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Re: flat caps

Tesla List wrote:
> 
> > Subject: Re: flat caps
> 
> Subscriber: pwac-at-flinet-dot-com Sun Feb 16 13:53:03 1997
> Date: Tue, 04 Feb 1997 13:08:51 +0500
> From: Jeremy Bair <pwac-at-flinet-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: flat caps
> 
> Tesla List wrote:
> >
> > Subscriber: Esondrmn-at-aol-dot-com Sat Feb 15 15:44:40 1997
> > Date: Sat, 15 Feb 1997 03:00:16 -0500 (EST)
> > From: Esondrmn-at-aol-dot-com
> > To: tesla-at-pupman-dot-com
> > Subject: Re: flat caps
> 
> <....
> ....>
> 
> > I recently made one that is .00475 mfd with two layers of .062 poly (for each
> > layer) and those plates are 47" long.  This cap just barely fits into a 6"
> > pipe section.
> >
> > Ed Sonderman
> 
> I have been taking apart small Capatictors just to get an idea of how
> they are arranged, I was amazed to open some of the rolled ones, they
> are awsome! After realizing it, all my caps I have ever used were rolled
> (About a year ago, I opened one for the first time to discover metal
> plates and tons of oil, now I can't find that one again!). I think
> rolled caps would be better for me. Let me ask you though, some small
> ones I opened (200V-at-220uf) had a sort of paper and metal materials
> soaked in oil, was this 'paper' like material a plastic? It was so
> soaked, I can't tell what it was, and how come to two windings of metal
> are different materials, one is shinnier than another.
> 
> And did you use an oil?
> 
> <...you would need plates of
> 1010 square inches / 14" (std. width of roof flashing) = plates
> 72.14"...>
> 
> Roofing Flashing? Wasn't that material thick? What is wrong with using a
> whole roll of Aluminum Foil?
> 
> Jeremy Bair

Jeremy,

The type of cap you opened was an electrolytic capacitor. This type
typically uses aluminum foil which is treated in a special manner to
have lots of surface area. The oily fluid was actually a conductive
electrolyte, and a very thin, transparent aluminum oxide layer was the
actual dielectric layer. That's why one foil plate had a different
appearance. Because this insulating layer is so thin, the amount of
capacitance is quite large versus the plate size. However, this type of
capacitor is "polarized" - it needs to always have the + and - leads
biased with the right polarity voltage in order to function best.
Applying AC to this cap would probably destroy it very quickly. Its not
suitable for Tesla Coil work other than perhaps as a filter cap in the
DC power supply section of a solid state coil. 

Most caps you'll encounter are rolled caps, except those made with a
mica dielectric. Paper capacitors may made from oil-soaked paper - the
oil is actually the primary dielectric, or the paper may be
"impregnated" with some other insulating material. Another common
dielectric is mylar film. None of these are suitable for serious Tesla
Coil work. The best dielectrics for our use are polyethylene or
polypropylene. Polystyrene is also very good, but not usually as
available in thin sheets and films as the first two materials. All of
these materials have low dielectric loss - this means that when used in
high-power pulse/AC circuits, the dielectric will not heat up as much or
as quickly as other materials, like mylar, paper/oil, or glass.  

The problem with making rolled capacitors from foil is in terminating
the ends to get a low-resistance, reliable connection, capable of
handling hundreds of amps of current. Commercial manufacturers have
specialized equipment and alloys not generally available to the novice
coiler. By using flashing, one can connect to the short-dimension ends
of the flashing, and because of the thickness of the material, not have
too much series resistance to the interior portion of the metal plates.
This construction DOES add inductance to the cap however, but not
usually enough to cause significant problems in Tesla Coil work. Since a
flat plate capacitor brings the foil ends of all parts of the cap out
the sides where they can all be connected in parallel, aluminum foil
works great for flat caps. Heavy Duty foil is usually easier to work
with.

In all cases though, its critical that some type of oil be used to
replace all the air between the plates and the dielectric, since the air
itself will get very hot and break down under the stresses seen in Tesla
Coil use. If not removed, the hot/ionized air will very rapidly cause
the nearby plastic film dielectric to fail, shorting out the cap. Go to
the funet site to get more detailed information information on building
caps:

      ftp://nic.funet.fi/pub/sci/electrical/tesla/  
 
                         OR

     http://www.funet.fi/pub/sci/electrical/tesla/


Safe coilin' and cappin' to you, Jeremy!

-- Bert H --