Triaxial wire build for power supply rails - diyAudio
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Old 26th April 2006, 03:31 PM   #1
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Default Triaxial wire build for power supply rails

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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File Type: jpg 1heatshrink.jpg (97.8 KB, 169 views)
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Old 26th April 2006, 03:32 PM   #2
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Here's the mike cable. I have a spool of about 600 feet left.
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File Type: jpg 2source mike cable.jpg (89.8 KB, 149 views)
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Old 26th April 2006, 03:34 PM   #3
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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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File Type: jpg 3naked braid.jpg (79.0 KB, 150 views)
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Old 26th April 2006, 03:37 PM   #4
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Here's the gutted cable, the limp braid..Take care when bunching it as you work it off the cable, as you want to keep the braid as pristine as possible.
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Old 26th April 2006, 03:39 PM   #5
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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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File Type: jpg 5first braid over inner core wire.jpg (80.8 KB, 136 views)
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Old 26th April 2006, 03:40 PM   #6
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Here, I've put on the first heatshrink and heated it.
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File Type: jpg 6first heatshrink finished.jpg (55.8 KB, 122 views)
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Old 26th April 2006, 03:41 PM   #7
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Here I've put the second braid layer on. I twisted it at the start end, and worked it tight to the other, the pic is in the middle of that process.
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File Type: jpg 7second braid layer.jpg (73.1 KB, 125 views)
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Old 26th April 2006, 03:44 PM   #8
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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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File Type: jpg 8triax.jpg (85.0 KB, 133 views)
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Old 26th April 2006, 03:54 PM   #9
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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
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Old 27th April 2006, 02:05 PM   #10
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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
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