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Re: Glass/poly and more...
From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: Sunday, September 14, 1997 10:18 AM
To: Tesla List
Subject: Re: Glass/poly and more...
Richard, Greg, Dr. Resonance, and others,
All the answers given to Richard Smit so far have been correct, each
in its own way.
Greg is right, the glass is really bad, and adding a layer of poly to
glass won't help much because the glass is still going to dissipate
excessive heat in a Tesla coil application. I also note that Greg
Leyh's reply was meant to be taken with a chuckle, and I enjoyed the
humor it contained.
Dr. Resonance brought up a good point regarding the resulting
non-uniformity of the electrostatic fields that could lead to corona
and breakdown. He rightly pointed out that a group of
series-connected capacitor sections is the way to go.
My own initial reaction to Richard Smit's post was:
* It ain't gonna do any real good, because the poly has a
* LOWER dielectric constant that the glass. The dissipation
* factor of the poly will be better than the glass, but since
* the glass is still the bulk of the dielectric material,
* there isn't much to be gained by using a poly/glass
* combination, since this baby is still going to run really HOT!
I was figuring, what the heck, I have some time free right now, so
I'll e-mail back and tell Richard all the things wrong with his
suggestion. But then I realized that doing that would not really
accomplish very much. The real question at hand was not so much what
did Richard get wrong, but what did Richard get right?
Richard Smit's idea of using poly with glass turns out to be
something of a real bummer. For several reasons. However, I would
like to take this opportunity to point out that while his choice of
materials was not the best, his BASIC IDEA is not really that bad.
Other members of this list have discovered that when they use an oil
with a high dielectric constant, it can RAISE the capacitance of
their homemade capacitor SIGNIFICANTLY, even though it represents a
SMALL part of the total dielectric material involved between the
plates. Most of the time the oil we use has a dielectric constant
between 2.0 and 2.2, and so it closely matches the dielectric
constant of the poly that we use. Thus we often don't see how much of
an effect the oil can actually have on our caps.
Here is a segment from a post to the Tesla List that I made on OIL
recently. Among other things, it contains info I gleaned from a post
by Ralph Down:
> Some coilers have used refrigeration oil and silicone oils.
> Ralph Down used SUNISO refrigeration oil (bought from local >
refrigeration repair shop for $30 for 4 litres.)
> He was pleasantly surprised to
> find that the high dielectric constant of this oil actually
> DOUBLED the capacitance value of his homemade capacitor.
Obviously such a capacitor would loose some of its "extra"
capacitance if it were physically compressed and the oil was pushed
out from between the plates and the poly. A way to prevent this from
happening is to use oil-soaked paper between the plates and the poly.
Personally, I also use oil-soaked paper between the poly/poly layers
whenever I use multiple poly layers between plates. (This is for poly
that is 20 mils thick or more).
By the way, Castor Oil has a dielectric constant of 4.67, whereas
transformer oil runs between 2.9 and 2.2. Has anyone here on the list
used Castor Oil for building a Tesla capacitor? I don't know about
YOU, but *I* am greatly interested in making use of such stuff if it
means I can cut the total physical size of my HV caps in HALF!
A question that *I* have, which I have not seen addressed on this
list before is: what is the long-term reliability and stability of
capacitors made using oil with high dielectric constants?
Ralph Down, how is your SUNISO oil cap working out? Have you measured
it recently to see if its capacitance value has remained the same?
Some people who have done post-mortems on their dead homemade caps
have reported that the oil is often greatly changed in appearance and
smell. Are we talking about transformer oil here, or something like
motor oil? Is the damage that we see to the oil the *result* of the
capacitor's death, or is it perhaps a partial *cause*? Some people
have dissected live, healthy capacitors. What have YOU guys found?
It sometimes takes guts to ask a question on this list. (It takes
guts to attempt to give an answer, too, because sometimes in the
process of answering, we make a mistake or two). But what I like most
about the members of this list is that when all is said and done,
there is a tremendous generosity at work here, a wonderful
willingness to share knowledge and enthusiasm. To ask questions. And
to attempt at least to answer them. Thanks!
Hope this helps.
Fr. Tom McGahee
----------
>
> From: Greg Leyh[SMTP:lod-at-pacbell-dot-net]
> Sent: Saturday, September 13, 1997 5:00 AM
> To: Tesla List
> Subject: Re: Glass cap with polyethylene
>
> Richard Smit wrote:
>
>
> > I was wondering! Glass is a bad dielectric for a Tesla Coil but
what if you
> > take a thin piece of polyethylene and place it between the foil
and glass.
> > Will it still be a bad cap or will it be better because of the
poly?
>
> It would be a far better capacitor -- provided that you remove the
glass, as well.
>
> -GL
>
From: DR.RESONANCE[SMTP:DR.RESONANCE-at-next-wave-dot-net]
Sent: Saturday, September 13, 1997 10:21 AM
To: Tesla List
Subject: Re: Glass cap with polyethylene
To: Rich
Using two different dielectrics in a cap is very bad. The
electrostatic
fields will not be uniform which could lead to local corona and hence
a
breakdown. Suggest using single sheets of plastic and then series
connecting each subsection.
DR.RESONANCE-at-next.wave-dot-net