Re: MMCs & Currents

["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>

Hi Terry,

Thanks for clearing things up some for me, today has been a downer since i 
was at a friends wake today :(  I finished the coil exept a large enough 
toroid, but i have some things that will work (a few small toroids and 
spheres).  Hopefully i will not blow any fuse!  Wish me luck,

Steve Ward.


>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: MMCs & Currents
>Date: Wed, 31 Jul 2002 12:28:13 -0600
>
>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
>Hi Steve,
>
>At 11:38 PM 7/30/2002 -0500, you wrote:
> >Hello list,
> >
> >Im looking for some opinions and answers here.  Im finishing up my large
> >coil, and have already obtained the capacitors.  They are GE .68uf 1200v 
>42L
> >series.  Looking at the Cornell Dubilier ratings for their same cap, they
> >list a I peak of 854A.  Im powering my coil with a 15000v 120ma supply.  
>I
> >am not using a rotary gap, but a oversized static gap with lots of
> >cooling(im still shakey about this).  So assuming my GE caps can handle 
>the
> >854A peak, does it sound like im ok with these?? Im using a single string 
>of
> >24 caps in series (.0283uf -at- 28800vdc).  I have a few other questions 
>about
> >caps while im at it:
>
>Yes, you are ok,  The RMS current rating is 14.7 amps which is just about
>what your coil will draw and the peak current may be just slightly less but
>it's ok.  I'll tell you a secret... The specs they list are about 1/2 what
>they really can handle :-))  The cap manufacturers generally use a 100%
>safety margin so people that run them a "little hot" won't have problems.
>
>Someone mentioned that it may be risky to use another manufacturers specs
>for this stuff.  As long as the caps are the same type and construction (be
>very careful here!), there is no magic between them.  The makers all buy
>the materials from the same place anyway.  Sometimes they buy the whole cap
>from the same place and just put a different label on it ;-))  The data at:
>
>http://www.cornell-dubilier-dot-com/misc/h942.htm
>
>is extremely useful to us!!  They have all the critical numbers we need in
>perfect simple charts.  The other manufacturers could learn a lot from how
>CD specs their caps in such a straight forward way...
>
> >
> >1. Do SRSGs cause more or less stress on the caps than with a static gap?
> >I'd assume less with less discharges per time period
>
>SRSGs will draw a little more current, but they are far more controlled.
>"things can happen" with static gaps that don't happen with rotaries.  Of
>course, rotaries can fail and send odd signals into things too.  Always use
>safety gaps in any case.  However, MMCs tend to be very over--voltage
>resistant, the NST is what will die from too much voltage.  MMCs are far
>more stressed by current, but now days, we are really good at being sure to
>use enough cap for a given coil system.  We now also have nice sources for
>those caps so it's not a problem.
>
> >
> >2. What is more stressful, 120bps or 240bps?(i think 240)
>
>240 BPS draws twice the current of 120BPS.  It is directly linear all else
>being equal.  People that run gaps at like 1000++ BPS with like a 10kVA pig
>behind it tend to blow caps into many small pieces unless they are using
>super powerful caps.
>
> >
> >3. Has anyone used static gaps with this much power?  Did it work good?
>
>Just a matter of keeping the gap cool.  Probably need forced air.
>
> >
> >Im lead to believe that i may be ok because of this scenario:  A friends
> >coil uses 12kv 120ma supply with 2 strings of .1uf 2000v CD caps(9 caps 
>per
> >string).  These only have a I peak of 285A.  We ran his coil for about 
>2-3
> >minutes with out any detectable warming of the caps.  He is also running 
>a
> >static gap.  We achieved 51" sparks :)  not bad!  Im guessing that since 
>i
> >have about 3X the I peak rating, that my single string could run better 
>than
> >his 2 strings.  Is there anything that im not understanding here??  And 
>just
> >in case anyone is wondering, the caps are 44mm wide and 27mm in diameter.
> >Im still awaiting my primary supports(in the mail), then i will basically 
>be
> >done!  Ill give all the details when im finished.
> >
>
>You have it correct.  These "big" 0.5uF and higher caps can handle a giant
>amount of current.  In general, the current is proportional to the value,
>so a 0.5uF cap will handle 5 times the current of a 0.1uF cap.  It is hard
>to burn these big caps up without a pole pig or some really serious power.
>Soon the wire leads get too small and you need the ones with the metal
>tabs.  Sometimes these caps can go to twice the current just buy getting
>the tab leaded versions.  Their disadvantage is the voltage is usually less
>so you need more of them.  Not a big deal if you can get the caps cheap.
>
>Cheers,
>
>	Terry
>
>
> >Steve Ward
> >
> >