Re: What caused capacitor failure?

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

Original poster: "Michael Rhodes by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rhodes-at-fnrf.science.cmu.ac.th>

At 14:09 22/10/01, you wrote:

>12kV 60mA NST
>MMC Built almost identically to the one Terry describes:
>http://users.better-dot-org/tfritz/site/MMCinfo/MMC.html
>
>I used the identical capacitors described ordered from DigiKey.  However, I
>only used eight in series as I only needed to support 12kV.  I built three
>strings, but only used two of them in parallel for a total capacitance of
>.014 microFarads (the transformer matches to .013 microFarads).
>
>I have a RQ-style cylinder spark gap with 10 .28 inch gaps in series.
>
> >>>>>>>>>snipped<<<<<<<<<<<<<<<<<<<<<
>Any ideas what would cause this?  My first guess would be too much voltage
>and they really did blow themselves up.  However, the caps are rated at 1600
>VDC each, so with 8 in series I should be able to handle 12.8 kV.

The 12kV for the NST is the rms rating and you need to select your capacitor
array to support the peak rating.  12kV * 1.4142 = 16.97kV.  So each cap
was seeing 2.12kV across it (16.97 / 8) or 25% above its rating.
Also be sure you have divider resistors to keep an equipotential across
each cap.  So you need "at least" 10 in series to satisfy the voltage
requirement (minimum).

This is some configurations produced by my program QuickTC
Assuming .1uF -at- 1600V DC Working voltage

Strings Caps/String     Capacitance     Voltage         V Rating        T 
Rating
1               8               12.5            12,800          Very 
Poor       Average
2               15              13.33           24,000          Good 
     Excellent
2               16              12.5            25,600          Good 
     Excellent

Strings Caps/String     Capacitance     Voltage         V Rating        T 
Rating
1               7               14.29           11,200          Very 
Poor       Average
2               14              14.29           22,400          Good 
     Excellent
2               15              13.33           24,000          Good 
     Excellent


>How can I prevent this problem in the future?  Needless to say, that was an
>awfully expensive 10 minutes (although immensely satisfying).  I'd like to
>figure out what I can do to prevent this failure before trying it again.
>
>Thanks in advance for your help.
>
>Brett
>
>
>
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   -------------------------------
   Michael Rhodes
   Senior Researcher
   Fast Neutron Research Facility
   Chiang Mai University
   Thailand
   -------------------------------