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how to pick thermistor based on data points?

Discussion in 'Electronic Design' started by [email protected], Jul 6, 2007.

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  1. Guest

    i have a temperature gauge which is designed for a NTC thermistor. I
    have the thermistor it works with, and I have measured the gauges
    indication to various resistances, as follows:

    79.4F 1773 ohms
    180F 192 ohms
    240F 78 ohms

    I need to buy lots more of the same thermistors but I dont know how to
    find them based on just the above datapoints. Is there a way I can
    come up with a thermistor spec so I can go thermistor shopping?
     
  2. From the numbers above it looks like a thermistor
    specified as R25= 1.9k and Beta = 4152 (average).

    R25 is the resistance at 25C. The nearest standard
    R25 value is 2k.
     
  3. Winfield

    Winfield Guest

    acannell asked,
    Thermistors are usually specified for their resistance at 25C,
    which is 77F, so you need to know your part's resistance at that
    temperature. Common thermistors drop about 4% per degree C (and
    this agrees with your 180F measurement), so your thermistor's
    resistance should be about 1870 ohms at 25C. Hmm, that's not
    near a common part value. For example, 2252 ohms at 25C is a
    common value, which would show about 9F too low on your gauge.
    2.0k is another common value, this would only be 3F low at 80F.
    It's possible your gauge is no more accurate than that anyway.
     
  4. Tom

    Tom Guest

    Converting those to Celsius, that corresponds to something like:
    R25=1900 ohm
    Beta=4233

    The (simplified) formula for the resistance of the NTC thermistor is
    then: R=R25*exp(beta*(1/T0-1/T)), where all temperatures are in Kelvin
    (Celsius + 273.15).

    greetings,
    Tom
     
  5. Guest

    You've already got a couple of good answers.

    Note that negative temperature coefficinet thermistors can be
    reasonably accurately modelled by the Steinhart-Hart three-paramter
    formula

    http://www.betatherm.com/stein.htm

    Betatherm makes "interchangeable" thermistors and Farnell stock their
    +/-0.2C parts, including one that offers a 2.252k (+/- 0.8%)
    resistance at 25C. They aren't cheap. Cheaper parts come in at +/-5%
    and 10% tolerance on resitance, and a temperature guage designed to
    use one of them would include some means of calibrating out this
    tolerance.

    There are other sources for interchangeable thermistors -
    Thermometrics and Yellow Springs Instruments (YSI) come to mind.
    Newark used to stock a +/-0.5C YSI part (at ten times the price of the
    Betatherm 0.2C units) and may still do.
     
  6. GPG

    GPG Guest

    http://www.epcos.com/web/generator/Web/Sections/DesignTools/NTCThermistors/Page,locale=en.html
     
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