Thermistor Circuit

Discussion in 'The Projects Forum' started by magikal, Nov 23, 2008.

  1. magikal

    Thread Starter Active Member

    Nov 23, 2008
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    I am working on a project that outputs What is the simplest way to get a thermistor circuit output of the temperature (or 1/10 of the temperature) in voltage? Is using voltage dividers the easiest way? Also, can anybody tell me a specific thermistor that operates in the 15C - 35C range?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    I dont suggest you voltage divider because you will read inaccurate values due to loading effects. I recommend you to use a non-inverting op amp configuration to read the voltage produced by the thermistor.
    Search in google for one, there are many out there.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    MIK3, I think you are confusing a thermister with a thermocouple. Thermocouples produce a voltage, a thermistor varies it's resistance to temperature.

    I'd use a simple constant current source, with a voltmeter across the thermistor myself. The problem is calibrating the range.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    Bill,
    I know the difference between a thermocouple and a thermistor. He will make a voltage divider with the thermistor and a fixed resistor and drive the input of the amplifier (or voltage follower if he don't want to amplify it).
     
  5. magikal

    Thread Starter Active Member

    Nov 23, 2008
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    Thanks for the help. The range needed is not very high, but accuracy is important. What sort of thermistor (or maybe some other device) does this nicely?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    The 15 - 35 C range is good for any thermistor. How about you tell us what resistance you want at 25 C? All thermistors have a stated resistance measured at 25 C. Then they have a response curve, which is non-linear.

    Perhaps you might be interested in the LM35?

    By the way, what does your project wish to output?
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Sorry, but where is the voltage from the thermistor supposed to come from? It's either a voltage divider circuit or a current source.
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    The power for the thermistor-fixed resistor voltage divider will come from the op amp's power supply. Its just an easy solution, not very complicated for a newbie :)
     
  9. steveb

    Senior Member

    Jul 3, 2008
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    If accuracy is important, then you will need a voltage reference with any type of thermister circuit. However, the nonlinearity is really a pain. You can make a linearizing resistor network which makes the response more linear over a narrow temp range, but still not straightforward. I think someone else mentioned a chip solution which is simpler since it provides a linear response to temperature.
     
    Last edited: Nov 23, 2008
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    Not only that, but the output is scaled in engineering units.
     
  11. magikal

    Thread Starter Active Member

    Nov 23, 2008
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    Sorry, I am not very experienced with circuits. I want the output to be a voltage that can be displayed on an ADC. What chips allows for a linear voltage output?
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    What you wish is a voltage that can be converted with some accuracy by an ADC. What is the number of bits in your ADC?

    Get a data sheet for an LM35.
     
  13. Andrew Leigh

    Well-Known Member

    Sep 8, 2008
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    Hi,

    you say you need accuracy but have not indicated what tolerance. To my mind a Pt100 sensor is the way to go, you can achieve ~ +- 0,35°C accuaracy in the lower temperature range. Have alook at the following article, it may be of assistance.

    http://beta.octopart.com/Microchip__TMPSNS-RTD1__0.pdf

    Cheers
    Andrew
     
  14. magikal

    Thread Starter Active Member

    Nov 23, 2008
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    Thanks for the help, but I cannot use micro-controllers for the project. An output by the ADC of 2-3 significant digits is enough. Also, are there any simple chips containing the summer circuit or a multiplication circuit that works with voltage?
     
  15. magikal

    Thread Starter Active Member

    Nov 23, 2008
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    BTW, is the LM35 a micro-controller?
     
  16. steveb

    Senior Member

    Jul 3, 2008
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    No, it's a temperature sensor that is very simple to use. You get a voltage output that in linearly related to temp in degrees C. I think 10 mV per degree.

    Do a search for LM35 datasheet and you should find the PDF for it.
     
  17. magikal

    Thread Starter Active Member

    Nov 23, 2008
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    How do i compare the difference, or find the product of two temperatures produced by a LM35?
     
  18. beenthere

    Retired Moderator

    Apr 20, 2004
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    You convert to a digital value for each and work with the numbers.
     
  19. magikal

    Thread Starter Active Member

    Nov 23, 2008
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    Conversion can be done with an ADC right? Can you inform me of the chips that can do some mathematical operations with these numbers after converted?
     
  20. steveb

    Senior Member

    Jul 3, 2008
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    If you want, you can take the difference between two temperatures with a differential amplifier made with an OPAMP. See Vol III in the E-books here.
     
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