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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: Tesla List <tesla-at-pupman-dot-com>
> To: 'Tesla List' <tesla-at-poodle.pupman-dot-com>
> Subject: Re: additional transformers [rolled caps]
> Date: Monday, August 04, 1997 11:11 PM
> 
> 
> 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