Re: Capacitor Question??

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Hi Dave,

The difference is due in part to the when the caps were manufactured versus 
the given state of the art in capacitor technology. There was a period of 
significant evolution in capacitor technology during the late 80's and 
90's. Much of the effort was focused on increasing capacitor energy density 
(i.e., how to squeeze more joules into a smaller package) and reliability. 
This culminated in the evolution towards much thinner, more highly stressed 
dielectrics combined with self-healing metallization systems.

Although it was known that the thinner dielectric system would experience a 
higher incidence of dielectric failures, with the thinner metallization 
systems, these failures would no longer be catastrophic. The gradual loss 
of capacitance over time was manageable. In fact, when the capacitance 
declined by ~5%, it was a useful and easily measurable predictor that the 
time had come for capacitor replacement, The GE PFN caps predate much of 
this development. So why is the GE cap relatively small? I suspect they 
used a Mylar/high-k oil dielectric system with a significantly higher k 
than polypropylene.

Best regards,

-- Bert --


Tesla list wrote:

>Original poster: DRIEBEN-at-midsouth.rr-dot-com
>Bert,
>You may very well be right. I asked the seller about their application
>and he said that he thoguht that they come out of an RF power supply
>and that he thought that they would be suitable for energy storage or
>energy discharge application. Speaking of relatively small physical
>size, the Maxwell 37667s are really quite small for their cap/voltage
>rating but yet they seem to be very suitable for coiling purposes (1000
>pps and high pk currents).
>David Rieben
>----- Original Message -----
>From: Tesla list <tesla-at-pupman-dot-com>
>Date: Saturday, April 17, 2004 9:05 pm
>Subject: Re: Capacitor Question??
>  > Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>
>  >
>  > Hi David,
>  >
>  > I've got a number of their single-eared 25 kV cousins that were
>  > originally
>  > part of a high voltage Pulse Forming Network in some older Varian
>  > medical
>  > equipment (a LINAC). These caps are almost certainly rated for
>  > moderate
>  > current pulsed duty but, unfortunately, it's not clear if they
>  > were
>  > designed for low pulse repetition rate (in which case they'd most
>  > likely be
>  > Mylar) or for high PPR (which would imply polypropylene). However,
>  > based on
>  > the comparatively small physical size and the age of the
>  > capacitors, I'd
>  > bet that they are Mylar.
>  >
>  > Pictures of the 40 kV cap and its 25 kV cousin are at:
>  > http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=3809614785
>  > http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=3810492088
>  >
>  > -- Bert --
>  >
>.