Re: FIRST capacitor, please advise/help

From: 	Adam[SMTP:absmith-at-tiac-dot-net]
Sent: 	Thursday, December 04, 1997 10:57 AM
To: 	Tesla List
Subject: 	Re: FIRST capacitor, please advise/help

>Hello all,  I sent an e-mail with attachments that got screwed-up,
>so here's my request again. Please send me any comments/thoughts.
>I'm building my FIRST Tesla Coil, 
>I have built, and destroyed, a couple of small/test capacitors.
>Now I'm building my FIRST real primary capacitor.

Did you learn anything useful from killing these caps?

>My power source is a 15 kV centre-tapped 30 mA 50 Hz NST.
>I calculated 6.3662 nF as the optimal value to match my NST.

>DIELECTRIC 22 pieces of 300mm x 300mm x 3mm HDPE.

Better to use 3 or 4 plates of thinner material, to help distribute any 
defects.  It would be still better to build a few high value caps with 
say 1mm of dielectric, and chain three of them in series.  You can even 
place the three new caps in the same container, with a fair amount of 
well placed insulation.

>PLATES     23 pieces of 280mm x 250mm x 0.4mm Aluminium.


>SPACERS    21 pieces of 1" x 1/4" x 300mm Aluminium stock.
>            2 pieces of 1" x 1/4" x 400mm Aluminium stock.
>                (300mm plus 100mm for external connection)
>            2 pieces of 1" x 1/8" x 300 mm Aluminium stock.
>                              ( for two ends of capacitor)

What are the spacers for? Did you not intend to just stack 
Al-PE-Al-PE-Al-PE ??? This is what I do in my caps and it works fine.  I 
did not bother with making multiple caps in series, as I am only running 
at 9kV, but at 15kV would certainly be wise to consider it.

>OIL         not yet decided.

USP mineral oil will work, as will a transformer grade mineral oil.  Make 
sure you design a good container to contain it though! I am on container 
#3 for my sad leaky cap. 

>1 Is 50mm at the ends enough?
>2 Is 25mm at the top/bottom enough?

Both OK.

>3 Based on your EXPERIENCE how many kV should such a capacitor
>   be able to handle.

With a multi-ply 3mm total dielectric, it should work well in a 15kV 
driven tesla setup, but no more than 15kV is going to be safe.  It 
depends a lot on the purity and uniformity of your PE.  All it takes is 
one or two weak spots and BANG!..fizzzzz... :-(

>I really want this to be a long-life component as I immagine other uses 
>in future such as high-power high spark-rate RSG Tesla use,
>can-crusher,magnetisation pulses etc.

Well, it will be a great capacitor for a small tesla coil, but you can 
forget about crushing cans unless you want to build, oh maybe 5,000 more 
just like it :-)


Make sure all the components are cleaned with isopropyl alcohol before 
assemble, and never subject the HDPE to any kind of stresses like high 
temperatures, scratches, etc... Also, you might want to find a way to 
hold the cap flat in the container, so it does not float apart (become 
sloppy) and change its value.


Adam Smith
Epoch, Inc. Digital Music Project

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