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Re: Maxwell Labs cap. questions
Hi Tony,
I have 5 of these capacitors in my 500 kV trigger Marx. Nice
capacitors. The specification sheet states derating based upon percentage
of current reversal for pulse operation. For RF applications the derating
is unstated. 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.
Some times the 5 megaVolt EMP generator has a bad day. It doesn't fire
exactly right. The Marx capacitors are subjected to about a 1 megaHertz
ring for about a hundred microseconds or so. After a few such shots they
start to die. Violently. Sometimes holes the size of a dime are blown in
the sides spewing capacitor goodies all over the place. In another
capacitor the whole top was blown off. The explosion was so violent that
it minced the capacitor top. All that could be found of it was a coarse
powder.
Putting capacitors in series with equalizing resistors. I built a
string of 10 mylar plastic capacitors rated at 0.1 uF in series for a
system with a 12 kV 30 ma neon. I reasoned that to withstand 60 Hz
transients (let alone 200 kHz transients) an RC time constant of about 8
milliseconds was needed. Well if C +AD0- 0.01 uF then R +AD0- 800 k Ohms.
That gives about 15 milliamperes average through all of the resistors.
The individual 80 k Ohm resistors would have to be rated at
I+ACoAKg-2+ACo-R +AD0- 18 watts average power each+ACE- This is a bit
much. I decided to use 20 megOhm 2 watt resistors across each capacitor.
A palliative at best. I also decided to arrange the capacitors in a Tesla
+ACI-equidrive+ACI- configuration (Circa 1981) with the lowest possible
inductance arrangement I could come up with. I used massive copper
strapping between all of the capacitors. I figured that with the high
dissipation factor of mylar that they could use all of the cooling !
that was possible. That was the best I could think of at the time. It
worked great+ACE- The thermal mass of the capacitors was so great that
they did not even get warm with a 5 minute run. By empirical qualitative
estimation they were very lossy. I would never use them again.
Barry
-----Original Message-----
From: Tesla List +ADw-tesla+AEA-pupman-dot-com+AD4-
To: tesla+AEA-pupman-dot-com +ADw-tesla+AEA-pupman-dot-com+AD4-
Date: Sunday, November 01, 1998 2:45 AM
Subject: Maxwell Labs cap. questions
Original Poster: NeonGlow+AEA-webtv-dot-net (Tony Greer)
Hello T-coilers everywhere +ACE- Recently, I
found three identical Maxwall Labs high voltage capacitors, and would
like to know how suitable they would be for TC operation, and, anything
else anyone knows about them. Here's what I do know about them: TYPE,
single-ended plastic case, low inductance, pulsed applications. SPECS,
catalog+ACM-31190, cap.+AD0-.08 uf , voltage+AD0-100 kv , series
ind.+AD0-.02uh , RMS
amps+AD0-25. SIZE, (overall) W+AD0-6+ACI- , D+AD0-2.5+ACI- ,
H+AD0-12.75+ACI- , About 8 Lbs. Just
wondering if anybody has used this type of cap. before, and what kind of
performance and reliability they gave, They seem very compact for their
ratings. I plan to use all 3 caps. in series (.0267 uf +AEA- 300 kv) in my
next coil with about 2.5kva input. Would voltage equalization resistors
across the caps be a good idea? If so, what resistance should be used
with an input voltage of 12+AH4-15kv ? Many thanks to anyone who can help+ACE-
Tony Greer in Lubbock , Texas.