Re: CD 942 failure - Found it!!

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

On 8 Mar 2004, at 18:08, Tesla list wrote:

 > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
 >
 > Hi John and All,
 >
 > It's a peak current failure.
 >
 > Our new failure mode...
 >
 > Although the CD942 caps have solid metal plates that never blow
 > out.  Apparently, the floating metalized layer can.
 >
 > http://hot-streamer-dot-com/temp/JohnRcap.jpg
 >
 > http://hot-streamer-dot-com/temp/JohnRcap2.jpg
 >
 > The entire area between the electrodes has bad current stress failure
 > areas much like a fuse.  About 1/4 of it is blown clear.  There does
 > not seem to be any RMS heat melting, but the peak currents probably
 > are twice what the cap is rated at:
 >
 > http://www.cornell-dubilier-dot-com/film/9422000.htm
 >
 > maybe 860 amps per cap or a total of 2600Amps peak in the four
 > strings!!  John mentions that the primary and such get very hot!!
 > What I don't know is why the high currents?  People use one string
 > with 12/30, two with 12/60 without trouble.  So it would seem John's
 > coil is drawing unusual very high peak currents.  But I don't know
 > why.
 >
 > Cheers,
 >
 >  Terry

Two possibilities spring to mind: the coil has a very low primary
inductance, or there is a safety gap connected directly across the
caps that is firing?

Malcolm