Thermistor

A thermistor is a type of resistor whose resistance varies significantly with temperature. It is widely used for temperature sensing and measurement applications due to its high sensitivity to temperature changes. Thermistors are made from semiconductor materials and are typically more sensitive than other temperature sensors like thermocouples.

Types of Thermistors:

1. NTC (Negative Temperature Coefficient):

Resistance decreases as the temperature increases.

Most common type used for temperature measurement.

2. PTC (Positive Temperature Coefficient):

Resistance increases as the temperature increases.

Often used for current-limiting devices (like circuit protection).

Key Features:

High accuracy: They are quite accurate over a limited temperature range.

Non-linear response: The change in resistance is not linear with respect to temperature.

Limited temperature range: They are typically used for lower temperature applications compared to thermocouples.

Example Applications:

Digital thermometers

Temperature control in home appliances (e.g., HVAC systems)

Battery temperature monitoring

Overheat protection

Thermistor Equation (for NTC):

The relationship between resistance and temperature can be approximated using the Steinhart-Hart equation for an NTC thermistor:

Where:

 is the temperature in Kelvin.

 is the resistance at that temperature.

, , and  are constants provided by the thermistor manufacturer.

A simpler, more common approximation is the Beta formula:

Where:

 is the temperature in Kelvin.

 is the reference temperature (usually 25°C or 298.15K).

 is the resistance at temperature .

 is the resistance at reference temperature .

 is a material constant (Beta value) provided by the manufacturer.

Practical Example:

If you have an NTC thermistor with a resistance of 10kΩ at 25°C, you could measure its resistance at different temperatures to calculate the corresponding temperature using these equations or using a pre-defined lookup table provided by the manufacturer.

Thermistors are especially useful when high precision is required over a small temperature range, making them popular for many everyday electronic devices.

Circuit

    

To connect a thermistor to an Arduino for temperature measurement, you can follow a simple voltage divider circuit. Here's the procedure to set it up and code the Arduino to read temperature values.

Components Required:

Arduino board (e.g., Uno)

NTC thermistor (e.g., 10kΩ at 25°C)

10kΩ resistor (for the voltage divider)

Jumper wires

Breadboard (optional)

Computer with Arduino IDE

Circuit Diagram:

You will create a voltage divider circuit using the thermistor and a resistor.

1. Thermistor and Resistor Configuration:

Connect one lead of the thermistor to 5V on the Arduino.

Connect the other lead of the thermistor to an analog input pin (e.g., A0) and one end of the 10kΩ resistor.

Connect the other end of the 10kΩ resistor to GND on the Arduino.

Schematic:

+5V ----> Thermistor ----> A0 (Analog pin on Arduino)

                |

               10kΩ

                |

               GND

Procedure:

1. Build the Circuit:

Place the thermistor and the 10kΩ resistor on the breadboard.

Connect them according to the voltage divider configuration described above.

The voltage at the analog pin A0 will change based on the thermistor's resistance as temperature changes.

2. Write the Arduino Code:

The Arduino will read the analog voltage and convert it to a resistance value, which can then be used to calculate the temperature.

// Constants

const int thermistorPin = A0;  // Pin connected to the voltage divider

const int seriesResistor = 10000; // Value of the fixed resistor (10kΩ)

const int nominalResistance = 10000; // Resistance of the thermistor at 25°C

const int nominalTemperature = 25; // Nominal temperature (25°C)

const int betaCoefficient = 3950; // Beta coefficient of the thermistor

const int adcMaxValue = 1023; // Maximum ADC value for Arduino (10-bit ADC)

void setup() {

  Serial.begin(9600);  // Start serial communication

}

void loop() {

  int analogValue = analogRead(thermistorPin);  // Read analog value

  

  // Convert the analog value to resistance

  float resistance = seriesResistor / ((1023.0 / analogValue) - 1);

  

  // Calculate temperature using the Beta formula

  float temperature = resistanceToTemperature(resistance);

  

  // Print the temperature to the Serial Monitor

  Serial.print("Temperature: ");

  Serial.print(temperature);

  Serial.println(" °C");

  

  delay(1000);  // Wait 1 second before next reading

}

// Function to convert resistance to temperature using the Beta equation

float resistanceToTemperature(float resistance) {

  float temperature;

  

  // Convert the resistance to temperature in Kelvin

  temperature = resistance / nominalResistance;      // (R/Ro)

  temperature = log(temperature);                    // ln(R/Ro)

  temperature /= betaCoefficient;                    // 1/B * ln(R/Ro)

  temperature += 1.0 / (nominalTemperature + 273.15); // + (1/To)

  temperature = 1.0 / temperature;                   // Invert

  temperature -= 273.15;                             // Convert from Kelvin to Celsius

  

  return temperature;

}

3. Upload the Code:

Connect your Arduino to the computer via USB.

In the Arduino IDE, select your board and the appropriate port under the Tools menu.

Click Upload to load the code onto your Arduino.

4. Monitor the Temperature:

Open the Serial Monitor (Ctrl+Shift+M) to see the temperature readings in real time.

Explanation:

Analog Read: The Arduino reads the voltage across the thermistor. The analog value ranges from 0 to 1023 (since Arduino has a 10-bit ADC).

Voltage Divider: The voltage divider formula is used to calculate the resistance of the thermistor:

R_{\text{thermistor}} = R_{\text{series}} \times \left( \frac{1023}{\text{analogValue}} - 1 \right)

Troubleshooting:

Make sure that the thermistor and resistor are properly connected in the voltage divider configuration.

Check that the thermistor has the correct beta coefficient and resistance values (typically provided in the thermistor’s datasheet).

If the temperature readings are unstable or incorrect, try adjusting the resistor value or check the wiring connections.

With this setup, your Arduino can measure temperature using a thermistor and display the readings in real-time.