[Amps] Choke anomoly

[Jim Thomson]

Date: Sun, 11 Nov 2018 17:49:09 +0000
From: Manfred Mornhinweg <manfred at ludens.cl>
To: amps at 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