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Re: additional transformers [rolled caps]


From: 	Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: 	Thursday, August 07, 1997 7:53 AM
To: 	Tesla List
Subject: 	Re: additional transformers [rolled caps]



> 
> From: 	Michael Smith[SMTP:md.smith-at-usa-dot-net]
> Reply To: 	md.smith-at-usa-dot-net
> Sent: 	Tuesday, August 05, 1997 12:40 PM
> To: 	Tesla List
> Subject: 	Re: additional transformers [rolled caps]
> 

> > 
> > 
> > From: 	Robert W. Stephens[SMTP:rwstephens-at-headwaters-dot-com]
> > Reply To: 	rwstephens-at-headwaters-dot-com
> > Sent: 	Monday, August 04, 1997 10:05 PM
> > To: 	Tesla List
> > Subject: 	Re: additional transformers [rolled caps]
> > 
> > 
> > > > > >
> > > > > > >Just though I'd throw in my two bobs worth after having
blown
> up a few
> > > > > > >rolled poly caps!
> > > > > > >
> > > > > > >All of mine failed at the edges (even being under oil)
so I
> reckon the
> > > > > > >secret is to put big overlaps between the plastic and
the Al
> plates i.e.
> > > > > > >at least an inch or more. Also here the thick poly sheet
is
> bloody
> > > > > > >expensive! so use multiple sheets of 8 mil concreters
plastic,
> > > > > > >preferably the clear stuff but if all else fails use
black
> plastic with
> > > > > > >engine oil! It ran ok on my coil for more than 3 months.
> > > > > > >
> > > > > > >Cheers,
> > > > > > >
> > > > > > >Peter E.
> > > > > > >
> > > > > > >Peter,
> > > > > >
> > > > > > I recommended leaving a full 2" edge separation metal to
edge
> of dielectric
> > > > > > in all rolled capacitors!
> > > > > >
> > > > > > Richard Hull, TCBOR
> > > > >
> > > > > Thanks for the info on that, I will go 2" on my future
overlaps.
> It
> > > > > seems strange to me that sparks seem to travel much further
along
> the
> > > > > surface of an insulator, no matter how clean it seems to
be.
> Maybe this
> > > > > is due to microscopic impurities on the surface of the
plastic?
> > > > >
> > > > > Another little trick that you probably have already figured
out
> is to
> > > > > insulate the input connections to the plates by rolling a
few
> layers of
> > > > > poly around them.
> > > > >
> > > > > Cheers, Peter E.
> > > > >
> > > > Peter, Richard, All,
> > > > 
> > > > I had the same problem with a rolled poly cap I made once,
and I
> > > > *was* using a 2 inch surround from the foil to the edge of
the
> poly.
> > > > Dismantling the cap after the failure showed these burns
> distributed
> > > > all along the edge of the foil outwards into the poly.
> > > > 
> > > > I had a chat with a capacitor engineer at Condesnser Products
about
> > > > this phenomenon and was told that this is why HV capacitors
are
> > > > generally not successful when one tries to apply more than
8-10 kV
> > > > across a single stage.  He explained that the energy exchange
> between
> > > > the plate and the local surface of the dielectric becomes too
> violent
> > > > and just completely overstresses the dielectric causing it to
> > > > break down.  Placing additional thickness of poly does not
> > > > significantly reduce this violent surface area exchange so
you will
> > > > still get failures no matter what you do if you try to run a
single
> > > > stage cap at 15 or 20 kV.
> > > > 
> > > > FWIW
> > > > rwstephens
> > > 
> > > Robert,
> > > 
> > > Do you advocate using several serial connected stages in the
one,
> rolled
> > > poly cap or several separate caps connected in series?
> > > 
> > > Peter E.
> > > 
> > Peter,
> > 
> > I'm no *expert* on homemade capacitors.  There are a number of 
> > coilers on this list who *are* experienced in this art enough to
be
> called 
> > experts.  In my opinion though I think placing a number of caps
in 
> > the same container all in series would be the way to go because
in 
> > doing so you reduce interconnection leadlength and help lower the
ESL 
> > of the cap which is what you desire in a pulse cap.  I have
thought 
> > about schemes employing flat stacking of poly and foil which can
give 
> > series capacitors all in one package.  Perhaps someone out there
has 
> > tried this and can chime in here.
> > 
> > rwstephens 
> 
> I am just starting on a flat capacitor that is two .02's in series.
I
> have 16x13 plates, and half have tabs that are 2x3 in the corners
and the
> other half have tabs centered on the 16" side. I will build with
tabs on
> one corner and the center untill I reach .02uf, then switch to the
other
> corner and the same center tabs for the other .02uf. I will bring
out all
> 3 connections, giving me the option of .01 or .02. The .02
sections'
> dielectric thickness is to be .069, so even alone, they should
stand up
> well. In series, they should be invincible, i hope.
> 
> I am shearing the plates 16x16 today and after building a suppost
> frame/clamp, I can mill out the 3" of waste off the plates all at
once.
> Well, once for the corner tab plates and once for the center tab
plates.
> 
> Perhaps this will help spark an idea. I have redesigned the
capacitor at
> least 10 times. It will be BIG, and heavy, but what the heck. The
same
> corner-center-then other corner idea will work on small ones too.
> 
> I intend to present the entire description of this capacitor when
done. I
> have utilized (i think) every bit of information gleaned from this
list
> and have taken (i hope) all precautions. This should be a great
capacitor
> for the anal retentive croud. More later. 
> 
> Michael Smith
>  
> 
> 

Michael,
You can actually get .01, .02, and .04 mfd from this arrangement. You
would get the .04 mfd rating by connecting the two outer tab sets
together. This effectively connects the two sets in parallel. 

Fr. Tom McGahee