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Old 02-09-07, 07:20 PM   #1
phantomcow2
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Would this shield a cable?

Am I correct that shielded cable is shielded because of the foil which wraps the cable (underneath the layer of insulation). Then the ground wire will be connected to something on either end to pass the "stray" charges.
Now, I have an application which carries a +5vDC pulse through 22awg wire into a drive. The manual of he drive warns that this line is sensitive to external noise. Frequencies of 100khz will likely be seen by this line.

So I am definitely routing the 16 wires which will be carrying 160vDC as far away as I can from those 5v lines. But, there's only so far I can separate them. At the closest point, they will be 2" apart from eachother. So I was wondering, if I carefully wrapped aluminum foil around the wire, would it shield it? The wire will not be moving, so I don't concern about breakage.
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Old 02-09-07, 07:22 PM   #2
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Quote:
Originally Posted by phantomcow2
Am I correct that shielded cable is shielded because of the foil which wraps the cable (underneath the layer of insulation). Then the ground wire will be connected to something on either end to pass the "stray" charges.
Now, I have an application which carries a +5vDC pulse through 22awg wire into a drive. The manual of he drive warns that this line is sensitive to external noise. Frequencies of 100khz will likely be seen by this line.

So I am definitely routing the 16 wires which will be carrying 160vDC as far away as I can from those 5v lines. But, there's only so far I can separate them. At the closest point, they will be 2" apart from eachother. So I was wondering, if I carefully wrapped aluminum foil around the wire, would it shield it? The wire will not be moving, so I don't concern about breakage.
It should, as a result of the Faraday effect. I'm sure there are others here that can either confirm or deny, but that's my opinion...
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Old 02-09-07, 07:25 PM   #3
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So it IS the faraday effect. I mentioned that today during a presentation, and I could not tell what the look on my teachers face meant.
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Old 02-09-07, 07:26 PM   #4
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Shielded cable is basically any kind of coaxial cable with the ground wire on the outside. Sometimes it's foil, sometimes it's braided wire, sometimes it's both. Usually it's to protect a weak or low-level audio or video signal from unwanted electronic 'noise'. Examples of shielded coaxial cables in everyday use include cable TV wire, audio cable used to connect audio components to a preamp or receiver, and electric guitar cords.
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Old 02-09-07, 07:28 PM   #5
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A braided shield usually works better. Can you replace the wire carring the 5V with a braid shielded cable? If you can, only ground one end of the shield.
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Old 02-09-07, 07:30 PM   #6
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I've got 6 conductor shielded cable already, with foil and the brade. All is grounded already on both ends. That reminds me, the 160v carrying cables are already using that shielded cable. That should mean no electric field will leave the cable housing then.
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Old 02-09-07, 07:38 PM   #7
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Just make sure you ground only one end. If you ground both ends you could have a ground loop carrying current that could make things much worse.
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Old 02-09-07, 08:01 PM   #8
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I thought ground on both ends was acceptable practice. One end is connected to the terminal on the drive made for ground, and the other end to a ground terminal on my mill.
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Old 02-09-07, 08:44 PM   #9
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Not all shielding is of equal quality. While an idealized Faraday cage is perfectly shielding, reality and idealized models don't perfectly coincide. Also note that how two lines come near each other can matter. Two parallel lines can wreak far more havoc on each other than two lines that cross perpendicular to each other but then never meet again. How you set up the lines is very important if you want minimal cross talk.

I saw this happen once back when I worked in a biotech. I was helping our IT guy set up a laptop for a presentation when he had trouble getting an Ethernet connection to be stable. I noticed his Ethernet line and power line parallel to each other and lying right next to each other. Sure enough, separating them by a few more inches (crossing only out of necessity near the laptop because of where the ports were) resolved the issue.
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Old 02-09-07, 09:59 PM   #10
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Heh, I know a funny instance with my TV downstairs. THis is a cheapo Sony, 10 years old, probably 10 year old cable too. Anyways, I plug in my stepper motor drivers for configuration, as soon as current is applied to the motor, instant fuzz on TV screen. Those drives are choppers, so they are chopping the current at about 20khz. And the coaxial cable for the TV is a good foot away from the step motor wires. It does not have this issue with other drives. But, the max I've put in with other drives is 28v. This drive direct rectifies the AC, so 160v!

Wonder if it will do that with my shielded cable, maybe I should go test. THen again, my 10+ year old coax might need to be replaced as well!
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Old 02-09-07, 10:33 PM   #11
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The cross talk issue is why ethernet cable is made up of twisted pairs. the fields cancel each other out.
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Old 02-09-07, 10:48 PM   #12
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Wonder if it will do that with my shielded cable, maybe I should go test. THen again, my 10+ year old coax might need to be replaced as well!
The understanding of how to shield a cable hasn't exactly changed in recent times. Nothing wrong with old cables.
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Old 02-10-07, 12:35 AM   #13
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I thought ground on both ends was acceptable practice. One end is connected to the terminal on the drive made for ground, and the other end to a ground terminal on my mill.
If you ground both ends, there could still be an electrical potential, depending on the relative potential at each grounded end. So I agree, if possible/necessary, ground only one end. I also agree that perpendicular wire crossings are better than parallel runs if you want to avoid interference...
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Old 02-10-07, 09:09 AM   #14
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You need to anchor the foil to a ground. Single ended as mentioned.

As a very small second job, I make quality audio cables, and I always anchor the ground on one end. That's also the REAL reason why you see directional arrows on some shielded audio cables....the shield is anchored on a single end, and the anchored end should be the source end, so that noise doesn't travel to the next component. The arrows should always be pointing towards the non-anchored end....it's easier for an untrained person to figure out "the sound comes FROM the CD player TO the amp...the the arrows should point TO the amp", than to have it the other way.
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