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Re: Maxwell Labs cap. questions
Tesla List wrote:
>
> Original Poster: RWB355-at-aol-dot-com
>
>
> Original Poster: "B**2" <bensonbd-at-erols-dot-com>
>
> Hi Tony,
> I have 5 of these capacitors in my 500 kV trigger Marx.They are
made with
> mylar and are therefore lossy and will heat up faster than a
polypropylene
> capacitor. They also contain castor oil which is viscus and not as
good at
> dissipating heat as oil. These will work in Tesla coil service but
will be
> lossy. You must keep a constant
> vigil on temperature. A slight feeling of warmth on the outside
could be
> very hot on the inside.
>
> Hi Tony, B**2, All
>
> If those caps are really built this way, forget about using them in
TC work. I
> blew several caps from the Platic Cap Co (which where built in a
similar
>> fashion). On my MOT test setup the caps got so hot, (in about 15 secs run
> time) that the glass cracked and melted the red label inside plus you
get a
> nice mess as the insides are turned outward. On an OBIT (200W) setup
any run
> time longer than about 40 sec spelled instant death to these caps.
>
> Coiler greets from germany,
> Reinhard
>
>
All,
I have three of those single ended type Maxwell caps (PN 31165) with
the
insulating blade rated at 0.04 uF-at-100kVDC wired in parallel. They were
used successfully in a 7-8 kVA, pole pige equivalent powered TC with a
max of 12 kV RMS applied. The factory rates these capacitors for 1 shot
per second. My break rate was 400 BPS. They would get slightly warm
to
the touch in their lower half where the guts are after about three or
four 30 second runs at full power. I limited my runs to about 30
seconds or less and gave equal time or more cooling time between runs,
over perhaps 5-6 runs average during a test session. Many such test
sessions were conducted and the capacitors did not fail or bulge. I
suspect the secret is partly and most importantly to keep the applied
RMS low enough that the resultant peak-to-peak voltage created by RF
ringing reversals stays well below the capacitor's voltage rating.
I have been able to replace these recently with double ended case style
40 kVDC Maxwells actually rated for 200 PPS or greater rep rates, which
I'm finding give me perhaps 10-15% better coil performance but go
somewhat longer before getting warm. Even these are not recommended for
use above 20% voltage reversal however (TC service is more like 85%+),
so I would not employ them in a commercial coil system. I'll be posting
pictures of this recently upgraded hobby coil system shortly on Chip's
website under "Latest pictures from the list" for those interested in
seeing a fine performer in action. It hurls 12 foot connecting
streamers
on just 7000 VA.
To summarize, I wouldn't pass on the 1 PPS, mylar(?) and castor oil
filled Maxwells rated at 100 kVDC for hobby level TC purposes if they
were obtainable at a good price. I have no confirming info that these
do indeed contain mylar dielectric and am suprised if they do as the
heating I experienced, especially at the power levels I employed, did
not seem to be commensurate with my mylar capacitor experiences in
general. I also did not see a detrimental tuning shift with these 1
PPS
Maxwells, another clue that their dielectric may not in fact be mylar.
Anyone out there know more about these?
I have also tried the red label end-foil PCI caps with the mylar
dielectric in NST powered systems and find that they heat so quickly
that the best tune point changes after only 15 seconds. I have never
ruptured or exploded one, but they have ALL gone to seeping oil as a
result of minor TC service. I do not recommend these at all for this
service. There however IS an identical case capacitor which should be
OK for TC service and these are the ones with the blue label inside the
clear glass housing. The blue is a code for polypropelene dielectric
as
opposed to the red for mylar. I haven't had the opportunity to try
these blue ones as they seem to be much more scarce in surplus circles.
Condenser Products is the original manufacturer to market this case
style cap as far as I know and their coined trade name for these is
"Glassmike".
Good luck.
Robert W. Stephens