LDPE a wonderful
> dust magnet, so you should begin assembly as soon after cleaning as
> possible.
> 
> Have all tie wraps and other stuff ready, and if at all possible,
> have a good helper with you. Explain the procedure clearly before
> beginning. Remember that once your hands are full with the rolling,
> you can't easily point to something or get it yourself. I always get
> the tie wraps linked up before I start rolling.
> 
> hope this helps.
> Fr. Tom McGahee