Date: Thu, 24 May 2012 11:33:19 +0000 (UTC)
From: Paul Decker <kg7hf@comcast.net>
Subject: Re: [Amps] Sensible filter Cap sizing
Hi Gang,
I've had a couple comments about 5x500V not being enough for 2488V and that I
should use 6 or 7 caps. When I was first building my ps, I was thinking along
the same lines, and if I had 6 or 7, I likely would have put them in just for
good measure. On the other hand I also rationalized it by thinking those
capicator designers have already put in a safety margin to meet their MTBF
stats, and I know that my p-p best case is 2488, even exceeding 2248 it
woudln't be a normal or prolonged use case, so why should I add yet another
safety margin to the system. So far, so good. I guess I could replace them with
the 8 3900uf 450V caps I got for free the other day.
73,
Paul, kg7hf
## Its gonna blow up in your face ! And you can forget about any “ 525 vdc
surge rating” either. Any surge rating is for less than 2-3 secs max.
With the caps maxed out, your leakage current will be sky high.
Did you even measure it. How do you know the voltage is dividing equally
between ALL the caps. Did u measure the V drop across EACH cap. What
exact value is EACH of the eq resistors across each cap. The value is not
important..but
they all gotta be the same ! IE: all 90 K.... or all 100 K. I use the
vishay brand 100K 3watt from mouser. These are 1% tolerance type..so no more
playing matchup by hand. 100 per box... and I bought several boxs of em..and
3 different
lot numbers too. I checked all 300 of em... and they are all within just a
few ohms of each other OK, now the v drop across each ca will be the same.
I instead, you use say 5% tol eq resistors..watch out. the caps are in
series..and so are the
eq resistors. The bledder current will be the SAME through all the resistors.
BUT the V drop will be ... bleeder current X resistor value. The resistor
with the HIGHEST value will also get the HIGHEST V drop..which is gonna be
way over 500 Vdc.
IF your line 240 vac line voltage from the street happens to wander to the
high side one day.... = kablamo. Loads of folks with AL-1500’s have had
that happen already.
Ideally, the bleeder current will be at least 15 x greater than the leakage
current of the caps. The leakage current on the caps will be largely
dependent on the V drop across each cap. Look up the eng specs and notes
yourself. Run the cap at 100% of
its V rating, and its leakage current is high. Run the same cap at 90% of its
V rating..and leakage current drops like a rock. I wouldn’t run caps at more
than 75-80% of their rating. 2488 div by .75 = 3317. 3317 div by 450 =
7.37
You need a min of 7 x caps... end of story. BTW... I have used those 3900
uf caps. They work damn good.
later... Jim VE7RF
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