Re: Good & Bad Maxwell Caps, MMC questions?

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Bill,

At 10:40 AM 7/16/2001 -0400, you wrote:
>I just blew my Maxwell cap (0.06uF, 30kV, catalog #37321 - type with
>terminals on top).

:-(

>
>I was never sure whether this was a good cap for Tesla use.  I've seen
>guidelines in the past that said those with the terminals on top
>(instead of on opposite sides) generally didn't have a long life.  It
>worked great for a while, but now it's dead (cap makes a sort of
>fizzling sound when I first power up).
>
>Can anyone confirm that this is indeed not a good cap to use, or is this
>what I could expect with any Maxwell cap?  And if mine was bad, which
>are good?  The reason I'd like to get another Maxwell (if I can find a
>more reliable one) is that it's "plug-and-play" - no assembly required,
>and compact.

"i" would smack it with a hammer and get inside to the guts and see why if
failed.  Poor connections, leaked oil, assembly/quality problem...  One I
saw had poor connections to the "packs" inside.  Casual contacts in oil for
like a 1000 amp peak part =:O

>
>Are MMC's really any better?  

Oh!  You bet! :-)

>To what extent are they "self healing"? 

They are "amazingly" self healing.  I have seen poly caps that have the
dielectrics "machine gunned" or "Swiss cheesed" that are still perfectly
good for service and still in spec.  Maybe 100,000 holes blown in them...
You can't just burn them alive, but the occasional over voltage (they take
3X their rating) is no problem at all.  Some manufacturers deliberately
over voltage them to find their failure point for SPC reasons prior to
shipping.

>To build a really sturdy MMC with the geek group caps (942c), for use
>with 15/180 NST (three 15/60's), what is the minimum string length and
>number of strings to handle the voltage and current they'll see?  

A single 15/60 would be hard pressed to tax a single string so three or
more strings would be fine.

>I can
>adjust upwards to get my desired capacitance, which is between 0.06uF
>and 0.045uF --- 0.06 because that's what I was using and it worked,
>0.045 because that's the "right" LTR value for 15/180 with a static gap,
>but I don't know (I haven't calculated yet) if my primary has enough
>extra turns that I'd still be able to tap it in tune with 0.045 (I
>currently tap around 7.5 with my biggest topload, and have 10 turns).  I
>understand that the strings need to be of adequate length to handle the
>voltage (though I'm not sure what that length is to get a long MMC life
>with 15kV AC in tesla coil use).  Am I right that there is also some
>minimum number of strings needed for adequate current handling (180ma,
>in my case)? 

A 15kV transformer has a peak voltage of 21kV.  10 caps in a string (2 kV
each caps) would be fine and will last forever voltage wise.  150nF / 10 is
15nF.  So, three strings of 10 for 45nF or four strings for 60nF is what
you want.

40 caps would run about $90.00...

>
>BTW (and I'm not sure, but I think I've heard this reported before), my
>coil seemed to be performing better than it ever had shortly before it
>died.

Maybe cause the voltage was turned up too high ;-))


Cheers,

	Terry


>
>	Thanks,
>	-  Bill Vanyo
>