Material with specific resistivity

Discussion in 'Physics' started by tshuck, Nov 24, 2012.

  1. tshuck

    Thread Starter Well-Known Member

    Oct 18, 2012
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    Hey folks,

    Since this isn't directly electronics, I figured I'd post in Physics....

    I am starting a new project for a simple control system and would like to know if anyone has an idea fora part.

    I need a bar, of some sort, that has resistivity of around 1KΩ-cm, in whatever shape it comes in.

    Does anyone know of such a material?
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    All the "resistivity of common materials" tables I can find jump right past the range of usefulness for you. I would recomment maybe graphite, ground to a size and shape the meets your requirement.
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    You may be able to find doped silicon with a bulk resistivity in this range. You might contact some of the wafer foundries (I don't know if any still exist in CA, but its possible) and see what they do with their scrap material.
     
  4. tshuck

    Thread Starter Well-Known Member

    Oct 18, 2012
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    Thanks for your replies, guys. I'm not sure that graphite would work... it has to be able to support a load as a cantilever. Considering graphite's structure, it would simply break... :(

    Si is pretty hard to find too...:(

    I suppose I'll have to keep looking...
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Are you wanting the resistance to vary as a function of strain? You can buy strain stickers (the basis of strain gauges) in almost every value, but I suspect I am misinterpreting your application.
     
  6. tshuck

    Thread Starter Well-Known Member

    Oct 18, 2012
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    Well, no. It needs to change resistance as a function of length...
     
  7. WBahn

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    Mar 31, 2012
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    Do you mean as it changes length because of some load on it (or because it changes temperature), or just that this one here is cut to length L1 and that one there is cut to length L2?
     
  8. tshuck

    Thread Starter Well-Known Member

    Oct 18, 2012
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    Fixed length whose resistance changes from point A to point B
     
  9. WBahn

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    Mar 31, 2012
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    How about using a thin resistive coating (perhaps even just a resistive paint).
     
  10. tshuck

    Thread Starter Well-Known Member

    Oct 18, 2012
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    I considered this, but it will experience a fair amount of friction, so I figured the paint would come off...
     
  11. WBahn

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    Mar 31, 2012
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    Okay. I'm done. Every time someone suggests something based on what you've told us so far, you decide to throw out another tidbit of your requirements that invalidates that suggestion. I don't tolerate that from paying customers, so I'm certainly not going to play the game for free.
     
  12. tshuck

    Thread Starter Well-Known Member

    Oct 18, 2012
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    Whoa, now, no need to be getting feisty... I'm trying to work through this too. It's not like I know everything you are going to suggest. I am simply trying to brainstorm with you here....

    What I've mentioned thus far has been concerns, not utter negations or nay-saying. I appreciate you trying to help thus far and I am only discussing concerns I have with possible implementations....
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I think what WBahn is saying is tell us what your doing and then you will get better answers. This is a help forum, not guess my application.
     
  14. tshuck

    Thread Starter Well-Known Member

    Oct 18, 2012
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    True, I suppose that would have been helpful, however I'm not on my normal computer and don't have access to all of my stuff...

    However, what I am trying to do is build a ball-balancing system where the ball will be restricted to 1 dimensional movement. The ball will move along two beams, one aluminum and the other I am trying to figure out... In order to sense the location of the ball, the aluminum will be attached to +Vcc while the other side will be attached to ground through a current limiting resistor... The ball is conductive so that when it moves along the bar, it changes the effective resistance the circuit sees. This way, I can tell where the ball is given a certain measured voltage across the resistor....

    Sorry for being short in my first post, I was pressed for time...

    EDIT: The beams are about 10 inches long....
     
    Last edited: Nov 26, 2012
  15. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    How about using nichrome wire wire or ribbon for your resistive element? Either real thin gage wire wrapped around the beam, like a coil spring with the ball making contact. Or thin ribbon formed into an "L" or "V" shape to act as the edge of the beam. Wire wound power resistors use the nichrome as the resistive element in them. You would need to experiment to find what the resistance is and adjust the circuit to match.

    This place sells both the ribbon and wire. It would take less of the ribbon than the wire. http://mpaksys.homestead.com/nichrome.html
     
  16. tshuck

    Thread Starter Well-Known Member

    Oct 18, 2012
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    Hmmmm... that sounds promising... I've never used nichrome, but I'll be looking into it, thanks...
     
  17. DenzilPenberthy

    New Member

    May 28, 2012
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    Cut a pencil in half lengthways. Pick different hardnesses to select the resistance/m you want. Job done. Alternatively you could use one of these bendy 'unbreakable' pencils.
    http://www.rowland-tools.co.uk/CHATTAHOOCHEE-CARPENTERS-PENCIL
    I don't have one with me at the moment to test how conductive they are but they might work for you.
     
  18. davebee

    Well-Known Member

    Oct 22, 2008
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    Here's another idea. It's not fully thought out, so don't quiz me for details because I don't have any, but here goes.

    Instead of resistance, what if your conducting sphere shorted a pair of wires to form a Lecher line, a resonant circuit whose resonance depended on the physical length to the short? By oscillating the resonant tank and measuring the frequency, you could determine the position of the sphere quite accurately, I would think.

    http://en.wikipedia.org/wiki/Lecher_lines
     
  19. Lilienfeld

    New Member

    Nov 8, 2012
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    Just a quick thought if the project is still ongoing:
    I guess you will have problems to find a suitable material, for whatever you want to do. The reason is that your specific resistance is I guess well in the semiconductor range such that you will have probably a hugely varying resistance with respect to temperature aka dissipation aka load. I suggest you try out to change the geometry to achieve a proper overall resistance and go for some metal or metal oxide.
     
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