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Re: Dielectric constants.
From: L.Robertson[SMTP:LWRobertson-at-email.msn-dot-com]
Sent: Friday, November 28, 1997 12:29 AM
To: Tesla Builders
Subject: Dielectric constants.
Hi ...
I see some discussion on the list recently about capacitor materials.
Some time ago while researching plastics to use in a rolled capacitor
I generated a short list and tried to get the constants at 1 Mhz and at a
thickness of 125 mils to make comparison easier. I discarded stuff
that was too expensive (Teflon) and the ones plainly unsuitable (Mylar
and alot of others).
I found the lists in the plastic company literature had plenty of errors,
but by cross referencing alot of sources I think these numbers may be
reasonably accurate.
The dielectric strength varies with thickness - reducing the thickness
by four approximately doubles the strength. So if the strength is 500 v.
at 125 mils, it is roughly 1000 v. at 32 mils.
So for what it is worth ...
Substance K -at- 1 Mhz. Dissipation Strength -at- .125
Kapton 3.4 0.3 % 570
Volts
Polyethylene 2.3 < 0.01 % 500
PET 3.0 1.6 % 500
Polypropylene 2.5 < 0.05 % 650
Polycarbonate 2.9 1 % 360
Acetal 3.8 0.5 % 500
I chose polypropylene, but it's a real bear to roll up, being much
stiffer than polyethylene. For flat caps though it is a good choice.
LR