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Re: Self healing caps

Hi Justin,

At 03:11 PM 12/29/1999 +1100, you wrote: 
>
>Does anyone know by what mechanism metallised film poly caps self heal? 
>If so, is there any way of incorporating this feature into home made caps? 
>Could be handy if at all feasible. 
>
>Justin 
>

	They evaporate (under vacuum) aluminum metal onto the bare polypropylene
film.  The layer is very thin (millionths of an inch).  When there is a
blow through, the metal simply evaporates into practically nothing around
the area of the hole.  Thus, it removes the metal in the shorted area.
Just like a fuse.  They have it figured out so that it does hardly any
damage to the cap other than removing the metal around the failed area.
They can take tens of thousands of blow throughs before the cap really
fails.  We have pulled apart over voltaged ones at work were 30% of the
metal has been blown out but the cap still appears good other than the
value has decreased.  they measure the value to judge how bad the metal
removal is.  That's why we try and monitor the value of MMCs to check for
damage.  However, they seem rock solid even in my highly stressed versions.
 The following paper is rather odd but has info too.  I have all the data
from many manufacturers but you probably don't want to know every last
detail...

http://www.peakpeak-dot-com/~terryf/tesla/misc/LT_POW_CAP.html

Unfortunately, the poly used in these caps is crystal clear and perfect.
They hold off 30kV with about 4 mil of poly.  The evaporation process is
beyond us.  You can actually buy spools of the roll cap material but you
need fancy machines to role them.  I used to have some links that showed
this equipment but I can't find them at the moment.  However, it was $100K+
stuff...


Perhaps if you could get very thin foil and not use oil, you could almost
try to make a homemade version.  However, the commercial caps start looking
really cheap very fast...

Cheers,

	Terry