Mysterious Conductor

Discussion in 'General Electronics Chat' started by BadBadger, Aug 30, 2014.

  1. BadBadger

    Thread Starter New Member

    Feb 1, 2014
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    Tonight I decided to attempt to splice together 2 electrodes used in a TENS (Transcutaneous Electrical Nerve Stimulator) unit. When I cut the molded connector from one of the lead wires, I discovered that, rather than being metallic, the stranded wire is composed of a bundle of dozens of ultra-fine, plastic-looking fibers within a pretty typical non-conductive sheath. About half of them appeared to be white, the other half black. And they were not intermingled. A cross section would show one side of the bundle is black fibers, the other white. Strange.

    Wondering how this was electrically connected within the connector, I dissected it with an X-acto. The copper conductor is crimped to this bundle of fibers. Obviously, I will not be able to solder two of these wires together.

    Does anyone know what this conductive material is and why it might be used in place of copper?
     
  2. tom_s

    Member

    Jun 27, 2014
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    thoughts.

    the electrodes (assuming your cutting pads) are meant to be placed over a wide area of skin to spread the pulses generated over a wide area of skin. hence carbon fibers spread over the pad. reason i would assume is to spread the charge rather than applying it to a pinpointed area.
     
  3. Externet

    AAC Fanatic!

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    Guessing, it is the type of cable that will survive years of manipulation, movement and abuse. Like telephone handset coiled cords, made of foil strands. Sort of a metallized mylar balloon material. Conductive but unsolderable.
     
  4. RichardO

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    May 4, 2013
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    I have seen cable like this before. The colored strands are like magnet wire. All of the wires of the same color are connected together. Each strand has a very thin varnish (or the like) insulation. This allows a very thin and flexible cable since there is no thick plastic insulation around the wire bundles.


    edit ***************
    After seeing the picture I realize that I have _not_ seen this kind of cable before.
     
    Last edited: Aug 30, 2014
  5. ian field

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    Oct 27, 2012
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    I was just thinking it sounds like that nasty stuff they used on telephone curly-leads.

    But if it went into a moulded connector, they're usually soldered at some point - the stuff I was thinking of is pretty much impossible to solder.

    Another possibility could be the way they terminate the self-suppressing carbon fibre ignition leads, the end terminal has a spike that pushes into the end of the cable - I don't see any reason why that couldn't be moulded after assembly.
     
  6. #12

    Expert

    Nov 30, 2010
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    Thanks for the information. I had assumed that the stick-on parts spread the current with conductive plastic. Now I know another way.:)
     
  7. BadBadger

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    Feb 1, 2014
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    Wow! Looks like I stirred up some interest. Thanks!

    Upon closer inspection, and a night of rest, I realize I grossly understated the number of fibers. It's more like hundreds if not a thousand. Also, what it looks most like is ultra-fine paint brush hairs. But plastic. In 2 colors. The black fibers could be carbon, but they look too shiny. But I can't imagine how the white ones could be carbon.

    I have a great composite closeup image I can offer up that will help tremendously. I'm new here. Is there a way to upload an image rather than an image link? If not, I'll have to find a place to park the picture so I can link to it.
     
    Last edited: Aug 30, 2014
  8. BadBadger

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    Feb 1, 2014
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    Ah, I just needed to upload file:


    TENS Electrode Conductor.jpg
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Thank you for sharing.

    This sort of 'conductor ' is normally used with high voltage, low current applications.

    What is the voltage output of the TENS unit?
     
  10. BadBadger

    Thread Starter New Member

    Feb 1, 2014
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    Thanks! I don't know the voltage, but I'm curious enough to look into it. I'll let you know what I can.

    Can you elaborate? What is this material? Why would it be used in this high-voltage, low-current application?

    Since my original intent was to splice (read that, SOLDER) 2 of these together, I didn't start digging into this thing until today. And I haven't dissected the junction inside the electrode itself, yet. That will be interesting.
     
  11. BadBadger

    Thread Starter New Member

    Feb 1, 2014
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    A picture is worth . . .

    TENS Specs.jpg
     
  12. #12

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    I think the white fibers are there to provide better tensile strength.

    Some of the TENs units I worked on could hit nearly 200 volts, unloaded, because the output was a pulse transformer. This never happens when attached to a person.
     
  13. ian field

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    It varies from one manufacturer to another, it can be as high as 180V but very narrow pulses - so only a miniscule amount of energy.

    Once I worked for a firm that made faradic muscle excercisers, its pretty much the same circuit as a TENS, but without the precise pulse timings to match biorhythm frequencies.

    One stupid customer ignored large print in the instructions; warning not to turn all the knobs to maximum and grab a pad in each hand.

    We got a visit from one of the standards agencies, so I shunted an output with an inverse parallel pair of LEDs to demonstrate there was so little energy they barely glowed. The agency inspector left fully content that the equipment was safe.
     
  14. ian field

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    Unless you break the skin under both pads - the resistance will be a fair bit more than 500 Ohms.
     
  15. studiot

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  16. BadBadger

    Thread Starter New Member

    Feb 1, 2014
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    First experiment: I separated the fibers and found that the black ones are conductive and the white ones aren't.

    Second experiment: No longer having the chemistry set from my childhood, I performed the most obvious, chemical experiment on the mysterious fibers. I put a lighter to them! The white ones melted into a nice, plastic blob. The black ones glowed red hot, but did not burn at all. In fact, they appear totally unscathed.

    I believe you are correct. And I'm thinking the black ones are pure carbon.

    Still, does anyone know the rationale behind using this method of conduction? At these voltages, currents, and freqs, wouldn't something simpler (and cheaper to make) do the trick? On the other hand, I'm thinking that maybe because the pad itself appears to be using carbon as a conductor, they just bring this carbon fiber from the wire and spread it out over the surface of the pad as part of the layering process during manufacture.
     
  17. BadBadger

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    Feb 1, 2014
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  18. studiot

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    Safety.
    The leads limit the current.

    The old safety adage
    "it's the volts that jolts but it's the mils that kills"
    applies.
     
  19. Kermit2

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    Feb 5, 2010
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    The carbon is a resistive conductor.
    Brilliant idea on the part of the seller.

    edit: studiot beat me to it.
     
  20. #12

    Expert

    Nov 30, 2010
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    As electrician for a band (many years ago) I believe there is no such thing as a copper connection that can stand being flexed every day. Well, maybe the phone company did it with those tiny ribbon wires. I think the fabric approach is more dependable...but then I never did more than half a dozen TENS machines. Whadaya say? Replace many of those?
     
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