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Re: [Amps] Choke anomoly

Subject: Re: [Amps] Choke anomoly
From: Manfred Mornhinweg <>
Date: Sun, 11 Nov 2018 17:49:09 +0000
List-post: <>

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.

Some multimeters, and often the better ones, use pulsed DC when
measuring resistance, to conserve battery power. These won't work right
with such a choke.

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.

Where magnetic flux leaks out of the core, ferromagnetic objects will be attracted. Instead where the flux stays well confined in the core, attraction of external objects will be very low, maybe unnoticeable. So either your choke has its air gap made on that side where it attracts stuff, or due to some asymmetry in the assembly it has an unintentional air gap or magnetic bottleneck on that side.

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.

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.

If instead the gap is distributed on both sides, then maybe it's asymmeric - intentionally or not.

Consider the possibility of the tow U pieces being symmetric, assembled with spacers between, but one of the spacers being steel and the other being non-magnetic ("air" gap). In that case of course the side with the non-magnetic spacer will attract objects.

All normal, constant-value chokes using iron lamination cores will have an airgap (or two), while transformers will not. Instead swinging chokes may use no airgap at all, or only a tiny one. If yours is a swinging choke, the asymmetry might be due purely to manufacturing tolerance, the two pieces meeting better on one side than on the other.

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.

I have designed some variable voltage, variable current power supplies, and I can tell you that it's mighty hard to get them to be stable when working into highly reactive loads! Using current-limited operation into a choke is the nastiest condition possible: The choke tries to hold the current constant, while the power supplies tries to force certain specific current. The two clash head-to-head! The control loop gain soars to almost infinity, together with a full 90 degree phase shift in the load, that adds to the 90 degrees phase shift of the supply's power stage, making a nice 180 deree phase shift and thus a formidable power oscillator. The power supply tries to force a square wave current through the choke, which reacts with voltage spikes that can easily reach kilovolts, at the full current that's flowing, thus putting out pulses that can exceed 10 kVA, and your poor little power supply had no chance to think what to do...

RIP, poor little power supply... :-)

how come the magnetism on left side and nothing on right side ? Is
this normal, or is choke defective ?

It's either due to intentional asymmetry (air gap on one side), or unintentional asymmetry (residual airgap on one side). I see no reason for worrying, the choke should still work fine.

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

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...


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