Re: CDE Caps

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Ian,

At 07:54 PM 2/7/2004, Ian McLean wrote:
>Hello Coilers,
>...
>When I inspected my cap strings, I first checked the capacitance of each
>string (12 x 0.15uF 2000V - 942C type), and each is OK, but I noticed some
>"yellowing" of the white epoxy encapsulant around the leads of a few of the
>capacitors.  There are no punctures or burn marks anywhere that I can tell,
>and the bleeder resistors look fine.  I have allowed 1/2 inch space between
>each cap, and they are mounted on a HDPE board with the bleeders on the
>opposite side of the board, so insulation should be fine.  Everything is
>wire wrapped and soldered with very firm connections.

The white epoxy used for capacitors stains very easily.  In this case, it 
is probably just the metal in the leads reacting with the epoxy chemicals 
or perhaps solder flux.  Ozone and NOx chemicals from the high voltage may 
cause this reaction too.  It is harmless.  After many years, the entire 
epoxy end will yellow just due to age.  Many cap makers just dye the epoxy 
yellow in the first place to hide this effect.


>Is this caused by internal dialectric failure & healing ?  Should I be
>concerned about this ?  Maybe I should increase my string lengths a little ?
>I am running strings of 12 which is 24kV rating for a 15kV/120mA NST stack
>(2 x 15/60 NST's).  I know this is cutting it a bit fine, as peak voltage on
>15kV NST's is about 21kV, right ?  I did however follow the guidelines from
>the Geek Group capacitor charts, so I thought I would be OK.

Don't worry about it at all.  I have some 20 year old white epoxy caps that 
have never been used that have turned all yellow too.

Cheers,

         Terry


>Rgs
>Ian