Date: Sat, 10 Apr 2010 22:00:40 +0800
From: Alek Petkovic <vk6apk@bigpond.com>
Subject: Re: [Amps] HV lytic max V ratings..some notes.
At 09:51 PM 10/04/2010, Carl wrote:
>I dont know if it still holds with some caps but they used to have a memory.
>IOW if run for a long spell (weeks, months) at say 60% of ratings and then
>run at max they had a tendency to fail.
>
>Carl
>KM1H
Yeah, that's what I was always lead to believe. I was told that it is
better to run your caps very close to their voltage rating. Dunno if
its a wives tale or not.
I usually sail pretty close to the wind. I work on having 1 spare
capacitor worth of headroom in a bank. For example, if the dc voltage
is 4500V, then I would use 11 x 450VW capacitors in the bank. Never
had a cap go bang on me by using that crude formula.
73, Alek.
VK6APK
## When firing up old caps that have been sitting around in box's, or these
surplus
ones I have been buying, this is the procedure I use.
## I insert a fluke dvm between fwb and B- of caps. [using 10 kv rated leads]
I TEMP bond the B- to chassis for safety purposes. This does not affect the
B+
meter, as it's wired directly between B+.. and B-. Using a small, FUSED
variac,
I bring the supply up slowly, over 10 x seconds.. to 500 vdc.. then let it sit
for
30 mins. Then repeat the process to 1 kv... then 1.5 kv etc.
## The DVM will show any leakage current. They sit as long as needed, till
any leakage
drops to zero. Then increase some more.
## I have never had a problem with the... 'memory issue'.. and this is the 1st
time I have even
heard of it. Now, say if the HB B+ supply USED to be run at 3 kv... and
caps were run at
75% of their ratings.... and now I want to run the caps at 90% rating.
4500vdc /4950vdc
= 90% using aleks example, then in goes the variac, [ step start temp
disconnected].. and the
forming process done again.... but to 90% of the strings ratings. Then all
temp stuff removed,
etc.. and B+ supply now run at the higher vdc. Never had any problems
using the variac
procedure... regardless of whether the caps were originally used at 65-75% of
their rating...
then run at 90+% or if old caps used, etc.
## There are a LOT of different series of caps these days.. in both CD's on
line catalog..
and also united chemi con /nippon chemi con's on line catalog. Heck, you can
still buy the
old style CGS series of hv caps... and also the old style caps from other
makers.
## Dunno if the 'memory' issue pertains to old style caps.. new style.. or
both... or if it's
an anomaly of one style of caps.. from one maker.. or just an anomaly. I
checked tech notes
and eng application notes from every cap maker... and also real old tech notes
from the same cap
makers... and can find NO references to the ..'memory issue'.
## In the past.. I didn't like the idea of running caps at 90% of their V
rating..esp with the
carbon comp eq resistor's drifting, etc. A transient off the street could
double in peak V.. b4 any
mov was fast enough to clip it off. The newer modern style, "high capacity in
the can"... caps.. used
with good close tol eq resistors, up to 90% of their ratings is no problem...
provided leakage is not
too high. If it is, then the value of the eq resistor has to be looked at...
and probably lowered.
## These super big uf caps have higher leakage current than the smaller caps.
[4800 uf vs 560uf]
Since I have excess capacitance to burn .. and CD's notes said for max long
term reliability, to run em at
75%.. that's what I have been doing. Then the leakage current is through the
floor... and I still have bucket
loads of uf available. If transients / spikes get past the movs, bypass
caps,etc... then the hv lytics HAVE to
absorb em. The caps, of course, look like a dead short to a high voltage
transient/spike.. and the caps will
never charge up to the higher value. In any event, you generally want the
bleeder current of the eq resistor's
to exceed the leakage current by a ratio of at least 15-1 . When I use CD's
formulae for value of the EQ resistor
[for caps in series].... it comes out a helluva lot lower than 100 k ohms. I
use 100K anyway. Then I always end
up with the 15-1 ratio... even if the caps were run at 100% of their V
ratings.
## For safety reasons... when experimenting with these high C filters... I
wanted PLENTY of vdc overhead.
[2000-5200 uf variety]
later........ Jim VE7RF
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