Temperature Sensors Explained: Thermistors, RTDs, Thermocouples, and ICs

Part of the Complete Guide to Sensors for Embedded Systems series.

Temperature is the most commonly measured quantity in embedded systems. But there are four fundamentally different sensor technologies to choose from, each with distinct trade-offs in cost, accuracy, range, and ease of use. This guide compares them all with practical recommendations for embedded engineers.

NTC and PTC Thermistors

Thermistors are temperature-sensitive resistors. NTC (Negative Temperature Coefficient) thermistors decrease in resistance as temperature increases — the most common type for temperature measurement. PTC thermistors increase in resistance and are mainly used as resettable fuses for overcurrent protection.

An NTC thermistor might have 10kΩ resistance at 25°C and 3.3kΩ at 50°C. You place it in a voltage divider with a known resistor, measure the output voltage with an ADC, calculate the resistance, then convert to temperature using the Steinhart-Hart equation or a lookup table.

Advantages: Very cheap ($0.05-$0.50), fast response time (seconds), small size, wide availability. Disadvantages: Nonlinear response requires complex math or lookup tables, limited accuracy (±1-2°C typical), self-heating with excessive current.

Best for: battery charging, motor winding temperature monitoring, simple over-temperature protection, and budget-constrained consumer products.

RTD (Resistance Temperature Detector)

RTDs use a pure metal element (usually platinum) whose resistance increases linearly with temperature. The most common type is PT100, which has exactly 100Ω at 0°C. At 100°C, the resistance is approximately 138.5Ω — a change of only 0.385Ω per degree.

This small change requires a Wheatstone bridge circuit and instrumentation amplifier for accurate measurement. Three-wire or four-wire configurations compensate for lead resistance. MAX31865 is a popular IC that handles all the signal conditioning for PT100 sensors and provides an SPI digital output.

Advantages: Excellent accuracy (±0.1°C), very stable over time, linear response, wide range (-200°C to +850°C). Disadvantages: Expensive ($5-$50), slow response (seconds to tens of seconds), requires excitation current and signal conditioning circuitry, susceptible to self-heating.

Best for: industrial process control, laboratory instruments, calibration references, and applications where long-term stability and accuracy are critical.

Thermocouples

A thermocouple consists of two different metal wires joined at a junction. The temperature difference between the measurement junction and the reference junction produces a voltage (Seebeck effect). Different metal combinations create different thermocouple types: Type K (chromel-alumel, -270°C to +1260°C), Type J (iron-constantan, -210°C to +760°C), Type T (copper-constantan, -270°C to +400°C).

The output voltage is very small — Type K produces about 41µV per degree Celsius. An amplifier like MAX31855 or AD8495 boosts this signal and provides cold junction compensation (the reference junction temperature affects the reading and must be corrected).

Advantages: Widest temperature range of any sensor type, very rugged, no excitation current needed (self-generating), fast response, small size at the measurement point. Disadvantages: Very low output voltage requiring amplification, needs cold junction compensation, lower accuracy (±1-2°C typical), susceptible to EMI.

Best for: furnace monitoring, exhaust gas temperature, welding equipment, and any application requiring extreme temperature ranges.

IC Temperature Sensors

Integrated circuit temperature sensors combine the sensing element, ADC, calibration, and communication interface in a single chip. They are by far the easiest to use.

Analog IC sensors: LM35 outputs 10mV/°C (250mV at 25°C). TMP36 outputs 750mV at 25°C with 10mV/°C. Just connect to an ADC pin — no external components needed (except bypass capacitor). Range typically -55°C to +150°C, accuracy ±0.5°C.

Digital IC sensors: DS18B20 uses 1-Wire protocol. Multiple sensors on a single GPIO pin using unique addresses. 9-12 bit programmable resolution. Accuracy ±0.5°C over -10°C to +85°C range. Ideal for multi-point temperature monitoring.

I2C sensors: TMP102, SHT31, BME280. Read temperature (and often humidity and pressure) by simply reading I2C registers. Factory-calibrated. Some include programmable alert thresholds.

Best for: consumer electronics, IoT devices, environmental monitoring, and any application where ease of integration matters more than extreme temperature range.

Choosing the Right Temperature Sensor

• Need to measure above 500°C? → Thermocouple (Type K or N)
• Need ±0.1°C accuracy in a lab? → PT100 RTD with MAX31865
• Need cheap temperature monitoring for a consumer product? → NTC thermistor
• Need easy digital interface for IoT? → DS18B20 (1-Wire) or BME280 (I2C)
• Need fast response for motor protection? → NTC thermistor in contact with winding
• Need multiple measurement points on one bus? → DS18B20 (up to 100+ on one wire)
• Need temperature + humidity + pressure? → BME280 or SHT31

For most embedded hobbyist and IoT projects, the DS18B20 or BME280 is the best starting point. For production designs, evaluate the total system cost including signal conditioning circuitry — an IC sensor at $2 that needs no external components may be cheaper than a $0.10 thermistor that needs a precision resistor, ADC channel, and calibration time.

📖 Related: Signal Conditioning for Sensors: Amplification, Filtering, and Level Shifting • NTC and PTC Thermistors: Working, Differences, and Applications

Vivek Bhageria

Vivek Bhageria — Lead Firmware R&D Engineer, 12+ years. Ex-Bosch (automotive powertrain), MusicTribe (real-time audio), medical devices. M.Tech BITS Pilani. I write at NerdyElectronics — practical, register-level embedded systems for engineers who want to understand what’s actually happening under the hood.

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