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Re: Blown Rolled "Hull" Capacitor Discussion


On Thu, 29 Feb 1996 tesla-at-grendel.objinc-dot-com wrote:

> >I also had problems until I used tranformer oil and used a vacuum
> >pump to remove all air.
> >
> >Scott Myers
> before I used one by itself.  I used mineral oil purchased in a 5 gal sealed
> container from the local Chevron distributor.
> 
> Ed Sonderman

A possible reason for the very different performance of the same design:
MINERAL OIL!! If I have understood correctly people used this and blew
their caps.. What if the different brands of "mineral oil" are very
different what comes to impurities, water content etc.. It might very well
be that as long as it's not lethal (harmfull to people) the manufacturers
don't mind about additional water etc. in the mixture.. At least in
xformer oils the voltage rating goes rapidly down as water content gets 
higher - or are there other (conductive?) impurities left to the "mineral 
oil" as it is manufactured? 

Other point to notice is that different brands of LDPE will have different
electric specs as well.. Some manufacturers may have better equipment,
better raw-material (and higher price:) and thus better quality.. 

These are just speculations but perhaps we could compare the brands
of mineral oil people use and list some that have worked ok.. Same
could be done on LDPE too..

I have used transformer oil and polypropylene meant for printing and
have had no problems with blowing caps - I have used 10kVAC max on
1.6mm di-electric -thickness.

btw: I don't have the boiling temperature of "mineral oil" handy but if
it's higher than 100C then boiling it would get extra water out.. Same
applies for xformer oil too.. The same could be done with a vacuum pump
too at a lower temperature.. When I pumped my caps I did let the pressure
go low enough (vacuum pump) to get the xformer oil to boil.. 


  Kristian Ukkonen.