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[Amps] Choke anomoly

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Subject: [Amps] Choke anomoly
From: "Jim Thomson" <>
Date: Thu, 8 Nov 2018 21:43:02 -0800
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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 
using my  fluke 87..and it measured exactly 10.0  ohms.    Then re-measured 
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  
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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