What is a thermistor?
The small, cheap part that tells your car, your phone battery, and your thermostat how hot things are — by changing its resistance as the temperature changes.
A thermistor is a thermally sensitive resistor — a semiconductor resistor whose resistance changes strongly and predictably with temperature (Wikipedia). Measure its resistance and you know its temperature. Most are NTC types, whose resistance falls as they heat up; PTC types do the opposite and are mostly used for circuit protection.
Resistance that tracks temperature
Every resistor’s value shifts a little with temperature. Ordinary resistors are engineered to minimize that drift. A thermistor is engineered to maximize it.
A typical NTC thermistor changes resistance by about −3% to −6% for every degree Celsius near room temperature (NTC engineering notes). Compare that with a platinum RTD at roughly 0.39% per degree, and you can see why thermistors are the go-to sensor when you need to resolve small temperature changes cheaply.
The catch is that the curve isn’t a straight line. It’s close to exponential, which is why thermistor math uses the β (beta) equation or the Steinhart–Hart equation to turn ohms back into degrees.
The two families
The temperature coefficient — negative or positive — decides what a thermistor is good for.
Resistance falls as it heats
Made from sintered oxides of metals like manganese, nickel, and cobalt (CTS). Smooth, steep, repeatable curve.
- Temperature sensing and measurement
- Inrush current limiting in power supplies
- Temperature compensation of other circuits
Resistance jumps past a set point
Usually doped barium titanate ceramic, or carbon-filled polymer. Nearly flat, then a sharp rise at a switching temperature (Wikipedia).
- Resettable overcurrent protection (“polyfuses”)
- Self-regulating heaters
- Motor starting and overheat detection
Everything about thermistors, in eight guides
Start at the top for fundamentals, or jump straight to the practical how-tos.
How thermistors work
The physics, the β equation, Steinhart–Hart, and self-heating — explained without the hand-waving.
02NTC vs PTC
Resistance that falls with heat versus resistance that jumps. Which one you need and why.
03Types & construction
Beads, discs, chips, glass, probes, and polymer PTCs — what each package is good at.
04Reading a datasheet
R25, B25/85, tolerance, dissipation constant, time constant — every spec decoded.
05Where they’re used
Battery packs, engines, thermostats, 3D printers, medical probes, and power supplies.
06Thermistor vs RTD vs thermocouple
Range, accuracy, sensitivity, and cost compared side by side.
07How to test one
A five-minute multimeter check that tells you if a thermistor is good, open, or shorted.
08Thermistor calculator
Convert resistance to temperature with β or Steinhart–Hart, and size a voltage divider.
TRY ITPlug in your own part
Enter R25 and β from any datasheet and see the full curve.
Probably within arm’s reach right now
NTC thermistors monitor engine coolant, intake air, and oil temperatures for a car’s ECU; they watch battery packs while charging; they sit in digital thermostats; and they regulate the hot end of nearly every hobby 3D printer (Wikipedia).
PTC thermistors work behind the scenes as resettable fuses, motor-start aids, and self-limiting heaters (TDK).
From silver sulfide to every circuit board
- 1833Michael Faraday notices that the resistance of silver sulfide drops sharply as it warms — the first NTC thermistor behavior, and the first documented observation of a semiconductor (Wikipedia).
- 1930Samuel Ruben patents a commercially viable thermistor, which he called an “electrical pyrometer resistance” (Scientific Reports).
- 1940sBetter, repeatable manufacturing makes large-scale production practical; thermistors spread into protection, regulation, and temperature compensation (Scientific Reports).
- 1968John Steinhart and Stanley Hart publish the equation that still defines precision thermistor math, developed for oceanographic measurements (North Star Sensors).
Thermistor FAQ
What does the word “thermistor” mean?
It is a contraction of “thermally sensitive resistor” — a resistor designed so its resistance depends strongly on temperature (Molex).
Is a thermistor the same as a resistor?
Electrically, yes — it is a two-terminal resistor with no polarity. The difference is intent: ordinary resistors are made to stay constant as temperature changes, while thermistors are made to change a lot, and predictably.
What is the most common thermistor?
The 10 kΩ NTC (10,000 Ω at 25 °C) is the everyday workhorse in hobby and HVAC electronics. Its β value tells you the shape of its curve; 3435 K and 3950 K are common, and 100 kΩ / 3950 K parts are typical in 3D-printer hot ends.
What temperature range can a thermistor measure?
Most are rated somewhere between −40 °C and +150 °C, although specialist parts span roughly −100 °C to +500 °C (Texas Instruments). Glass-encapsulated beads are the usual choice above about 150 °C (Penn State).
How accurate is a thermistor?
Within a narrow band, very. Using the Steinhart–Hart equation, a well-characterized NTC is typically good to about ±0.15 °C from −50 to +150 °C, and better than ±0.01 °C is achievable over 0–100 °C (EEPower).
Do thermistors wear out?
Ceramic thermistors are very stable when used within their ratings, but they can drift with sustained high-temperature exposure, moisture ingress, or mechanical stress on the leads. Glass-sealed parts offer the best long-term stability.