Triaxial wire build for power supply rails

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Made a triax out of some cheap materials. The goal being the complete containment of magnetic fields which are produced by the current that is pulled from the supply. This solution avoids all the normal inductance based fields that can interact with all the low level signals, and keeps the inductance to the chip low.

It is 8 consecutive pictures, so hold on till I finish. The filenames essentially tell the story. The internal wire is just some stuff off the shelf, the braid I used was harvested from some parts express microphone cable.

After the last pic, I'll post the as designed inductances, as well as the measured inductances, capacitances, resistances, and the dielectric constants of the materials.

Here's the first: purchased heatsink from pe..66 cents per 4 foot length, up to 99 cents, I think..

John
 

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Here's the braid after I have pulled it off the wire. It can be tricky, as the wire pair has some paoer around it, and if you catch the paper, it will bunch up inside the braid. That makes it a bull to get it off.

Watch out with the razor, as if you go too deep, you will nick the copper wire of the braid. This makes pulling the braid over the wire a pain when the nicked braid sticks ya..
 

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Here's the braid stretched over the inner wire. Slide the braid over the entire wire, and when it's all there, twiste one end past the end of the wire. This gives an anchor to the braid, and you can work the braid towards the other end. Try to keep the braid against the wire, and keep the braid uniform. This will minimize inductance.
 

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Here's the final assembly, the yellow shrink is the final coat, and it's been shrunk.

I used 3/16 shrink for both layers. Different wire sizes will require different shrinks of couse, I purchased 3/16 up to 3/4 size for various experiments.
 

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I had promised the wire to Peter for a trial in sumptin, but I had some testing problems.

I measured the diameters of the wire, calculated the inductances..then, when tested, the inductances were not right. Took a while to figure it out.

I thought I used 16 guage wire, that is 50 mils diameter. The caliper I used was off, as it did measure 50 mils...when I measured the resistance of the wire, turned out to be 8.53 milliohms per foot...18 guage wire...duh..got a good caliper, confirmed 18 awg..

RE-calc'd the inductances, they are much closer.

Measurements were performed at 100 Khz for resolution. I believe it also dropped the internal inductance of the inner solid wire due to skin effect, I intend to persue that further..

Results:

Inner to middle:

L 60.25 nH per foot

C 107 pf per foot.

Z cable..23.7 ohms

Dielectric coefficient of the wire insulation: 6.23 as tested.

If the 15 nH per foot is removed from the reading, the lowest the DC can be is 4.68.

Middle braid to outer braid:

L 34.5 nH per foot.

C 167 pf per foot.

Z of 14.37 ohms

Dielectric coefficient of the heatshrink: 5.57.

Inner to outer:

L 93.75 nH per foot.....78.75 if you subtract the 15 nH per foot (max)

Composite C: 65.25 pf per foot. Note, this is the series capacitance of the two dielectrics in this combination.

DC: this is a composite structure dielectric coefficient...5.91 as measured, 4.96 if you neglect the inner wire internal inductance.

Z cable: 37.9 ohms.

R inner wire: 8.53 milliohm per foot. 18 awg.

R middle braid: 5.71 milliohm per foot, 16.5 awg.

R outer braid: 5.91 milliohm per foot, 17 awg.

Note that the outer braid is a little more resistance...this is a result of needing a little more of it to cover the larger diameter.

Cheers, John
 
I hate to break the news to you after all that effort, but shielding the wire will not contain the magnetic field that surrounds it when current is flowing through it.

Here's a simple experiment. Get any cheap compass, even one that comes from a cereal box. Put the compass near the wire while running some current through your double shielded wire. If the needle moves, the magnetic field is "escaping" your shield.

Shields only prevent capacitive coupling to/from the wire. They do not stop magnetic fields.

I_F
 
I_Forgot said:
I hate to break the news to you after all that effort, but shielding the wire will not contain the magnetic field that surrounds it when current is flowing through it.

Here's a simple experiment. Get any cheap compass, even one that comes from a cereal box. Put the compass near the wire while running some current through your double shielded wire. If the needle moves, the magnetic field is "escaping" your shield.

Shields only prevent capacitive coupling to/from the wire. They do not stop magnetic fields.

I_F

You are incorrect with regard to the application I am speaking of.

The purpose of this triax is NOT an attempt to shield the external world from a net current within. If the outer braid were to be used as a shield, then your statement would be correct. The braid will be ineffective for magnetic shielding.

However, this cable is designed to provide an axially symmetric current distribution. What this means is that all the currents within this cable are symmetrically centered about the geometric center of the wire. Since all currents from and to the load run through this cable, both rails and ground, there will be no external magnetic field present outside of the structure.

If you use the middle braid as ground, for example, and the inner core wire as the negative rail, when the amp is drawing current through the neg rail, that current returns through the middle braid. This produces no external field.

When current is being pulled from the positive rail, the same occurs...no external magnetic field. In point of fact, there will be no internal field present from either braid regardless, and the externals all cancel.

My atavar is a depiction of the field strengths resulting from current through the middle braid which is returned via the outer braid. Note that there is no magnetic field within the inner braid, and none outside of the outer braid. Also, note that the field strength between the braids is dropping off as 1/r.

You incorrectly assumed that the braids are being used to shield, they are not.

This geometry is far better at eliminating coupling than a mere shield..

Cheers, John
 
I_Forgot said:
Just the same, I'd be interested in the results of the compass test...

I_F

It is not necessary to do. There is no external field in all cases.

If one wished to, they could duplicate this by running a current through a coax, with the opposite end shorted. This will fail to move a compass needle.

That is the reason coax exists, to eliminate the external fields, as well as prevent coupling.

Cheers, John
 
poobah said:
I_forgot,

Do keep in mind the presumed application for this is where the sum of currents flowing through all 3 conductors is zero...
Yup..

poobah said:

Morning John,

;)


Mawnin dude..

How's it shakin?



Oh, forgot to add. This is the chart I use to find the effective wire guage from the resistance per foot, it only works for copper at room temperature..the data points are from some wire table, belden I think..

Cheers, John
 

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Curious if this would be at least somewhat effective for a pre-amp or in a headphone amp to get at least some amount of reduction of the magnetic fields. It might not be complete containment but it is certainly affordable at a buck and a half/foot. I've used some of their other cable product before and it worked well.

Gepco® Brand -- Audio and Video Cable Products --
 
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