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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