I picked up a used Dahl 4 Henry choke from a fellow across town yesterday.
Thing weighed a bunch, 127 lbs. Rated at 3A CCS. I measured the DC
resistance,
using my fluke 87..and it measured exactly 10.0 ohms. Then re-measured
using
my B+K 875-A... and it spits out 8.7 ohms. Then used the B+K 875-A to
measure the chokes
inductance, and it measured 3.56 H. I also have a newer B+K 875-B, and it
measured
3.58 H. The B+K 875-B does not measure DC resistance, but AC resistance,
using I believe
a 1 khz square wave, hence it cant not be used on chokes, or xfmrs, readings
come out a blank.
So far, so good. Then used my small 0-60 vdc @ 5A lab supply to feed
0-30 vdc into the choke,
just to see how warm or hot it gets, with 3A of DC flowing. With exactly 10
vdc applied, the current
meter on the power supply displayed 1.15 Amps. Using 10 / 1.15...that
equates to 8.7 ohms,
so it appears the old B+K 875 read the DC resistance correctly.
Then I noticed the 4 x 5 x 12 small lab supply leaned over... and STUCK to
the left side of the choke!
The dahl choke uses the same pair of hypersil C cores as their 10 kva CCS
plate xfmrs, identical, except the
choke, of course has just a single winding. I had a 8 inch long, chrome
plated letter opener close by, and it
stuck to the left side of the choke, like a magnet. Heres the kicker, the
same metal letter opener, would NOT stick
at all when placed on the RIGHT side of choke. Choke is 13 inches wide x
8 inches deep, x 11 inches tall.
I have not played with a HV power supply choke in a long time, and this is the
1st time I have ever tried feeding DC
current through a choke. So what gives here, how come magnetism on left
side, and none at all on right side ?
The choke is in perfect shape and condition, I can see where they have done the
cut on the cores, then polished, the
with winding in place, the tops of each core are put back on, aligned, then the
pair of cinching straps used to mash
each top of each core to the mating lower cores.
Then I turned down the current limit on the lab supply, and all hell breaks
loose on the digital display voltage and current
on the the lab supply. I quickly disconnect the pair of test leads from the
lab supply, and get a small spark. Then with nothing
connected to supply, 3 secs later, smoke billows out of the lab supply, so
yanked the plug out of the 120 vac outlet. My guess
is the back EMF cooked the supply, should have had a rvs connected diode,
and or a MOV across the choke, and or output
of lab supply.
Chokes DC resistance and inductance still measures the same. So how come the
magnetism on left side and nothing on right side ?
Is this normal, or is choke defective ?
The plan was to use it as an experiment in a C-L-C setup on my latest B+
supply, thats under construction. I also have a Kilovac
25 kv rated, 65A CCS ceramic vac relay, in SPST format, to shunt out the
choke, on the fly, as part of the configuration.
Using the PSUD software, the C-L-C config has merit, for reducing ripple
down to extreme low values. Interestingly enough, a
C-R-C, using a simulated 8.7 ohm resistor, also works very good, not as good
as a choke, but extremely effective. The C-R-C
config would reduce ripple by a factor of 10. And thats with the same dcV
drop across the 8.7 ohm resistor as the 8.7 ohms across
the choke. The C-L-C config would provide for a ripple reduction with a
factor of 550.
You also get the same ripple reduction effect if a 10-50 ohm glitch resistor
is used in the B+ line..on the output, but only if a fair amount
of C is used, like 10-20 uf, which of course defeats the purpose of the
glitch resistor. I also discovered the same thing on the proposed
C-L-C config. Unless a fair amount of C is used for the 2nd cap, the choke
has zero ripple reduction enhancement.
I was also going to try placing the choke in the B- lead, to relieve the B+
stress. On paper it sounds good, but when you look at it more closely,
its a wasted effort. IF RVS connected safety diodes are wired from B- to
chassis in both B+ supply..... and also in second cabinet, that houses the
RF deck, the 2 x pairs of diodes are shunting the choke in between them.
The dc Vdrop across the choke will bias the diodes on !
A choke in the B- lead wont work for another reason, the dc V drop across the
choke is now unwanted and extra varying BIAS on the GG Triode !
So choke has to go into the B+ lead. To be on the safe side, I will float it
from chassis, using either glastic insulators or a sheet of either micarta,
or thick UHMW, etc, etc.
The choke was originally used in a marine HF setup B+ supply, custom built.
AFAIK, the choke did not use a resonating cap across it.
Any comments would be greatly appreciated.
Jim VE7RF
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