How to Use a Temperature Sensor with Arduino for MOSFET Thermal Management

Reading a temperature sensor and printing the value to the serial monitor is a beginner milestone. But when you are switching high currents through a power MOSFET or dropping voltage across a linear regulator, knowing the ambient air temperature is useless. To prevent catastrophic failure, you need to know the junction temperature hidden inside the silicon. Learning how to use a temperature sensor with Arduino for active thermal management means bridging the gap between the physical case temperature you can measure and the internal junction temperature you must control.

The direct answer for power electronics: attach a digital sensor like the DS18B20 directly to the component's metal tab using thermally conductive epoxy, read it via the Arduino's OneWire bus, and use the component's thermal resistance specifications to calculate the internal junction temperature in real-time.

Why Basic Temperature Reading Isn't Enough for Power Circuits

Silicon has a hard physical limit. For most standard power MOSFETs and BJTs, the absolute maximum junction temperature ($T_J$) is 150°C to 175°C. However, you cannot physically place a sensor inside the silicon die. You can only measure the case temperature ($T_C$) or the heatsink temperature ($T_S$).

The Math: Junction-to-Ambient Thermal Resistance (RθJA)

To use an Arduino to predict failure before it happens, you must understand the thermal path. Heat flows from the silicon junction (J), through the case (C), through the interface material, into the heatsink (S), and finally to the ambient air (A). This is modeled using thermal resistance, measured in °C/W.

The core equation for junction temperature is:

T_J = T_A + (P_D × R_θJA)

Where $P_D$ is power dissipation in Watts. But $R_{θJA}$ is a composite value:

R_θJA = R_θJC + R_θCS + R_θSA

If your Arduino reads an ambient temperature ($T_A$) of 30°C, and the MOSFET is dissipating 10W, the junction temperature is: 30 + (10 × 9.2) = 122°C.

Derating Curve Interpretation: A datasheet might claim the part can handle 50W. However, that is only true if the case is held at exactly 25°C. Above 25°C, you must apply the linear derating factor (typically ~0.38 W/°C for a TO-220). At a case temperature of 100°C, the maximum allowable power dissipation drops to roughly 21W. If your Arduino code does not account for this derating curve, it will allow the component to destroy itself under heavy loads in a hot enclosure.

Heatsink Selection and Airflow: Buying Thermal Headroom

Let's say your Arduino calculates that $T_J$ is creeping toward 130°C. How do you buy thermal headroom? You must lower $R_{θSA}$.

For a 10W load in a 30°C ambient environment, keeping $T_J$ under a safe 110°C requires a total $R_{θJA}$ of no more than 8.0 °C/W. Subtracting the fixed $R_{θJC}$ (1.5) and $R_{θCS}$ (0.5), you need a heatsink with an $R_{θSA}$ of 6.0 °C/W or better.

Real-World Heatsink Pick: The Aavid Thermalloy 577202B03300G is a standard extruded aluminum TO-220 heatsink. At natural convection (still air), its $R_{θSA}$ is roughly 7.2 °C/W. This is slightly too high for our 6.0 °C/W target.

What Airflow Buys You: You don't necessarily need a larger, more expensive heatsink. Adding forced air changes the physics. Moving air at just 200 LFM (Linear Feet per Minute) across the fins of the 577202B03300G drops its thermal resistance to approximately 5.0 °C/W. You can achieve 200 LFM using a compact 40mm fan like the Sunon EE40101S1 MagLev (~$9.00). This brings your total $R_{θJA}$ down to 7.0 °C/W, resulting in a safe $T_J$ of 100°C.

Enclosure Changes: If this circuit is inside a sealed NEMA box, natural convection is dead. The ambient temperature inside the box ($T_A$) will rise significantly above the room temperature. In enclosed systems, you must mount the heatsink to the outside wall of the enclosure or use an internal fan to circulate air across an external heat exchanger.

Wiring the DS18B20 and Arduino Thermal Cutoff Code

To implement this, we use the Maxim DS18B20 digital temperature sensor. Unlike thermistors, it does not suffer from wire-resistance errors over long runs and provides a direct digital Celsius readout.

Wiring:

• VDD to Arduino 5V
• GND to Arduino GND
• Data to Arduino Pin 2
• 4.7kΩ pull-up resistor between VDD and Data

For power components, use the waterproof probe variant and strap the metal probe tip directly to the MOSFET tab using a hose clamp or high-temp thermal epoxy (like Arctic Alumina). Do not use standard superglue; it acts as a thermal insulator.

The following code reads the case temperature, calculates the junction temperature based on our 10W / 1.5 °C/W $R_{θJC}$ model, and triggers a relay to cut power if the silicon exceeds 120°C.

