Date: Sun, 21 Mar 2010 12:11:21 -0800
From: "Zeitler, Lane LT, FST-1" <zeitlel@cpr3.navy.mil>
Subject: [Amps] how much C in PS is too much?
I am curious. I see some 10,000uf 450volt electrolytic caps for sale on
Ebay. 8 of these would give you 3600volt capability at more or less
1250uF total capacitance. Is this too much?. Are there any dangers using
this much C in a filter for a PS? In the past I have used eight 560uF
caps and other values similar to this. Just curious, thank you. Lane
Ku7i
## well, I didn't find any on ebay, 3 mins ago..so probably snapped up.
## I have been running various tests using high amounts of C in B+
supplies.. and so far, so good. But not with 10,000 uf units though.
## those things are typ 3.5" diam x 9" long. We have tried 8x 2500uf
caps in series.. = 312 uf. [2" x 4"] Also what has been tried is 30 x
3900 uf in series..and also 24 x 3900 uf. I also tried 20 x 2000 uf.
## later on this year, we will be trying 24 x 2300 uf to make a 'bank'...
then paralleling 9 x banks = 862 uf.
## what else will be tested is 8 x 2500 uf to make one bank.. then
paralleling 2 x such banks. 624 uf.
## some precautions have to be observed though. I use a real 50 ohm glitch R,
which is preceded by a fast HV fuse. The glitch R LIMITS the fault current,
while the hv fuse, INTERRUPTS the fault current. I also install a 2nd hv
fuse,
in one leg of plate xfmr sec... between sec and FWB. I use either 6A10
or 10A10 diodes. [ 1 kv @ 6/10A... 400A surge, 70/100A rep rating.]
## glitch R's that have been tried... and work... [can be cro-barred
repeatedly]
are 2 x 100 ohm, 100 w ww in parallel. 2 x 100 ohm, 225 w ww. 4 x 200 ohm,
275w type CX globars. 4 x 200 ohm, 225 w ww. My latest is 4 x 200 ohm,
type AS globars, each is 1.5" diam x 18" long. Just abt all are fan cooled.
## I also use a fast, 'instant trip' magnetic hydraulic breaker, with no
fluid
in the reservoir [ that's the instant trip part] . Also, since the banks of
caps
are sometimes[95%] in a separate box from the main B+ supply, the box of
caps gets it's own dedicated B+ meter.... strapped directly between B+ and
B- . Ditto with RF deck,.. and also ditto with B+ supply. If the HV
fuse in the
B+ blows open.. the B+ meter in the B+ supply will read full voltage...
meanwhile
the B+ meter in the RF deck reads zero. Box of caps also reads full B+. I
use 10 x
1 meg, ohmite , 1% slimox resistor's, 15 kv rated [each]... in series with the
0-1 ma
meter... to make each B+ meter.
## the step start procedure is different as well. I use huge 20 ohm Arcol,
metal
finned units. [300w] For real high C stuff, the step start R has to be
progressively
lower, so the sec surge doesn't blow out the 2nd HV fuse... [Between xfmr sec
+ FWB]
On the 7700 vdc @ 862 uf set up... 6 x 20 ohm arcols are strapped in parallel.
## The problem is the magnetizing current of the bigger xfmr's.. flows through
the
step start R.. creating a V drop.. so the caps never can get fully charged up.
## step start R goes into one leg of 240 line. I use surplus 3 x pole
contactor's..
and strap all 3 x poles in parallel.. to make on big SPST contactor. 3 x
such assy's are
built. One for each leg of the 240 line.. and the 3rd one does the step
start. A 0-15
or 0-30 second timer module does the step start timing. The 10 contacts of
the
timer are used to apply 120/240 to the solenoid coil of the step start
contactor.
## energy storage goes up to the square of the voltage. Another method of
step start
involves using a small variac [ big enough to handle the mag current] to ramp
up the
B+ to max.... then the variac is quickly switched out of the circuit. [no
step start R
used at all] I have not tried using a triac to ramp up the B+ over several
seconds.
## To make the high C filter work right... you need a xfmr with low esr on
both pri+sec.
And also big ga wire back to main panel. Still, with just 8 x 2500 uf
caps.. and a slightly
tweaked up step start, and stock small xfmr, the results are amazing. Zero
ripple, and superb
dynamic regulation. Static regulation improves too..esp with heavy loads.
Pep output
on wattmeter is up... like 100-150 w pep.
## I used 100 k 3 w mof 1% tol eq resistors from mouser.. across each
cap. Also, a 6A10
safety diode is rvs connected across each lytic. 4-6 parallel 6A10's are
wired between
chassis and B-. And 4-6 more, flipped 180 deg. [rvs connected, 8-12 in
total] . This way,
B- can't wonder more than +/- .7 V Just realize you are dealing with
one helluva
lot of stored up energy.
Later........ Jim VE7RF
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