Re: DeltaV/DeltaT vs capacitor roll length.

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

Original poster: "Virtualgod" <mike.marcum-at-zoomtown-dot-com> 

so for a given cap short n fat is better than long n skinny?

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, April 10, 2004 7:43 PM
Subject: Re: DeltaV/DeltaT vs capacitor roll length.


 > Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>
 >
 > Tesla list wrote:
 > >Original poster: Finn Hammer <f-h-at-c.dk>
 > >All,
 > >On Kreso`s pictures of the Maxwell 37667 caps, it is obvious that the
 > >individual capacitor rolls are as long as the inside of the case, so they
 > >are about 5" long.
 > >On the spec. sheets that I have seen of (What we now call MMC caps) the
 > >dV/dT is dependant on the length of the cap. The small types have a
better
 > >rating than the bigger types in longer cases.
 > >I am wondering how Maxwell manages to produce a cap with superiour dV/dC
 > >and still from individual caps that are 3 times as long as the ones we
use
 > >for MMC`s.
 > >Cheers, Finn Hammer
 > >
 > >.
 >
 > Finn,
 >
 > With metalized polypropylene pulse capacitors, it's not so much the length
 > of the cap as it is the surface area at the ends of the cap, since the end
 > cap area governs the allowable current density for the cap. The sprayed-on
 > end contacts are made from a low melting temperature metal, typically
zinc.
 > Unfortunately, the electrical contact between the sprayed metal and the
 > metallization layer on the dielectric is difficult to control and is the
 > weak link for this style of capacitor. Under high pulse or RMS currents,
 > marginal contact points heat up. Heating of the capacitor body can also
 > induce physical shrinkage problems within the PP dielectric, which can
also
 > contribute to progressive detachment. Rapid dv/dt conditions can also
 > create mechanical stress due to slight dimensional changes within the
 > capacitor winding through Coulomb attraction, particularly in capacitors
 > with insufficient winding tension. If any of these problems cause the
 > effective contact area to further decrease, heating can intensify in the
 > areas still making contact, leading to progressive deterioration, lead
 > detachment, and internal arcing.
 >
 > This phenomenon is not a problem with foil-film construction since ohmic
 > contact to the foil can be made considerably more robust. The best of both
 > worlds is to use foil electrodes at each end combined with self-healing
 > metalized film floating layer(s). This appears to be the construction used
 > in the Maxwell 37667 cap and in many high current snubber caps.
 >
 > Best regards,
 >
 > -- Bert --
 > --
 >
 >
 >
 >