#include <OneWire.h>
#include <DallasTemperature.h>

// Pin assignments
const int ONE_WIRE_BUS = 2;
const int RELAY_PIN = 8;
const int STATUS_LED = 13;

// Thermal constants for IRFZ44N (TO-220)
const float R_THETA_JC = 1.5; // Junction-to-Case thermal resistance (C/W)
const float POWER_DISSIPATION = 10.0; // Expected worst-case Watts
const float MAX_JUNCTION_TEMP = 120.0; // Safe cutoff threshold (C)

OneWire oneWire(ONE_WIRE_BUS);
DallasTemperature sensors(&oneWire);

float lastCaseTemp = 0.0;

void setup() {
  Serial.begin(115200);
  sensors.begin();
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(STATUS_LED, OUTPUT);
  
  // Ensure system starts in a safe, OFF state
  digitalWrite(RELAY_PIN, LOW); 
  digitalWrite(STATUS_LED, LOW);
  Serial.println("Thermal Management System Initialized.");
}

void loop() {
  sensors.requestTemperatures();
  float caseTemp = sensors.getTempCByIndex(0);
  
  // Handle disconnected sensor fault
  if (caseTemp == -127.0 || caseTemp == 85.0) {
    Serial.println("FAULT: Sensor disconnected or read error. Cutting power.");
    digitalWrite(RELAY_PIN, LOW);
    digitalWrite(STATUS_LED, HIGH); // Blink or solid LED for fault
    delay(1000);
    return;
  }

  // Calculate Junction Temperature
  float junctionTemp = caseTemp + (POWER_DISSIPATION * R_THETA_JC);
  
  // Hysteresis to prevent relay chatter
  if (junctionTemp > MAX_JUNCTION_TEMP) {
    digitalWrite(RELAY_PIN, LOW); // Cut power
    digitalWrite(STATUS_LED, HIGH);
    Serial.print("THERMAL CUTOFF: Tj = ");
    Serial.println(junctionTemp);
  } 
  else if (junctionTemp < (MAX_JUNCTION_TEMP - 10.0)) {
    // Re-enable only when 10C below threshold
    digitalWrite(RELAY_PIN, HIGH); 
    digitalWrite(STATUS_LED, LOW);
  }

  // Telemetry output
  if (millis() % 1000 == 0) {
    Serial.print("Tc: "); Serial.print(caseTemp);
    Serial.print(" C | Tj Est: "); Serial.print(junctionTemp);
    Serial.println(" C");
  }
  
  delay(250);
}

FAQ: Advanced Arduino Temperature Sensing

How to use a temperature sensor with Arduino without a breadboard?

Breadboards introduce high contact resistance and poor thermal mass, making them unsuitable for permanent thermal monitoring. For a permanent installation, transition to a custom PCB or perfboard. If you are monitoring surface-mount components (like an SMD MOSFET or an LDO regulator), use an 0805 or 0603 NTC thermistor (like the Murata NCP series). Solder the thermistor directly to the copper thermal via or ground pour adjacent to the hot component. The copper acts as a heat spreader, giving the Arduino a highly responsive, localized temperature reading without needing bulky epoxy or probes.

How to use multiple temperature sensors with one Arduino pin?

The DS18B20 utilizes the 1-Wire protocol, which allows you to daisy-chain up to 127 sensors on a single Arduino GPIO pin. Wire all VDD pins to 5V, all GND pins to ground, and all Data pins together to Pin 2. You still only need a single 4.7kΩ pull-up resistor on the shared data line. In your code, the DallasTemperature library will assign an index to each sensor based on its unique factory-burned 64-bit ROM address. Use sensors.getTempCByIndex(0) for the MOSFET, getTempCByIndex(1) for the voltage regulator, and so on. This saves pins and simplifies wiring in complex power systems.

How hot is too hot for the Arduino Uno itself?

When managing external thermals, do not forget the microcontroller's own limits. The ATmega328P chip on the Arduino Uno is rated for an ambient operating temperature of -40°C to +85°C. However, the bottleneck is usually the onboard 5V linear voltage regulator (often an NCP1117 or similar). If you power the Uno via the barrel jack at 12V, the regulator must drop 7V. At just 100mA of current draw (from relays, sensors, and LEDs), the regulator dissipates 0.7W. Because the Uno's regulator lacks a proper heatsink, its thermal resistance is roughly 70 °C/W. This causes the regulator case to hit 70°C above ambient. In a hot enclosure, the Uno's voltage regulator will hit its internal thermal shutdown (typically ~150°C) and reset the board long before the ATmega328P silicon fails. If your enclosure exceeds 45°C, power the Uno via the 5V USB pin, bypassing the onboard regulator entirely.