Thermistor maths and connection

Discussion in 'General Electronics Chat' started by clintonb, Apr 20, 2015.

  1. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    I moved onto the ADC using a thermometer that is supposed to give the temperature as a function of resistance.

    I took the connection diagram in the datasheet and connected a 330 ohm resistor from the temperature vout pin and ground.

    I obtained the thermistor resistance using the equation I found on a tutorial site for a thermistor with a resistor in series.

    Resistance=(5/(Adc_val/1023*5)*330)-330

    I used the resistance-temperature table to get the equation in excel that could convert the resistance of the thermistor to a temperature.

    When I run the program I get a value much lower than the mercury bulb(+-10 degree less) and as the temperature goes higher the error seems to get smaller.

    Is it perhaps necessary to sit with the mercury bulb and do my own calibration with my own values.
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Without knowing what thermistor you used, and without a schematic, all I can do is guess. First, 330 ohms seems very low; most commonly used is a 10k thermistor and a 10k resistor. Second, the formula you used assumes a 10 bit ADC; is that what you used?

    Or maybe you aren't using a microcontroller?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    You also need to use 1% resistors.
     
  4. kubeek

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  5. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Can you get the "coefficients" for your thermistor? The data sheet usually has them.
     
  7. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    I
    It is in fact a 10 bit number read on a microcontroller's AdC pin. The thermistor is a 5k and the best I have in my shelf is a 3k so I tried both a 3k and a 6k combination as a resistor since the data sheet doesn't say much more than it needs to be a resistor.

    Seems to make a huge difference as it now seems to be about 5 degress over but definatley a huge impact - thanks for the tip

    Edit : eventhough it is reporting 5 degree more than actual, it is 5 degrees more consistantly so by decreasing the calculated temp by 5 degrees seems to always give the accurate temperature.
     
    Last edited: Apr 20, 2015
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You can substitute a pot for the fixed resistor, and calibrate to agree with the bulb thermometer.
     
  9. kdillinger

    Active Member

    Jul 26, 2009
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    Another option is to use a remote diode temperature sensor rather than a thermistor. It depends on your required temperature range.
    Remote diodes sense -40 to +125.
     
  10. DickCappels

    Moderator

    Aug 21, 2008
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    Would you please post the datasheet for your thermostor or tell us the name of the manufacturer and the part number?
     
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