On Sun, 16 Oct 2005 02:19:32 GMT Rick Frazier
wrote:
Wow, with cap banks like you describe, there's some significant energy
there... What are you working on, a pulsed cyclotron?
Plasma physics research facility. 2-4 minute charge cycle, 70 mSec
discharge. These banks supply the crowbar, or "follow-on," current
once the big 5 kV banks have discharged down. Yes the charging
supplies are large, and the future ones will have to be even larger.
We tend to go for some kind of constant current scheme, up to the
preset voltage limit.
I don't think our leakage current is nearly as high as you suggest.
The caps tend to form nicely after awhile and the leakage current
ramps down with use. Until one fails, of course. ;-)
A bank of 90 caps at 16000uf each looks like 1.4 farads. That's an
incredible amount of storage at first look. The leakage alone on a bank like
that would be about 400ma.
I rounded. We're actually shooting for individual banks of .3F at
900V, so that takes 1.2F of 450V caps, times 16 of these banks.
Charge/discharge is as above, leakage should be less that what you
suggest, but I could be wrong about that.
In the linear amplifier, there are really a couple of things that tend to
kill the caps. First, the ripple current tends to heat the caps, which are
in a relatively warm location to begin with, sitting right next to a fairly
large transformer, and a full wave bridge just above them.
I don't know how much internal heating you'll get, but temp is
certainly something to be concerned with. Our application stays pretty
much at room temp.
However, given that I live in an area with less than perfect regulation of
the incoming AC line, I tend to be a bit gun-shy about what I see personally
as a "marginal" design. At my last QTH, the line voltage per our local
Hawaiian Electric Light Company (HELCO) should have been between 230 and 240
volts. I was the last house on the end of a 1 mile run of utility poles, and
the transformer, which was over 300 feet from the building, served only my
house and workshop at the time. In my workshop, I ran a single phase to 3
phase solid state inverter for a 3 horsepower wood lathe. During a period of
two years, even with rather sporadic use after the first year, I had four
inverters fail, and the failure was always the input diode bridge.
Did you consider upgrading the input bridge? I agree that excessive
line voltage is a problem, but the bridge was clearly your weak link.
After the
first failure, I instrumented the AC line and kept graphs on it. The line
voltage ranged from 235 to 243 volts over the typical 24 hour period, and
tended to be around 239 volts much of the time, which I see as the nominal
voltage for that location. Unfortunately, there were times when the voltage
would rise to as much as 255 volts for brief periods of time (usually only a
few seconds, but one prolonged instance went over 15 seconds).
I think the surge ratings for Al caps are for a 30 second surge.
OK, not a long time, considering the really big picture,
but looking inside the amplifier and seeing that three of the bridging
resistors (50K ohm, 7 watts) and 4 of the capacitors were replaced at some
time in its history, I am tending towards getting more margin into the
design.
I agree that there's often room for improvement in amateur gear.
Upping the resistor power ratings might be useful, too. If they run in
the same ambient, then they should also be derated.
My tolerance for failure is low. I would prefer to get this amplifier up to
snuff, put it into service, and never have to open the case again.
I agree with your goal, and I don't think running 450V caps at 450V
will come back to haunt you. Just make sure that the 30 second surge
rating is above the highest voltage you think you'll ever see.
Of course, in the back of my mind (and not previously mentioned) is the
possibility of running the amplifier from the 220 volt tap on the
transformer, which would give me 3925 volts nominal on the bank at 240 volts,
and 4010 volts on the bank at 245 volts, and 4175 if I see the same sort of
255 volt spike at the new QTH.
With 10 caps, that's still less than 420 V/cap. I don't think that's
any problem at all.
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Jim Adney
Madison, WI 53711 USA
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