Heating Foil temperature regulation project

[nani261]

Hi,
I have this project where I need to keep a small heating pad (10x10cm) at 37° Celsius.

I have set up a DS18B20 to check ambient temperature and an NTC temperature sensor to check the heating pad temperature.

Chatgpt tried helping me but I havent been successful with the temperature regulation, it turns on and increases temperature steadily but never regulates or goes down. My idea is to control the heating pad with a mosfet P55NF06L, turning the heat off once it reaches 37.5 and turn it on at 36.
I have 2 power sources, a barrel jack in my breadboard providing 12v to heating pad and 5v from arduino for temp sensors.

I'd appreciate any help, I'm completely lost.

[bobcousins]

@nani261

an NTC temperature sensor

Which one? I assume you mean an NTC thermistor.

In general you connect the thermistor in a potential divider, and read the analog value. There are several ways to convert to temperature: use a lookup table, use the beta formula, or for a more accurate result use the Steinhart-Hart formula. You may also want to compensate for self-heating. Whatever method, it is a good to idea to calibrate the result against known temperature points.

Probably to start I would use the beta formula. You may find a simple off-on approach does not provide good regulation, typically you will get swings around the set up. A more precise approach would use PWM control and a PID controller.

[docdoc]

Post your code here (inside "<CODE/>" tags please) and add a schematic diagram for helping us understand what's going on (even a photo of a pen and paper schematic would be helpful).

[Railroader]

Use a logic level N channel MOSFET. Ordinary MOSFET will not work.

[jim-p]

@nani261

I'd appreciate any help, I'm completely lost.

Which arduino are you using?
Forget the NTC and use a digital temperature sensor.

[nani261]

Hi! Thank you for your help. Yes, I'm using an NTC thermistor. I cant get the mosfet to turn off the heating pad, no matter what I try.
Here is the circuit:

And this is the code I have right now:

// Include necessary libraries
#include <OneWire.h>
#include <DallasTemperature.h>

// Pin definitions
#define NTC_PIN A0          // Analog pin for NTC thermistor
#define HEATER_PIN 9        // MOSFET control pin (PWM)
#define ONE_WIRE_BUS 2      // DS18B20 data pin

// Temperature control settings
#define TEMP_SETPOINT 37.0  // Target temperature (°C)
#define HYSTERESIS 0.25      // Prevent rapid switching

// NTC Thermistor properties
#define SERIES_RESISTOR 10000.0 // 10kΩ pull-down resistor
#define NOMINAL_RESISTANCE 10000.0 // NTC resistance at 25°C
#define NOMINAL_TEMPERATURE 25.0 // Reference temperature (°C)
#define BETA_COEFFICIENT 3950.0 // Beta coefficient

// Initialize DS18B20 sensor
OneWire oneWire(ONE_WIRE_BUS);
DallasTemperature dsSensor(&oneWire);

void setup() {
    Serial.begin(9600);
    pinMode(HEATER_PIN, OUTPUT);
    dsSensor.begin(); // Start DS18B20 sensor
}

// Function to read NTC thermistor temperature
float readNTC() {
    int rawValue = analogRead(NTC_PIN);
    float voltage = rawValue * (5.0 / 1023.0);

    // Adjust resistance calculation
    float resistance = SERIES_RESISTOR * ((5.0 - voltage) / voltage);


    // Steinhart-Hart equation
    float steinhart;
    steinhart = log(resistance / SERIES_RESISTOR) / BETA_COEFFICIENT;
    steinhart += 1.0 / (NOMINAL_TEMPERATURE + 273.15);  // 25°C reference
    steinhart = 1.0 / steinhart - 273.15; // Convert to Celsius

    // Print results **before** returning the value
    Serial.print("Raw ADC: "); Serial.print(rawValue);
    Serial.print(" | Voltage: "); Serial.print(voltage, 3);
    Serial.print("V | Resistance: "); Serial.print(resistance, 1);
    Serial.print("Ω | Temp: "); Serial.print(steinhart, 2);
    Serial.println("°C");

    return steinhart;  // Return the calculated temperature
}

void loop() {
    // Read DS18B20 sensor
    dsSensor.requestTemperatures();
    float ds18b20Temp = dsSensor.getTempCByIndex(0);

    // Read NTC thermistor
    float ntcTemp = readNTC();

    // Display sensor values
    Serial.print("DS18B20 Temp: "); Serial.print(ds18b20Temp);
    Serial.print(" | NTC Temp: "); Serial.print(ntcTemp);
    
    // Use NTC temperature for control
    if (ntcTemp < TEMP_SETPOINT - HYSTERESIS) {
        Serial.println(" | Heater ON");
        digitalWrite(HEATER_PIN, HIGH);
    } 
    else if (ntcTemp > TEMP_SETPOINT + HYSTERESIS) {
        Serial.println(" | Heater OFF");
        digitalWrite(HEATER_PIN, LOW);
    }
    
    delay(1000); // Update every second
}

[Dave_Lowther]

What does the serial output show? Can you post a relevant snip in code tags. Relevant would be the condition that should turn the heating pad off, because you say it never turns off.
[Edit. Which mosfet are you really using? ...]

@nani261

mosfet P55NF06L

@nani261

Here is the circuit:

[bobcousins]

@nani261

Here is the circuit:

It appears you may have the MOSFET connected wrongly. Source should be connected to ground.

Note if you have damaged the MOSFET, it might be always on.

[DaveEvans]

Your schematic is wrong: it shows the DS18B20 data pin connected to 5v and the Vcc pin connected to pin D2