2.27 Thermistor

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Overview

In this lesson, you will learn how to use thermistor. Thermistor can be used as electronic circuit components for temperature compensation of instrument circuits. In the current meter, flowmeter, gas analyzer, and other devices. It can also be used for overheating protection, contactless relay, constant temperature, automatic gain control, motor start, time delay, color TV automatic degaussing, fire alarm and temperature compensation.

Components Required

Component Introduction

Thermistor is a sensitive element, and it has two types: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC), also known as NTC and PTC. Its resistance varies significantly with temperature. The resistance of PTC thermistor increases with temperature ,while the condition of NTC is opposite to the former. In this experiment we use NTC.

The principle is that the resistance of the NTC thermistor changes with the temperature of the outer environment. It detects the real-time temperature of the environment. When the temperature gets higher, the resistance of the thermistor decreases. Then the voltage data is converted to digital quantities by the A/D adapter. The temperature in Celsius or Fahrenheit is output via programming.

In this experiment, a thermistor and a 10k pull-up resistor are used. Each thermistor has a normal resistance. Here it is 10k ohm, which is measured under 25 degree Celsius.

Here is the relation between the resistance and temperature:

R_T =R_N exp^{B(1/TK – 1/TN)}

R_{_T} is the resistance of the NTC thermistor when the temperature is T_{_K}.

R_{_N} is the resistance of the NTC thermistor under the rated temperature T_{_N}. Here, the numerical value of R_{_N} is 10k.

T_{_K} is a Kelvin temperature and the unit is K. Here, the numerical value of T_{_K}is 273.15 + degree Celsius.

T_{_N} is a rated Kelvin temperature; the unit is K too. Here, the numerical value of T_{_N} is 273.15+25.

And B(beta), the material constant of NTC thermistor, is also called heat sensitivity index with a numerical value 3950.

exp is the abbreviation of exponential, and the base number e is a natural number and equals 2.7 approximately.

Convert this formula T_K=1/(ln(R_T/R_N)/B+1/T_N) to get Kelvin temperature that minus 273.15 equals degree Celsius.

This relation is an empirical formula. It is accurate only when the temperature and resistance are within the effective range.

Fritzing Circuit

In this example, we use the analog pin A0 to get the value of Thermistor. One pin of thermistor is connected to 5V, and the other is wired up to A0. At the same time, a 10kΩ resistor is connected with the other pin before connecting to GND.

Schematic Diagram

Code

#define analogPin  A0 //the thermistor attach to 
#define beta 3950 //the beta of the thermistor
#define resistance 10 //the value of the pull-up resistor

void setup()
{
  Serial.begin(9600);
}
void loop()
{
  //read thermistor value
  long a = analogRead(analogPin);
  //the calculating formula of temperature
  float tempC = beta / (log((1025.0  * 10 / a - 10) / 10) + beta / 298.0) - 273.0;
  float tempF = 1.8  * tempC + 32.0;
  Serial.print("Temp: ");
  Serial.print(tempC);
  Serial.println("degree Celsius");
  Serial.print("Temp: ");
  Serial.print(tempF);
  Serial.println("degree Fahrenheit");  
  delay(200); //wait for 200 milliseconds
}

After uploading the code to the Mega2560 board, you can open the serial monitor to check the current temperature. The Kelvin temperature is calculated according to the formula T_{_K}=1/(ln(R_{_T}/R_{_N})/B+1/T_{_N}).

Phenomenon Picture