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How Easy Is It to Use a Thermistor?! © GPL3+

Thermistor as thermometer.

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How easy is it to use a thermistor?!

In this article I will explain how to use a thermistor. First of all, what is it a Thermistor? A thermistor is a type of resistor whose resistance is dependent on temperature. There are two opposite types of thermistor:

  • PTC (Positve Temperature Coefficent), resistance increases as temperature rises
  • NTC (Negative Temperature Coefficent), resistance decreases as temperature rises

In this case I use NTC.

A little bit of math.

To calculate the thermistor resistance using a simple formula called equation with parameter B (with only NTC termistor).

Where:

  • e is the base of natural logarithm
  • R0 is the resistance of the thermistor measured at the temperature T0
  • B is a constant coefficient that depends on the characteristics of the material, it is a constant expressed in K, and its value is indicated by the manufacturers on the technical sheets

To calculate the temperature we need know the resistance RT using the Ohm's laws.

This is a schematic version of circuit.

RT = VRT / (VR/R)

Now we have all the data to calculate the temperature.

Remember to convert all parameters (for example T0) to Kelvin before the calculations, and also the result is in Kelvin.

This is the result.

Code

ThermistorArduino
//Thermometer with thermistor

/*thermistor parameters:
 * RT0: 10 000 Ω
 * B: 3977 K +- 0.75%
 * T0:  25 C
 * +- 5%
 */

//These values are in the datasheet
#define RT0 10000   // Ω
#define B 3977      // K
//--------------------------------------


#define VCC 5    //Supply voltage
#define R 10000  //R=10KΩ

//Variables
float RT, VR, ln, TX, T0, VRT;

void setup() {
  Serial.begin(9600);
  T0 = 25 + 273.15;                 //Temperature T0 from datasheet, conversion from Celsius to kelvin
}

void loop() {
  VRT = analogRead(A0);              //Acquisition analog value of VRT
  VRT = (5.00 / 1023.00) * VRT;      //Conversion to voltage
  VR = VCC - VRT;
  RT = VRT / (VR / R);               //Resistance of RT

  ln = log(RT / RT0);
  TX = (1 / ((ln / B) + (1 / T0))); //Temperature from thermistor

  TX = TX - 273.15;                 //Conversion to Celsius

  Serial.print("Temperature:");
  Serial.print("\t");
  Serial.print(TX);
  Serial.print("C\t\t");
  Serial.print(TX + 273.15);        //Conversion to Kelvin
  Serial.print("K\t\t");
  Serial.print((TX * 1.8) + 32);    //Conversion to Fahrenheit
  Serial.println("F");
  delay(500);

}

Schematics

termistore_bb.jpg
Termistore bb
termistore_schem.jpg
Termistore schem

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