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Re: rolled caps
From: Geoffrey Schecht[SMTP:geoffs-at-onr-dot-com]
Sent: Sunday, September 07, 1997 1:31 AM
To: Tesla List
Subject: Re: rolled caps
>
> From: Mad Coiler[SMTP:tesla_coiler-at-hotmail-dot-com]
> Sent: Saturday, September 06, 1997 3:22 PM
> To: tesla-at-pupman-dot-com
> Subject: Re: rolled caps
>
> >From: Geoffrey Schecht[SMTP:geoffs-at-onr-dot-com]
> >Sent: Saturday, September 06, 1997 9:46 AM
> >To: Tesla List
> >Subject: Re: rolled caps
> >
> >Answers to questions:
> >
> >1.) Having a PE-al-PE-al cap will give you about about half of the
> >capacitance of a PE-al-PE-PE-al cap since capacitance is directly
> inversely
> >to plate separation
>
> I think you meant double the capacitance.
Guess I got that turned around. Sorry! Not enough Everclear in my coffee
this morning.
>
> >(I think that I understood the question...maybe you
> >should repost with a layer scheme like the one I just used).
> >
>
> I dont think you eactly understood. The actual distance between plates
> should stay approximately the same - three thicknesses of 30 mill PE. I
> will attempt to show the two chioces I am facing:
> -- plate A
> O PE 1
> O PE 2
> O PE 3
> -- plate B
>
> OR
>
> -- plate A
> O PE 1
> -- AL (spacer)
> O PE 2
> -- AL
> O PE 3
> -- plate B
>
> Since the AL inbetween the PE layers is electricaly shorted from one
> side to another it shouldn't add to the total seperation of the plates.
> Another way to see this is that as the thickness or the dielectric
> increases the capacitance goes down. In both models the thicknes of the
> dielectric is constent - 90 mills. I just thought that having conductive
> plates between might improve the efficiency of energy storage. Having
> the extra plates would allow smaller charges to act on each other at
> closer distances, perhaps allowing it to be better at pulse type
> discharging? Hmmm, is that any clearer?
Ok. _Now_ I have the picture. Agreed, the capacitance would not be
affected. Adding the extra plates may actually worsen the pulse risetime
capabilities; with the extra plates, you're introducing additional
resistance in series with the displacement currents through the capacitor.
It's easy to visualize, wherever you have an AL in the above diagrams,
insert an R on either side to represent parasitic series resistance...the
more plates you add the worse it gets. Increasing ESR is not conducive to
good fast-pulse discharge characteristics. I don't have any feel for how
bad the impact would be, it may in fact be negligible.
The extra metal in the caps also adds eddy current losses; again, I don't
know if that would be a significant factor.
Energy storage in a cap is simply 0.5*C*V^2. There's no special technique
to change that situation.
How you roll the caps has some effect on the parasitic inductance, too. The
aluminum electrolytics that we use in switching power supplies are low ESR
devices to begin with but some of them minimize ESL by how they're wound
(i.e. they're flat wound and then folded in half to partially cancel the
magnetic fields).
Let me know how the things work, in any case. Unless I can get Maxwells or
some other commercial HV caps cheaply, I'll probably wind my own someday.
<megasnip>
Geoff
(P.S. After I wrote this, I looked a bit further down the list at some
unread stuff, Tom McGahee mentioned that thicker dielectrics increase
losses. That's something else to consider.)