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Can a thermocouple be used as a power cable?

  1. Aug 15, 2012 #1
    I am trying to find out if type K thermocouple is capable of transmitting power from an external power supply to a camera and some lamps attached on the end. I have 3 pairs of thermocouples. One pair for the camera supply. One pair for the video output. One pair for the lamps.

    I have tried google for answers but have found very little useful information.

    Thermocouple specs:
    Length approx 75m
    type K
    22AWG
    loop resistance 0.9368Ω/foot

    Camera specs:
    11.5V to 15V DC
    400mA to 500mA

    Video output specs:
    2V p-p into 150Ω

    Lamps Specs
    0.5A
    12V DC
    LEDs

    Analysis i have done so far:
    Resistance of thermocouple- 246x0.9368=231Ω
    Voltage drop across the thermocouple- V=IR=0.4x231=92.4V
    If the current was to decrease by 10mA this would cause the voltage at the camera to decrease by 2.3V- 0.39x231=90.1V
    If the resistance was to increase by 1Ω then the voltage will increase by 0.4V- 0.4x232=92.8V
    At a lamp supply of 0.5A the power dissopated over the length of the thermocouple would be approx- P=IIR=0.5x0.5x231=57.75W
    Equating to 0.77W/m

    I look forward to your responses.
     
    Last edited: Aug 15, 2012
  2. jcsd
  3. Aug 15, 2012 #2

    NascentOxygen

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    Hi ECI Engingeer! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    I don't understand the meaning attached to length of your thermocouple. What does 75m tell us https://www.physicsforums.com/images/icons/icon5.gif [Broken]
    Maybe you have a thermopile? And what source of heat do you have in mind?

    From http://en.wikipedia.org/wiki/Thermocouple#K :
    So a single thermocouple of this type at a junction temperature differential of 100°C will generate approx 0.004 volts, so I conclude you must have many more than the three pair you mention. http://img140.imageshack.us/img140/7701/questionicon.gif [Broken]
     
    Last edited by a moderator: May 6, 2017
  4. Aug 15, 2012 #3
    The thermocouple cable is 75m long. Which enables us to work out the loop resistance of the cable among other things.

    Sorry I didnt explain very well before earlier...
    Im not concerned about the temperature differential characteristics, just with the visability of using this cable as a means of transmitting power though from an external power supply to a carmera and some LED lamps attatched at the other end. I know this is un-conventional and not the intended use for any thermocouple (hence my post). I am just looking for peoples opinions on whether it is possible or not. I suspect it would'nt work and would just end up destroying the cable.
     
  5. Aug 15, 2012 #4

    NascentOxygen

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    Oh, now it's less mystifying. http://img210.imageshack.us/img210/9088/smilgb.gif [Broken]

    As you've shown, for a power cable that resistance is extraordinarily high. To deliver 12v or so to the camera you'd have to supply over 100v to the cable. That doesn't sound a very good payoff. The cable might not even be able to tolerate 100v, since such a voltage was not in its design. (I presume it is a shielded cable?) As you've pointed out, the power dissipation will be 1000s of times higher than the cable was intended to pass. What gauge is the conductor in the cable? EDIT: I see 22AWG, so it's single conductor?

    Not knowing the voltage rating is a bit of a handicap. Otherwise you could rig it up to a 0-100v power source and pass current through it, slowly stepping the current up every 15 mins and noting the temp of the cable by feel. When it starts to feel warm, I'd stop. In the absence of any specs, I'd call that the limit. It's not heat resistant cable is it, intended to lead to a furnace or something?

    If the cable could pass 0.5A, you could feasibly include a voltage regulator at the camera end to iron out voltage variations as current changes, so keeping the camera safe. You are envisaging constant operation, are you, passing 0.5A for hours on end?

    Is there some reason for not wanting to buy more-suitable cable?

    A close-up photo of the end of the cable might be informative. I don't suppose there are any part numbers on the cable, that would allow you to ask the manufacturers of its ratings?
     
    Last edited by a moderator: May 6, 2017
  6. Aug 15, 2012 #5

    CWatters

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    I think 22awg of copper wire is normally limited to about 0.9A so very unlikely this wire could be used.
     
  7. Aug 15, 2012 #6
    Yes single conductor and shielded. I would imagine it would get very warm at 100V stage but I don't think temperature transients will be a big issue because I think k type thermocouple is rated to around 1200 degrees Celsius. I am more concerned about the risk of arcing or the damage that could be inflicted on the cable connectors. Or even damage caused by the peltier effect at the cable connections and junction boxes.

    We are hoping to use the existing thermocouple cable rather than using conventional cable because the area is not easily accessible and it would be a lot more cost effective just to use the existing thermocouple. However I am doubtful this will actually happen.

    Thanks for advice. I will look into setting up an experiment and the use of a voltage regulator.
     
  8. Aug 15, 2012 #7

    sophiecentaur

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    Is the 'cold' end of the thermocouple at 'your end' of the connecting lead? This would mean that the two wires could be of different metal with very different resistance. If this is the case, then why not just use the copper (?) halves of two of the pairs to supply camera and lamps (in parallel) or even parallel up the two sides of each couple to provide two conductors of even lower resistance? The other pair could be used for the low power signal with not detriment as the signal current would be low and you could use a video amp to compensate.
    This would all hang on knowing the resistances of the various combinations of conductors (individual or in pairs) but you can, presumably measure that.
     
  9. Aug 15, 2012 #8

    NascentOxygen

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    So it's still installed in the high temp location? I assumed it was a roll of disused cable you had lying in the corner and you wanted to use it for some unrelated job to save buying more cable. :smile: So its life as a thermocouple is over?

    You have "three pairs" of thermocouple cable? So that's a total of 6 solid conductor cores? Are these enclosed in 6 lengths of separate copper braid for shielding, or just 3? Does the braid form the outside cover of the cable, or is it covered in an electrical insulator?
     
    Last edited: Aug 15, 2012
  10. Aug 16, 2012 #9
    It will ony enter high temp conditions periodically, approx 20-30 times a week for an hour max at any 1 time. The intention is to keep the functionality of the thermocouple whenever the camera equipment is not being used/connected.

    In terms of shielding I believe there is just 3 pairs of shielded conductors, each incorporated into 3 larger bunches of separate cables, with elctrical insulation sheath around the outside.
     
  11. Aug 16, 2012 #10

    sophiecentaur

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    Why nor use the shields as DC conductors? They are bound to have a decent cross sectional area and a pair of the shields could easily carry enough 12V power for the whole lot.
     
  12. Aug 16, 2012 #11

    NascentOxygen

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    My thinking is that you might be able to use the shielding as the return conductor, powering the camera and LEDs with it as their common earth. That will leave you the 6 conductors to combine as you see fit for your 3 signal lines. Just would need to check that the shielding braid is substantial enough to can carry the 1A current, though it is probably bonded to the shielding of all those other cables so giving it a respectably-low impedance. As sophiecentaur pointed out, there is probably one conductor in each pair with a lower resistance, so use that to carry the 0.5A. By making low path resistances, you won't need anything like 100v to power the camera, thus no concern about voltage stress in the cables.
     
  13. Aug 16, 2012 #12

    sophiecentaur

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    @NascentOxygen
    Snap!
     
  14. Aug 17, 2012 #13

    NascentOxygen

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    ..... http://img840.imageshack.us/img840/6112/wink5.gif [Broken]​
     
    Last edited by a moderator: May 6, 2017
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