Date: Wed, 18 Apr 2012 13:47:07 -0600
From: "John Lyles" <jtml@losalamos.com>
Subject: [Amps] More Capacitors vs LC filters
One more thing about big oil-filled capacitors: When the voltage gets above
about 10-12 kV, manufacturers build their capacitors with series capacitor
packs. Each one is wound paper or film with foil, connected by a jumper wire or
strip.
If you figure a 45 kV cap is made out of 3 series-connected 15 kV assemblies,
and the total cap is, say 5 uF, then each cap must be 15 uF internally. All is
fine and good until one of those packs shorts out. Lets say you are operating
at 30 kV for convenience and this happens.
Now you have two capacitors, of 15 uF each, operating at 15 kV each (approx),
very close to their death zone. Doing the calculation, the original 5 uF
capacitor could store up to 4 kJ at 40kV (1.3 kJ per internal cap), close as
you would dare run to ratings. But is is derated to operating at
2.25 KJ. Now, in the failure mode, which is, by the way, realistic, in a pulsed
system over time, it is operating with two assemblies and the total stored
energy went up to 3.37 KJ. This is part of the problem that will cause a 4 KJ
capacitor to 'rupture' and expel oil and flames, in addition to slitting metal
case.
I don't know how those big 'Lytics are made, the 3900 uF 450 VDC jobbies, but
if there is are any multiple-connected caps inside, then failure scenarios like
this warrant being thought about.
John
K5PRO
## Points well taken. The 3900uf @ 450 vdc caps are just ONE cap inside,
same deal with the even bigger caps
More Capacitors vs LC filters
. Some where along the line, cap makers figured out how to stuff a lot of uf
into a smaller package. Reading through the tech notes,
what they did was dimple the foil, etc on the electrolytics. Much like
dimples on a golf ball, except they dimple on both sides of the foil, etc.
By doing this, they managed to increase the total C by a huge amount.
## The 3900 uf caps are not small either, but small for their total C. The
2300uf caps I have are 2.5 inch od x 4 inch tall. The 2500 uf caps are
smaller, at 2 inch OD x 4 inch tall...and weigh even less, a mere 14 ozs
each.
The biggest ones I have are 10,000 uf each @ 450 vdc. These are 3 inch
diam x 9 inch tall.
## cap makers also terminated them internally differently these days.
They now use wide, flat connections, instead of small diam, round connections.
That reduced the series XL by a whole bunch. Then they can operate them
on higher freqs. If you look at old 1980 handbooks.. the switching supplies
always used like nine caps in parallel..since one big cap would not handle
the ripple current.
## On modern day style lytics, the ripple current rating now increases with
increasing freq. The old style lytics were the opposite. I only operate
these caps at 70-75% of the max V rating. With 7650 vdc no load,,,and 24
caps.... no big deal. 7 caps could short out and then with 17 left, they are
100% maxed out. Still, you have to use some common sense with the amount of
energy involved. I have never had a cap short out since 1972 when I got into
this hobby. The worse case is the uf will drop off on real old lytics,
when they dry out. Never seen one go open either.
later... Jim VE7RF
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