5 Replies Latest reply on Jul 24, 2018 10:54 PM by user_342122993

    Thermistor (103AT-11) Sample Project

    user_13463998

      This time, actually a while ago, I came across a chance to make a sample project of thermistor.

      IMG_2503.JPG

      Although the circuit seemed to be easy, the calculation was not as easy as I was expecting.tempSensor.JPG

      According to the web page of the thermistor, we can calculate the resistance of  a thermistor with following equation.

       

      equ1.JPG

      Rt : Resistance at temperature t (kelvin)

      R0: Resistance at T0

      B: Thermistor related constant. For 103AT-11 it is 3435 +/- 1%

      Conversion from Tk (Kelvin) to Tc (Celsius) is Tc = Tk - 273.15

       

      Dividing both sides by R0, we get

       

      equ2.JPG

      Replacing e to log

      equ3.JPG

      Dividing both sides by B

      equ4.JPG

      From the Datasheet the resistance value of this thermistor is 10.0K at 25 degree Celsius,

      so if we replace R0 to R25 and T0 to T25 (Klevin for 25c) the equation  is now

      equ5.JPG

      Moving 1/T25 from the right side to the left side

      equ6.JPG

      Reducing the left side

      equ7.JPG

      Then reforming the equation according to T

      equ8.JPG

      Finally we get

      equ9.JPG

      In the figure below

       

      Rt = R1 x Vt / (VDDA -Vt)

      circuit1.JPG

       

      If we normalize the ADC value (ADin)  of input voltage 0V ~ VDDA V to 0.0 ~ 1.0

       

      Rt = R1 x ADin / (1 - ADin)

       

      Now we can write a code

       

      Rt = R1 * ADin / (1 - ADin) ;

      K = 1.0 / (log(Rt / R25) / B + 1.0/T25) ;

      T = K - 273.15 ;

       

      To make the long story short, here is the whole enchilada!

      (aka, project attached...)

       

      moto