Re: MMC cap preferences? Foil/Film thing... (fwd)

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Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Sat, 12 Mar 2005 12:01:19 -0700
From: Terrell Fritz <terrellfone@xxxxxxxxxxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: MMC cap preferences? Foil/Film thing...

At 10:38 AM 3/12/2005, you wrote:
>.............
>
>   Hmm, Ross Overstreet says metallized film is better than metal foil
>due to its "self healing" properties.  You and Terry say that metal
>foil is better due to increased current capability.
>
>So, who's right, or is this one of those religious arguments that may
>never be settled?
.......

It's all cool ;-))

Metal "foil" caps have a center floating plate that is still metalized film 
and still takes care of the self healing thing.  Even the foil caps still 
have a metalized film layer.  I always call them "foil" so there are no 
mistakes!!

The problem with the "all" or "fully" metalized film caps where all the 
layers are 1/1000000 inch metal film is that they cannot reliably connect 
the metal film to the end plates.  The film to end plate connection is a 
very weak spot and they fail there under high current.  Using "foil" 
(500/1000000 inch thick) plates in that area solves the problem completely.

For high pulse current high dI/dT stuff, you definitely want to spend a 
little extra on the metal foil caps.  They can easily take "insane" 
currents without failure where the metalized caps barely meet their 
specs...  The only advantage the metalized caps have is that they are 
cheaper and have higher values/volume.

A few people have gotten metalized caps to work with Tesla coils but the 
caps are not very stressed in those specific coils.  There are many sad 
tails of the all metalized film caps burning up, but very few people have 
managed to destroy the metal foil caps.

The big Cornell Dubilier 942 series (NOT "940") caps are very popular, well 
known, and tested and have good data sheets that can be trusted.  Rell.com 
sells them with a minimum order of about $75 and DC Cox may be able to 
supply them now too from Resonance Research, but he just got past a bad 
illness so not sure he is up to it yet.

Newark probably sells them too but you tend to pay a lot more there...

I usually try to have about 30 CD 942 caps laying around here to grab as 
needed.  They never seem to stay around long!!  There is no reason to use 
other caps unless you already have some other known good type or got a good 
salvage deal on some other type.  But you are on your own in that 
case...  There is a list of known good and bad caps at:

http://hot-streamer.com/TeslaCoils/MMCInfo/good-bad.txt

Everyone uses these caps at there "DC rating" for Tesla coils too with no 
problem at all.  For Tesla coil use, the ~630VAC rating can be 
ignored.  I"" often use them at %150 voltage with no problem to save a few 
bucks too...

Cheers,

         Terry