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Re: Good cheap large caps?



Original poster: "tesla by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla-at-paradise-dot-net.nz>

Team and Duck
Thank you for your response Chris.
As some list readers will be aware I've long thought the bigger 942C series
caps with a little less voltage were a better option

While I'm not in a position to compare the two caps in service I've done a
few sums for a small, medium and a large MMC based on the two caps.
As you would imagine in each case the bigger caps require far fewer units
BUT there  is always less Ipk and Irms capability using the larger caps as
fewer parallel strings are needed.

Following are some numerical comparisions using minimum components and
allowing current capability specs to vary. Ctotal values are made as close
as possible

20nF/30kv using 150nF/2kv  Units reqd 20, Irms 27A, Ipk 864A, 2 strings  of
10
18nF/30kv using 330nF/1.6kv Units reqd 18, Irms 13.3A Ipk 633A  1 strings 18

112nF/16kv using 150nF/2kv  Units reqd 48, Irms 81A, Ipk 2.59kA, 6 strings
of 8
99nF/16kv using 330nF/1.6kv Units reqd 30, Irms 40A, Ipk 1.9kA, 3 strings of
10

50nF/32kv using 150nF/2kv Units reqd 80, Irms 67.5A, Ipk 2.16kA, 5 Strings
of 30
49nF/32kv using 330nF/1.6kv Units reqd 60 Irms 40A, Ipk 1.9kA, 3 Strings of
20

96nF/50kv using 150nF/2kv Units reqd 400, Irms 216A, Ipk 6.9kA, 16 Strings
25
96nF/50kv using 330nF/1.6kv Units reqd 279, Irms 120A Ipk 5.7kA, 9 Strings
31

For a design with similar the following numerical example shows that it is
more ecomomic to use the larger cap(less units reqd)  with the benefit of an
increase in Voltage capability resulting for the need to have more strings
of less C per string.

96nF/50kv using 150nF/2kv Units reqd 400, Irms 216A, Ipk 6.9kA, 16 Strings
of 25
104nF/56kv using 330nF/1.6kv Units reqd 385. Irms 146A, Ipk 7kA, 11 Strings
of 35

Clearly the best component to use will depend on your specific coil but I
believe the forgoing illustrates a good case for the 330nF/1.6kv unit on the
basis of cost. I can purchase the 330nF/1.6kv unit for USD$2.38 at qty=27
units which is comparable or less than the 150nF/2kv caps

Hope no number crunching errors above and it gives food for thought
Best to list
Ted Linney in NZ