Date: Sun, 11 Nov 2018 17:49:09 +0000
From: Manfred Mornhinweg <manfred@ludens.cl>
To: amps@contesting.com
Subject: Re: [Amps] Choke anomoly
<Jim,
> 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.
Such a big choke can easily fool a multimeter, due to the high
inductance and low resistance. If the multimeter uses anything other
than a constant, regulated DC for measuring resistance, the inductance
will affect the reading and make it useless.
## Not quite useless. The B+K jived with the indicated vdc / current
as displayed on the lab supply.... b4 I blew it up.
## What I was told by engineers at hammond..who own the Dahl xfmr and
choke line is.... the choke requires DC bias to measure its inductance. With
the
correct test gear used, the choke is actually 4H..with 3A flowing through it.
But the
2 pieces of test gear they use is beyond cost prohibitive for typ hams.
http://www.waynekerrtest.com/datasheet/instruments/wk3255.pdf inductance
meter
http://www.waynekerrtest.com/datasheet/instruments/wk3265b.pdf mating 25 A /
125A bias units.
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 understand this is a double-U core made from wound tape. So it has
just two magnetic joints, rather than the three of a doble E or an E-I
core. Either one or both of those magnetic joints must have a gap in
between. ALthough called airgap, it's normally filled with some
non-magnetic spacer, rather than air. Try to see if your core is gapped
just on one side. If so, then of course that side will attract objects,
and the other side will not, or at least much weaker.
### Problem solved, You are correct of course, there is a insulated
gap on BOTH U shaped cores, aprx 1/4 the way down from the top.
Like an idiot, I knew abt the gaps, but did not realize I had placed the
blade of the letter opener way too low on right side core. Once aligned on the
gap, it sticks like concrete, mother of all magnets. Identical on both sides.
## I obtained another lab supply to continue the testing above... but lab
supply #2
is just a 0-37 vdc @ 3A type. The blown up one is fixable. I got
smart this time,
and installed paralleled 6A10 diodes across the choke to kill any back EMF.
Zero issues
with current limit control, or voltage adjust.
## I ran 3A CCS through the choke for 2.5 hrs straight. ZERO heat at all
from the windings
for the 1st hr. Barely luke warm after 2.5 hrs. DC resistance had risen
from 9.7 ohms cold,
to 10.62 ohms after 2.5 hrs. Inductance rose slightly from 3.68 to
3.74 H.
## with the blown up supply, I could have stuffed a lot more current that 3A
through the choke..
and probably could get it hot... like 50-60 volts at 5A. You increase
current so much, then it goes into thermal runaway,
and stuff heats up very quickly. At least thats my experience with plate
chokes. IE: 1.5A,
the plate choke runs at room ambient temp, then gets toasty warm at 1.6A..and
hot at 1.7 A...
and blazing hot to touch at 1.8A.
<RIP, poor little power supply... :-)
## Its an expensive...poor little power supply, that requires
fixing asap.
> 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.
<Nice - but a modern switching power supply would reduce the ripple much
more, give you a highly regulated voltage even while the line voltage
varies by 30% or more, and as a bonus give you almost perfect power
factor - all while being about 50 times lighter.
And of course, about 200 times more complex, too... ;-) Nothing comes
free in this life...
<Manfred
## 40 Millivolts P-P with 7 kv under a load of 3 A is superb in my book,
thats like .00057 % RMS ripple. However, I cant do anything about the lousy
power factor.... nor the incoming line voltage variations when loaded. You
are
correct though, its all stupid heavy, bulky etc.
## One drawback I see on all switching supplies is they cant be operated at
more than
a tiny bit above their rated current. IE 20A CCS switcher, typ good for
22A ICAS. No big issue
just use a bigger switcher.
## I did notice that continental electronics in Dallas Texas, who builds
SW BC TX, uses
a string of series connected switchers for their B+ supply. Typ they
would total 11-12 kv in the steady
state, with an Un modulated cxr... and double to 22-24 kv, when modulated
100% on AM... and even
more with 125% modulation used. Then down to zero volts, when modulated
100% negative. I Think
each switcher was aprx 700 vdc in the idle condition...and rising to 1400+
VDC on positive mod peaks.
Typ 17 used. So high B+ levels can be done with series switchers.
## For now, Im just using what I have readily available. Xfmrs, chokes, HV
lytics, 6A10 diodes,
metal and glass tubes, manual tuned Pi nets, surplus vac tune and load caps,
vac relays... copper tubing
from home depot, aisle three, all old school stuff.
Later... Jim VE7RF
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