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Re: Liquid Layered Capacitors

Original poster: "Chris Rutherford" <chrismrutherford@xxxxxxxxxxxxxx>

Hi Chris,
I'm also quite new to coiling, however I did manage to produce a 1KW system that used a home made bi-directional rotary spark gap (<http://www.hackinghardware.com/blog/files/tc.JPG>http://www.hackinghardware.com/blog/files/tc.JPG).
Given that capacitors often fail due to dielectric breakdown and over heating it is likely that you would have to think very carefully about how to keep your system cool when you run it. Without looking in to detail at the breakdown characteristics of oil it is impotent to think about the equations that give rise to capacitance.
(i.e. "Capacitance is calculated by multiplying the relative dielectric constant by the permittivity of free space(epsilon0 = 8.9 x 10-12 farads/meter) and electrode surface area, then dividing by the thickness separating the electrodes. Units are expressed in farads.")
As you can see, the capacitance is proportional to the thickness of the dielectric, therefore in order to get a useful capacitance this needs to be very thin, and constant. With oil and freezing etc it is likely that it would have to be quite thick and uneven, resulting in problems. Also, as far as I know liquid based caps are messy and time consuming.
Most people with a limited budget go for multiple miniature capacitors (mmc). I have however seen a couple of salt water capacitor banks that looked successful. I once looked at making a tinfoil/plastic capacitor by placing sheets of tinfoil inside a4 folder sleeves, but having done the calculations and thought about having to evacuate it to remove reactive o3 i decided against it. MMCs work well, and dont cost much. 

Hope this helps,

Chris R



On 11/26/06, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx > wrote:
Original poster: "Breneman, Chris" <<mailto:brenemanc@xxxxxxxxxxxxxx> brenemanc@xxxxxxxxxxxxxx> 

Hello,

I'm new to building tesla coils, and am trying to get my first one
built.  The problem is that I have very little money available to
spend on materials.  The main thing I'm trying to get now is a
capacitor for the primary circuit, and it looks like my best option
is to build one.  I've seen a number of designs for homemade
capacitors, but came up with one myself, and was wondering if anyone
has tried it, or if there is any inherent problem in the design.
I was thinking that the most efficient and easy-to-build designs were
liquid capacitors where at least one of the plates is a liquid such
as salt water.  The most common design of a liquid capacitor seems to
be filling bottles with the solution and putting them all in a vat
with another conductive solution.  But isn't this wasting space due
to the large plates?  The best space-savers seem to be layered
capacitors, which also seem to be pretty efficient (from what I've
read), but the recurring problem is apparently that the solids don't
get extremely close contact with each other.  So I got thinking ...
what if a layered liquid capacitor could be built?  Or more like
liquids that could dry.
Here's one specific design idea that I had: Inside of some kind of
bottle, put a small amount of salt water, then a layer of olive oil
as a dielectric.  Olive oil will float on top of the water and
generally has a higher freezing point than water
(<http://www.oliveoilsource.com/olive_chemistry_freezing.htm>http://www.oliveoilsource.com/olive_chemistry_freezing.htm) so it 
should be possible to freeze this or at least make it harden somewhat
without causing the water to freeze and expand - this could
potentially be done in a refrigerator.  After the layer of oil
hardened, a small layer of hot glue could be placed on top of it to
prevent it from mixing with above layers.  Then more salt water could
be added, and the process repeated, until there were enough layers to
fill the bottle.
One problem with this is that I can't find the breakdown voltage of
olive oil.  Does anyone happen to know this?  Also, is there any
mechanical or other problem with this design that would prevent it
from working?

Thanks,
Crispy