McMaster Carr Mica Sheets

From:  richard hull [SMTP:rhull-at-richmond.infi-dot-net]
Sent:  Sunday, April 05, 1998 8:53 PM
To:  Tesla List
Subject:  Re: McMaster Carr Mica Sheets

At 08:57 AM 4/5/98 -0500, you wrote:
>From:  Hollmike [SMTP:Hollmike-at-aol-dot-com]
>Sent:  Saturday, April 04, 1998 11:51 AM
>To:  tesla-at-pupman-dot-com
>Subject:  Re: McMaster Carr Mica Sheets
>Hi All,
>    The McMaster-Carr mica cap failed today.  I was working for the first ten
>minutes or so and then shorted.  This was using three layers of mica and using
>my 12kV neon.  I was still trying to tune the coil and may have created a
>spike that punctured it.   I will perform an autopsy after I finish this
>Mike Hollingsworth
I have not chimed in on this as I was sure someone else would.  Capacitor
grade mica is graded and very very expensive.  It is usually graded from V1
thru V4 grades.  V3 and V4 is unacceptable for HF or pulse work. V1 grade is
currently unobtainable in sections much larger than 1" X1".  V2 can still be
found in sizes up to 2" X4" at some mica mining and vabrication facilities.
In short, the very finest Ruby Muscovite mica is just about all gone in
"books" which can yield large sheets of mica in moderate thicknesses.

V1 grade is absolutely water clear, with no tinting, inclusions or staining
over the entire surface!  V2 is also absolutely water clear, but can have no
more than 2 very small inclusions per sq. inch.  V3 grade is clear but can
have a moderate uniform browning which is barely detectable and a few
inclusions per sq inch. V4 can have non-uniform coloration, but still must
be totally transparent and can have a number of larger inclusions which
don't break the surface.

Mica caps must be made to exclude all air and run dry!  No oil!  Most
normally they use alternating stacked flat plate construction with ultra
thin .0005" or thinner lead foil plates.  The capacitor is usually
compressed to a tight stack under hundreds of pounds of pressure and then
molded or sealed in a cast casing to seal out moisture, etc.

The best and only survivable construction is the use of multiple series
capacitor setups within the stack.  This involves a hookup
plate/mica/floating plate/mica/floating plate/mica/floating plate/
mica/hookup plate.  The normal amateur hookup plate/mica/hookup plate
construction is doomed.

Assuming the Mc Master Carr mica is real high grade capacitor mica (which is
almost certainly not the case), then the capacitor construction itself is at
fault.  Watch out for reconstituted mica!  It is just no good for pulse work
at all.

The breakdown for mica in volts per mil is given usually for a .5 or 1 mil
thickness!  2 mils thick and the voltage standoff drops sharply.

Richard Hull, TCBOR