Date: Fri, 10 Jan 2014 11:16:32 +0100
From: peter chadwick <g8on@fsmail.net>
To: km1h@jeremy.mv.com, amps@contesting.com
Subject: Re: [Amps] Russian HV Supply Capacitors
## I concur. I once used 4 x 30uf at 3500 v OIL caps...rated for discharge
use in a 4-1000 amp back in the late 70s. Wired series parallel with careful
eq across the caps. caps were wired series-parallel.
IE: 30 uf at 7 kv. B+ supply put out 4400 vdc no load. Worked for 18
months...then 3 of em leaked oil badly. 4400 / 7000 = 63% of max V rating.
max plate current on the 4-1000 amp at the time was 600 ma. Loaded B+ was
4400 vdc. :
## The problem with that russian cap is....its missing the two most important
specs. ESR....and CCS ripple current rating. My B+K 875B measures esr at
1 khz. Some esr measuring devices will measure at freqs like
100 hz, 300, 1 khz, 10 khz etc. Watch out. On modern design lytics, the ESR
will drop as test freq increases. This is due to the lower internal XL
from wider conductors being used. Switching supplies operate at high freqs.
On the old style caps, it was the reverse. Old style caps had low esr at low
freqs.....which got worse at freqs increased. On old switching designs in the
ARRL books, they used several caps in parallel to handle the ripple current
at the higher freqs.
## If we knew the ESR at 120 hz.......and also CCS ripple current rating at
120 hz....... one could then properly evaluate the Russian oil cap. Carl is no
doubt correct. Don’t operate discharge caps at any more than 50% of their V
rating.
## Years ago, Far electronics was selling high ESR lytics, designed for photo
flash use. Most reported that their 8 caps in series would overheat, spew crap
out the vent holes, etc... when used in the typ 1200-1500w pep out SSB amp.
Jim VE7RF
I agree with Carl here. I had a string of 220mFd 500 volt electrolytics meant
for photoflash work: even in a choke input filter, they didn't last very long
before squirting stuff out of the vent holes. The Sprague 36D computer grade
electrolytics I replaced them with lasted for years, and have only been taken
out of service because there's an Acom 1500 due to arrive on Monday!
Capacitors meant for SCR commutation are another matter - they are designed for
continuous high currents
73
Peter G3RZP
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