PTC Thermistor Probe/Cylinder Identification help

Discussion in 'General Electronics Chat' started by gjames, Mar 4, 2015.

  1. gjames

    Thread Starter New Member

    Mar 4, 2015
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    I am trying to identify what Thermistor I currently have. I believe it to be of the PTC type. Since the resistance is rising as the temperature rises.

    There are plenty of charts out there for NTC type thermistors but not much on the PTC type.

    This one is potted inside a cylinder that I believe is aluminum. Two wires come out of one end.

    At 67 degrees Fahrenheit I am getting a reading of 36.34 M Ohms.

    The unit this is installed into can reach a temperature of around 150 degrees Fahrenheit.

    I am attaching a picture of the Thermistor and a picture of it on the Schematic (there are two thermistors on the Schematic they are both identical).


    How can I determine what this is so I can buy a replacement?

    Thanks.
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK, does "36.34 M" ohms means 36.34 meg ohms or 36.34 milli-ohms.
     
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  3. gjames

    Thread Starter New Member

    Mar 4, 2015
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    MΩ Megaohm.. My fluke displays it as M Ω (see attached picture)
     
  4. gjames

    Thread Starter New Member

    Mar 4, 2015
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    Okay I'm an idiot. Apparently I was testing the Thermistor that is malfunctioning.

    These might actually be NTC type.
     
  5. #12

    Expert

    Nov 30, 2010
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    This is not in the range of any thermistor I have met.
    5 volts into 36 million ohms causes 139 nanoamps of current. This is simply not practical for a real world sensor.
     
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  6. gjames

    Thread Starter New Member

    Mar 4, 2015
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    So this is actually an NTC type. However I'm not exactly sure which one.

    Assuming the "good" thermistor is still good and based on the readings I'm not seeing anything that specs out.

    I am getting a reading of 160 k ohms (160000 ohms) at 65 F (18.33 C).

    To me it seems like it should be around a 100k NTC Thermistor. But could it be more like a 120k NTC Thermistor?
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    So, before you read 36 meg on the bad part and now you are reading 160K on a good part. Is this correct?
     
  8. gjames

    Thread Starter New Member

    Mar 4, 2015
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    Yes this is correct.
     
  9. Lestraveled

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    May 19, 2014
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    Does the resistance increase as it gets hotter, or decrease?
     
  10. gjames

    Thread Starter New Member

    Mar 4, 2015
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    Resistance decreases as it gets hotter.
     
  11. bwilliams60

    Active Member

    Nov 18, 2012
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    Then it is NTC. What exactly are you working on? Have you tried the manufacturer?
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK
    It looks like your sensor is consistent with a standard 100K NTC thermistor. A Murata NXRT15WF104FA1B040 can be purchased at digikey for about 43 cents. This should do it.
     
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  13. gjames

    Thread Starter New Member

    Mar 4, 2015
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    It's a heater for a machine. Yes the manufacturer won't sell the components for the assembly and they are not interested in sharing specifications because the part is around 10K USD.
     
  14. gjames

    Thread Starter New Member

    Mar 4, 2015
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    That is what I was figuring. I just wasn't getting exact measurements based on the charts out there.

    If I use the Murata NXRT15WF104FA1B040 would I just Pot it myself in an aluminum cylinder?

    Any recommendations on assembly and potting material that works well with Thermistors?

    Thanks.
     
  15. Lestraveled

    Well-Known Member

    May 19, 2014
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    Well, I am not going to pot it for you. Look at your failed part. What did they use? Copy it as best as you can. At 43 cents a piece, you can try several ways for not much money.
     
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  16. gjames

    Thread Starter New Member

    Mar 4, 2015
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    Okay. :)

    I will research potting materials tonight.

    Just wasn't certain if there was a particular material that worked best to normalize temperature from the thermistor to the cylinder container.


    Thank you for all the assistance. I really appreciate it!
     
  17. #12

    Expert

    Nov 30, 2010
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    Thermal conductivity of the potting compound has everything to do with time lag. Do the best you can with something that feels cold to your touch. If it's too slow, the machine will, "hunt" for the balance point.
     
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  18. RichardO

    Well-Known Member

    May 4, 2013
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    A tip on potting... After you pour the potting material into the casing, you should put the unit in a vacuum chamber to pull out the air bubbles. If you don't remove the air, the thermal resistance will be a lot higher than it could be. This might be a problem -- I am not sure.
     
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  19. gjames

    Thread Starter New Member

    Mar 4, 2015
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    Good point.

    I read that this potting material (Polytec TC 437) flowed really well for putting a thermistor into a small space. The aluminum cylinder the current thermistor is in is 1 inch long x .1875 inch diameter.

    Doesn't leave a whole lot of room. Unfortunately where the thermistor is placed it's in a machined groove on an aluminum part. So it probably can't exceed 5mm in diameter.

    So far these are the materials I am looking into.

    Polytec TC 437 - by Polytec PT - http://www.intertronics.co.uk/products/thermally_conductive_adhesives_01.htm

    Duralco 4703 - by Cotronics Corp - http://www.cotronics.com/vo/cotr/ea_ultratemp.htm

    832TC - by MG Chemicals - http://www.mgchemicals.com/products...capsulating/thermally-conductive-epoxy-832tc/

    My interest in MG Chemicals is that it's readily available through many electronics distributors. Duralco and especially Polytec appear to be superior. The article on the Polytec usage is here: http://www.newmaterials.com/News_De...thermocouple_cavities_12929.asp#axzz3TYpHndvP


    Any thoughts to weigh in with?
     
  20. #12

    Expert

    Nov 30, 2010
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    Polytec 301 is the obvious winner on that page because it HAS a thermal rating and the number is, "3".
    (Higher number is good.)

    Would you like to convert BTU inches per hour, per square foot, per Fahrenheit into Watts per meter per Kelvin?
     
    Last edited: Mar 5, 2015
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