Re: DeltaV/DeltaT vs capacitor roll length.

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

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

Yes.. with metalized polypropylene pulse capacitors. Doesn't matter with 
foil-film.

-- Bert --

Tesla list wrote:
>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 --
>  > --
>  >
>  >
>  >
>  >
>
>.