Re: MMC Caps

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

Original poster: "Steven Ward by way of Terry Fritz <twftesla-at-qwest-dot-net>" <srward16-at-hotmail-dot-com>

Well really,

you have a bit more than 21kv peak.  The caps will take the voltage for a 
while, but they will get warm from the current.  I used a single string of 
similar CD caps (.22uf 1600v) with only one string.  But i only had a 12kv 
transformer, and the string was of 15 caps for 24kv for a .0146uf cap.  They 
would get a very slight warmth after a few minutes.  Im not sure how long 
they will last, but they have been used for at least 10 or 12 minutes so 
far.  I was getting 3 foot sparks with this coil, so the caps were working 
good.  If you dont have the money, and you must have a MMC, then i would 
just go ahead with what you want to do, but maybe keep that gap closed up a 
bit more.  And watch for temperature on the caps.  If they get pretty warm, 
they probably wont last too long.  Futhermore, for my large coil i use the 
same caps, but with 16 in a string for 25.6kv, with 2 strings thats .0275 uf 
for a 15/120 PSU.  They stay very cold, and i have been running the coil for 
extended periods of time lately (about 1 minute per run, and about 6 runs 
usually a night).

Your other CHEAP alternative is to make your own cap.  Im thinking i will 
make a bucket cap someday.  These look very tough with a good capacitance.  
And i need a good tough cap that i can subject to a lot of abuse for 
whenever i have any crazy ideas.  Im sure you will figure something out, i 
would go with the MMC caps that you are looking at.  I should also mention 
that Richardson electronics has many CD caps.  I got the .22uf 1600v caps 
for i think $2.93 each, so roughly the same cost for the same bank of caps 
since you will need more.

Steve Ward.




>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: MMC Caps
>Date: Thu, 24 Oct 2002 17:43:51 -0600
>
>Original poster: "Rob Judd by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><canska-at-a5-dot-com>
>
>I'm looking to build an MMC and The Geek Group caps look like a good deal 
>to
>me. The caps are Cornell Dubilier 942C20P15K, 0.150uF +-10%, 2kV each. See
>http://www.cornell-dubilier-dot-com/misc/h942.htm and
>http://www.cornell-dubilier-dot-com/film/9422000.htm for more details on the
>caps.
>
>I've got 3 NSTs, each 15kv 30ma. So for 15kv 90ma the resonant tank cap
>would be 0.0159uF. In theory, one string of 10 of these caps should do it.
>The actual capacitance would be .0150 and should take 10 caps * 2kv = 20kv
>max (assuming the DC volt rating is acceptable as the AC rating for tesla
>coil use). But the peak voltage of a 15kv RMS transformer should be
>something like 19kv. Is this pushing the caps to the limit? Is there any 
>way
>to guestimate the life expectancy of a string of these caps used this way?
>
>I realize I could use longer strings to tolerate more voltage and add more
>strings until reaching the desired capacitance, and also reduce the current
>on each string. But I'm not rich, so if just one string of 10 will work for
>a reasonable lifetime, I'm inclined to go with that!
>
>Any help at all is appreciated!
>Rob Judd - rob-at-a5-dot-com - canska-at-a5-dot-com


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