Re: Hurray, I destroyed my homemade cap!

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: Hurray, I destroyed my homemade cap!
From: Tesla List <tesla-at-stic-dot-net>
Date: Sat, 29 Nov 1997 20:29:17 -0600
Approved: tesla-at-stic-dot-net

From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Saturday, November 29, 1997 3:10 PM
To: 	Tesla List
Subject: 	Re: Hurray, I destroyed my homemade cap!

Tesla List wrote:
> 
> From:   Adam[SMTP:absmith-at-tiac-dot-net]
> Sent:   Thursday, November 27, 1997 12:56 PM
> To:     Tesla List
> Subject:        Re: Hurray, I destroyed my homemade cap!
> 
> Bert wrote:
> >Sooner or later, your dry caps will fail due to microdischarges, surface
> >oxidation, and spark tracking all stemming from electrical  breakdown
> >effects along the air-LDPE interface, and particularly around the edges
> >of the plates. While corona may look innocuous, it's actually extremely
> >hot and energetic.. and hot ozone does very nasty things to LDPE. Please
> >keep us informed on how long your cap stays operational.
> 
> Yeah, I know all about that :-)  Back in the eighth grade, I made some
> caps from pepsi bottle material (PET) for my first TC ever, and after a
> few weeks use the plastic had these fractal-esque lightning-like
> branching patterns all over it, especially from the corners of the
> plates.  It was actually quite beautiful and artistic, and I included the
> etched dielectrics as part of my display.
> 
> So then, how about solid corona protection, like paraffin wax?  I could
> get this cheaply at the grocery store, melt it in a double boiler and pot
> the cap.  This would make the cap a lot more difficult to repair, but if
> I over engineered it in the first place it should survive a few years of
> Tesla use, right?.  I am using 120mil of very high quality LDPE (4 x
> 30mil) and operating at 9kV, so my cap is not likely to blow anytime
> soon.  Mostly I just want to make something without that horrible oil!!!
> 
> -Adam
> 


Adam,

While pariffin is quite a good dielectric, the problem you'll encounter
is getting it to displace all entrapped air without solidifying first.
LDPE has a relatively low softenning and melting temperature, limiting
the degree of preheating you can do. 

There may be a way to reduce the edge stress and corona a bit. One way
would be to construct your cap as a series of identically sized modules,
and then connect them in series. By making them identical size, you'll
evenly spread out the dielectric stress along the edges of the plates.
For example, if you constructed your dry cap as a group of 10
identically sized modules, each with 12 mils of dielectric, and each
sized at 10X the desired final capacitance, when you connect them in
series you'll end up with a cap having an effective dielectric thickness
of 120 mils, but each cap would "see" only 1/10 of the total applied
voltage. While this won't eliminate the corona, it should reduce it,
possibly extending the life of your caps. 

BTW, mineral/transformer oil is REALLY not all that bad to work with
once you've got a container that doesn't leak! And it works VERY
effectively at stopping corona. :^)

Safe cappin' to you!

-- Bert --