Re: defective caps?!?!

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Steve,

I'll pass this to the whole list since you ask a key question in all this...

At 12:12 AM 9/14/2002 -0500, you wrote:
>About this metalized foil, what makes it worse than regular foil??  

"Foil" caps have end plates that are real aluminum foil.  Just like the
kind one uses to wrap fish but much thinner at say 0.00025 inch.
"Metalized film" is a vacuum plasma process that deposits aluminum on poly
film at only 0.000005 inch...  So metalized aluminum is far far thinner
than solid foil.  The place it really counts is at the end cap connections.
 Foil has like 100 times the contact area where the metalized have far less
and then has a bunch of poly in there too messing things up right at the
critical end connections.  the end connections seem to be a "fuse".

So... We really want the "thick" aluminum foil to make the connection with
the end caps.

Now is were it gets goofy...  "All" MMC caps have a "metalized" inner
section.  This is inside the cap and provides a very thin "fusing" layer in
case the caps have to self heal from an over voltage.  Here is a chart ;-)

"All metalized" (not so great MMC caps)
	Blows like a fuse, lower current.  "All" layers thin metal film.  Fat caps
get lucky ;-)

"Foil metalized" (best MMC caps)
	Higher current self healing.  Critical end cap connection thick foil but
still has thin inside layer to allow for over voltage healing.  The best...

"All foil" (commercial caps)
	Highest current, but no self healing.

This is why we came up with the Good/Bad list.  It is sort of complicated
and it is hard to know for sure...  It appears the GE 42L series is indeed
all metalized.  The "fat ones" my be tough, but not as tough as the foil
Geek group versions.  This is why the GE version become surplus/extinct and
the Cornell-Dubilier version is on back order ;-)))

Cheers,

	Terry

>It seems 
>tougher to me, but what do i know :(.  Also, im really confused in that on 
>your website, you say to get metalized polypropylene caps.  I think you may 
>want to revise this and perhaps give some insight about the reason not all 
>metalized caps will work.  I notice a difference in the construction of the 
>2 caps i have.  The .68's have more clearance on the edges of the center 
>plate, and i can pull the entire layers apart.  The .33s wont come apart, 
>and didnt have much clearance on the edges.  Also, when pulling the layers 
>off of the .68's, the edges of the foil tear differently than the .33's did. 
>  The .68's had a rough foil edge, like foil does, the .33's ripped almost 
>perfectly.  Maybe im just trying to find some justification here, in the 
>hopes that not all is lost.
>
>Steve Ward.
>
>