Thermistor
A thermistor is a temperature-dependent resistor. NTC (negative temperature coefficient) types are the most common: resistance drops as temperature rises, typically by about 3–4 % per °C. When used in a voltage divider with a fixed resistor, the output voltage tracks temperature.
The resistance follows the Steinhart-Hart equation: 1/T = A + B·ln(R) + C·(ln(R))³, where A, B, C are material constants. An NTC bead is made of metal-oxide semiconductor ceramic whose grain-boundary conductivity rises steeply with temperature. The relationship is nonlinear, so firmware typically either uses a lookup table or computes Steinhart-Hart directly.
In plain terms
Imagine a resistor made of a material that gets lazy when it warms up (NTC: resistance falls with temperature) or stiff when it cools (PTC: resistance rises with temperature). One analog pin is all you need to read a temperature.
Why designers use it
- Measure PCB and battery temperature with a single analog pin and two resistors.
- Trigger over-temperature shutdowns in phone chargers and battery packs.
- Control fan speed in a laptop — read temperature, output PWM, no digital temp sensor needed.
- Set the hot-end and bed temperature in a 3D printer with millimetre-level accuracy.
Best for
- Thermal protection
- 3D printer control
- Battery temp sensing
Key specifications
- R25: 100 Ω – 1 MΩ
- B-value: 3,000 – 4,500 K
- Tolerance: ±0.5 % – ±10 %
- Range: −55 °C – +200 °C
- Self-heating: 1 mW/K – 10 mW/K
When not to use it
- When you need better than ±0.5 °C accuracy over a wide range without calibration — a digital sensor like the PCT2075 or DS18B20 is more accurate and linear.
- When temperature changes are very slow and the 1-second ADC polling rate wastes MCU time.
Common mistakes
- Using a generic 10 kΩ pull-up for all thermistors — the divider sensitivity depends on matching the pull-up to the thermistor's nominal resistance at mid-range.
- Applying a linear interpolation across the full −40–125 °C range and accepting ±5 °C error at the extremes.
Where you will find it
- A lithium-ion battery pack uses a 10 kΩ NTC thermistor bonded to the cell surface: the charger IC reads the thermistor voltage and pauses charging if the cell temperature rises above 45 °C, preventing the thermal runaway that can cause a fire in a fast-charging session.
- A 3D printer's hot-end controller uses a 100 kΩ NTC thermistor in a voltage divider: the firmware samples it 10 times per second, computes temperature via a Steinhart-Hart lookup table, and adjusts the heater cartridge PWM duty cycle to hold the nozzle at the setpoint ±1 °C for consistent filament flow.
- A bathroom scale's strain-gauge amplifier uses an NTC thermistor as part of a temperature-compensation network: as the amplifier's offset drifts with ambient temperature, the thermistor adjusts a trim-voltage to cancel the drift, keeping the zero-point stable from the cold bathroom floor to a warm summer day.
A short history
The first commercially viable negative-temperature-coefficient (NTC) thermistor was invented by Samuel Ruben in 1930 (US patent 2,021,491, filed 1930, issued 1935). Bell Labs (G. L. Pearson, J. A. Becker, C. B. Green) refined thermistor materials and applications through the 1940s, including their use as temperature-sensitive resistors for telephone-line voltage stabilisation. Positive-temperature-coefficient (PTC) thermistors, made from doped barium-titanate ceramic, followed in the 1950s. NTC thermistors—commonly with B-values of 3950 K and resistances of 10 kΩ at 25 °C—have become the dominant low-cost temperature sensor in white goods, automotive coolant temperature, and battery-pack thermal monitoring. PTC thermistors function as resettable fuses (PolySwitch by Raychem, 1980s) and inrush limiters in switch-mode power supplies. Modern thermistors achieve ±1 % tolerance over wide temperature ranges in 0402 SMD packages.
Good to know
- An NTC thermistor's resistance drops about 4 % per °C — you can put one in a voltage divider and read 0.05 °C resolution with a 12-bit ADC.
- Steinhart–Hart equations (1968) convert thermistor resistance to temperature with three calibration constants — accurate to 0.01 °C across 100 °C span.
- Inrush-limit thermistors live in the AC inlet of every PC power supply: cold they're 10 Ω (limiting capacitor charge), hot they drop to 0.5 Ω (low loss in steady state).