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Re: MMC experience please

Hi John,

	I have done a "lot" of work in this area and have a few programs to answer
such questions.  However, your question points to some serious shortcomings
with those programs that I need to get fixed ASAP!  Mainly, using them on
many types of caps and some user friendly issues...  Reinhard's version at
the below may be better, but I can't quite find it...

http://users.better-dot-org/rbuchner/


There are two sources "I" have for this:

http://users.better-dot-org/tfritz/site/programs/MMCCALC2.ZIP

Is a computer program that does MMC calculations but it is tailored to the
56nF Panasonic caps.  It is fairly easy to modify but it could be vastly
improved.  I have not updated it.  I was so happy to get it to work at all...

http://users.better-dot-org/tfritz/site/MMCinfo/MMCPower4.html

Is sort of the master document I use for this stuff.  The computer program
is based on it.

So putting my next computer project aside ;-) lets talk of your situation...

Your proposed caps have a 22.5mm lead spacing and thus have a rated
dissipation factor of 67 degrees/watt.  However, Panasonic "pads" that
number so it is really closer to 40 C/W.  This is entered into line 305 of
my program (which needs to be changed to a pleasant user input :-))  The
value and cost needs to be changed as well...  I will assume 300kHz and
500BPS as some fairly worst case numbers too.

The program then gives:

MMC Calculator  Ver. 2.1  2/19/2000  Terry Fritz
Transformer voltage =  15000 
Transformer current =  .03 
Firing voltage =  21213.15 
Fo =  300000 
Break rate =  500 
Cap value =  1E-08 
 
Strings  Caps/Str  Capacitance  Voltage   Temp C    Cost
  1        2          5.00         3200   758.95    1.64  :-((  :-(( 
  2        4          5.00         6400   189.74    6.56  :-((  :-(( 
  3        6          5.00         9600    84.33   14.76  :-((  :-(( 
  4        8          5.00        12800    47.43   26.24  :-((  :-(( 
  5       10          5.00        16000    30.36   41.00  :-((  :-(( 
  6       12          5.00        19200    21.08   59.04  :-|   :-(( 
  7       14          5.00        22400    15.49   80.36  :-))  :-(  
  8       16          5.00        25600    11.86  104.96  :-))  :-|  
  9       18          5.00        28800     9.37  132.84  :-))  :-)  
--------------------------------------------------------------------
 10       20          5.00        32000     7.59  164.00  :-))  :-)  

Yipes!!  You need a 9 x 18 $132 cap for such a modest NST!!

The problem is that a 10nF cap has only 1/5th the current handling of a
50nF cap (it is proportional).  So you really don't save any money buy
using the smaller value but cheaper caps.  "I" think caps at about the
100nF 2kV level are the best spark for the buck but rather hard to find.
That UK buy may be a darn good deal (UK MMC bulk buy) even if we are on the
other side of the ocean.

So let's say you use 47nF 1600 volt Seacor caps like D&M's high voltage
sells for $1.30 each for 50.

http://home-dot-netcom-dot-com/~davmckin/DMsCapacitors.html

I don't quite know the dissipation factor for these as well as the
Panasonics (working on that! :-)) so I will use the conservative number of 40.

MMC Calculator  Ver. 2.1  2/19/2000  Terry Fritz
Transformer voltage =  15000 
Transformer current =  .03 
Firing voltage =  21213.15 
Fo =  300000 
Break rate =  500 
Cap value =  4.7E-08 
 
Strings  Caps/Str  Capacitance  Voltage   Temp C    Cost
  1        9          5.22        14400    39.58   11.70  :-((  :-(( 
  2       18          5.22        28800     9.90   46.80  :-))  :-)  
--------------------------------------------------------------------
  2       19          4.95        30400     8.88   49.40  :-))  :-)  

OK!  Now were talking.  36 of D&Ms caps do that same thing for $65 plus
some left over caps.  36 caps at a $1.40 each would be $50.40 .  If Dave is
"sold out", DigiKey would still run you about $100 for the equivalent :-(
Maybe wait till Dave gets them...

Personally, "i" would string ten 56nF 1600V Panasonics together in series
for 5.6nF at 16000V and just let them caps deal with it! :-))  At DigiKey's
10 piece price of $27.48 total, that is still a good, and not "too" risky,
option.  If they blow up, "I" just grab ten more at "my" $14 price :-))  If
you want to "innovate", I think blowing a fan across them would vastly
improve the survivability of such heavily loaded capacitors.  Forced air
cooling, is an area of MMC (EMMC) science yet to be explored...

"I" am trying to get out of the "cap business" so I do not have any plans
for anymore big buys of caps.  In the wild and wooly days of early MMC
testing, having lots of caps was a necessity and I sold the extras.  But
now it is time for those better at "sales" to take over MMC selling...

I will try to get the MMCCalc3 version fixed so such calculations are
easier for first time users and others that do not have the insight gained
from writing the program...

Cheers,

	Terry



At 11:30 PM 7/20/00 -0500, you wrote:
>
>
>Hello,
>
>I am looking into purchasing some panasonic vigh voltage metallized
>polypropylene film caps (pulse caps) to make a MMC.  I will be powering
>it with a 15kv 30ma NST.  Going by the tradition formula the cap value
>should be .005ufd.  I was looking at making the MMC using .01ufd,
>1600vdc caps.  It would have 7 strings of 14 caps.  Which gives .005 -at-
>22,400 vdc.  I understand it is better to use more strings then less.
>Does this arrangement sound ok to you seasoned MMC'ers, or am I missing
>something?  Digikey has these guys for 82 cents each in quantities over
>100.  Under $100 for a great pulse cap sounds good to me!
>BTW I will be using a static gap arrangement for the time being, if that
>makes any difference.
>
>Does the above sound ok or i guess I should ask if you were to be using
>a 15/30 NST and static spark gap how would you design your MMC.
>
>
>Thanks much coilers,
>John M.
>
>