Re: Dielectric constant table

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Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, September 18, 2003 4:23 PM
Subject: Dielectric constant table


 > Original poster: The MCP <ejkeever-at-comcast-dot-net>
 >
 > I'm wondering how long it's been since the dielectric tables present on
many
 > tesla coil sites have been updated.
 >
 > I'm also wondering about some of the values on them. For example, the
table
 > says that Mylar is good for 7.5Kv *per mil*. However, one of my early
 > attempts at a cap using 10 mils of mylar failed at 5000V rms. Something is
 > wrong.

Dielectric strength is much higher (in a V/mm sense) for thin films than for
bulk material.
The databook value is usually for an idealized material, very carefully
prepared, in a precision test cell. Almost anything will reduce the
breakdown voltage.. small surface defects,small irregularities, etc.

5000V RMS is about 7kV pk, which works out to 700V/mil for your 10mil
polyester... considering that air is 70V/mil, that's not bad
Also, mylar is quite lossy... a bit of heating can dramatically change
breakdown behavior.

 >
 > Also, a suggested addition... the giant table at
 > http://www.asiinstr-dot-com/dc1.html says that hydrogenated castor oil has a K
of
 > about 10 at 80* F.

Indeed, the high dielectric constant of Castor Oil is why it's used in
Maxwell Labs capacitors.
 >
 > Speaking of which - Anyone ever considered novel materials to build a
 > capacitor? For example, Iodine? Iron oxide? Copper oxide?

You name it, someone's used it.  Someone on the list (many moons ago) was
contemplating making their own ceramic insulators..

 >
 >