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Re: rolled caps
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.
> Question (2): I assume the plastic should be clean. Is LDPE
sensitive
> to soap, or should a damp cloth work fine to get the dust off?
> Question (3): Will three layers of 30 mill also work for 15kV or
do i
> need more?
>
> 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