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Why is telfon wire silver coated?

Discussion in 'Electronic Basics' started by Harry Muscle, Dec 5, 2003.

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  1. Harry Muscle

    Harry Muscle Guest

    Why is teflon wire usually silver coated? Does this affect the resistance
    of the wire?

    I'm working on a project where I'm gonna be using a length of small gauge
    telfon wire as a heater in water. I'm thinking of using either 24 or 26
    gauge wire, but I noticed that most places that sell this stuff mention that
    it's silver coated. Why is that? Does the silver coating affect the
    resistance of the wire? I need to know this since I'm using the resistance
    of the wire to determine which gauge to go with and how long of a wire to
    use. Anyone know approximately how much (in %) the resistance would go up
    or down? I've read somewhere that it probably decreases resistance,
    especially with AC due to the skin effect (which is where the silver is),
    but I don't fully understand what that means. I'm gonna be running about 8V
    12.5A AC through this thing in case it matters.

    So the thing the I especially need answered is how much (in %) it would
    affect the resistance of the wire. Also, is all teflon wire (in 24-30
    gauge) silver coated as a general rule. In other words, can I assume it's
    silver coated even if a store doesn't mention anything about it.

    Thanks,
    Harry
     
  2. Yes, it makes it lower by a tiny bit. Teflon is a high temperature
    capable insulation, and if you use the wire near the upper temperature
    limit for the teflon, it is hot enough to rapidly oxidize the surface
    of the copper. And teflon is expensive, so if you are already paying
    that much for wire, you may as well go all out with the conductor.
    The effect is so small (except at high frequencies, when the current
    is confined to just the surface of the wire) that you needn't worry
    about it. The effect of temperature on the resistance will be bigger
    than the effect of the silver plating.
     
  3. Harry Muscle

    Harry Muscle Guest

    I didn't know that temperature affects resistance. Would you mind
    elaborating on this a little. I'm not trying to heat the water up much,
    actually just a little above room temp (ie: aprox 25-30C). However, I don't
    know how hot the wire will actually get, especially since it will be covered
    by about two inches of gravel. But I don't think it would reach higher than
    50C, probably less. Would that start to make a difference on resistance?
    Is there a formula I could use to get an idea of how resistance would change
    with temperature? Btw, when temp goes up, does resistance go down or up?

    Thanks,
    Harry

    P.S. I realize that if I only expect the wire o reach 50C using
    teflon might be overkill, but I can get teflon wire pretty cheap, so for a
    few extra bucks I'd rather play it safe.
     
  4. John Larkin

    John Larkin Guest

    The wire resistance will increase about 0.4 per cent per degree C.
    Since the teflon insulates the wire thermally as well as electrically,
    the wire may get quite a bit hotter than the water you're heating.

    You could use bare nichrome wire at this low voltage.

    John
     
  5. http://www.cirris.com/testing/temperature/copper.html
     
  6. David Harmon

    David Harmon Guest

    I don't actually know the answer, but I'd be surprised if it was anything
    other than its behavior in making contact to the wire-wrap pin in the
    original intended application.
     
  7. Steve Dunbar

    Steve Dunbar Guest


    Google turned up the answer at this site:
    <http://www.logwell.com/tech/shdwe/teflon_wire>.

    Tin plating, as is used on ordinary wire, can dissolve into the copper and
    become difficult to solder when exposed to high temperatures. Silver
    plating is used on wires rated for high temperatures to avoid this problem.
    Also, tin plating will melt at the temperatures used in extruding the
    teflon insulation onto the wire, thus fusing the strands of the wire
    together.
     
  8. MG

    MG Guest

    It is usually difficult to make resistors with Copper wire because is too
    good a conductor.
    It is NOT impossible but you have to work out the Gauge Length Resitance
    Current and Voltage to produce the right amount of heat at the right voltage
    (the voltage available 120AC?, 12V? etc.)

    John Larking suggested bare nicrome, that would be fine at low voltage but
    at 120V there are safety concerns, even if the wire is buried.

    You do not indicate the project purpose, so is difficult to help or say
    whether a solution is practical or not. Looks like you are trying to keep
    some Dry Well from freezing, or is for a pond with turtle???

    Wire 24-30 Gauge is pretty delicte to be buried under gravel.


    MG
     
  9. Steven Swift

    Steven Swift Guest

    Copper changes resistance by about 0.4%/C. or 0.4ohms/100ohms/C (4000ppm).
     
  10. DarkMatter

    DarkMatter Guest


    No, you cannot.
    The designation for silver plating on wire is referred to as "SPC"

    Silver Plated Copper

    TPC is Tin Plated Copper

    It most assuredly has a low OHMs per 1000 ft number.

    The cladding depth will affect that though. There are about 3
    cladding depths usually.

    SPC is typical, but PTFE coated wire comes other ways as well.

    SPC is the most common, because the wire type is typically used in
    mil applications. It would be hard for you to find a teflon wire with
    a UL rating. They don't rate it. A 1000 VDC mil rating is at 600V
    for our illustrious cert lab.

    Anyway. Tinned copper inside teflon oxidizes on a shorter shelf
    life plan, and is very hard to wet with the solder once it does.
    Perhaps another reason why SPC is seemingly always what is chosen, or
    found in surplus.
     
  11. DarkMatter

    DarkMatter Guest


    It's called temperature coefficient. It effects many things in
    electronics, and the effect is referred to as temperature drift.

    Capacitors drift with T as well.
     
  12. DarkMatter

    DarkMatter Guest


    Wire wrap pin? You have dated yourself, but that is not the reason.

    Wire wrap wire is typically solid. I'd be willing to bet the wire
    he is referring to is stranded. As in not for wire wrap.
     
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