A thermistor is a type of temperature sensor made from semiconductor material whose electrical resistance changes significantly when the temperature around it changes. The name is a combination of “thermal” and “resistor.” Thermistors are found in everything from car engines to medical devices to home thermostats, and they’re one of the most common ways electronic systems detect and respond to temperature.
How a Thermistor Works
Every thermistor operates on a simple principle: as its temperature changes, its resistance to electrical current changes in a predictable way. A circuit sends a small current through the thermistor, measures the resistance, and converts that reading into a temperature value. The resistance change per degree is much larger in a thermistor than in other temperature sensors, which makes thermistors especially sensitive to small temperature shifts.
Most thermistors are made from metal oxide ceramics, typically compounds of nickel oxide and manganese oxide. These materials are shaped into tiny beads, discs, or rods, often encased in glass or epoxy for protection. The semiconductor nature of these materials is what gives thermistors their strong temperature response. As heat energy increases, it changes how easily electrons can flow through the material, which directly changes the resistance.
NTC vs. PTC: Two Types of Thermistors
Thermistors come in two varieties based on how they respond to heat.
NTC (Negative Temperature Coefficient) thermistors decrease in resistance as temperature rises. This is the more common type. When heat agitates the electrons inside the semiconductor, more of them become available to carry current, so resistance drops. NTC thermistors typically measure temperatures from about –80°C to +150°C and are the standard choice for temperature monitoring in most electronics.
PTC (Positive Temperature Coefficient) thermistors increase in resistance as temperature rises. They behave a bit differently: below a certain threshold called the Curie point or switching point, their resistance changes gradually. Once that threshold is crossed, resistance spikes dramatically, sometimes by several orders of magnitude within just a few degrees. This sharp switching behavior makes PTC thermistors useful as safety devices. They’re often built from barium titanate and similar ceramic compounds.
Both types have an exponential (nonlinear) relationship between resistance and temperature, meaning the change isn’t proportional. A 10-degree rise at low temperatures produces a different resistance shift than a 10-degree rise at high temperatures. Engineers use mathematical models to account for this curve when converting resistance readings to accurate temperatures.
Where Thermistors Are Used
NTC thermistors show up wherever a device needs to know the temperature of something and respond accordingly. In your car, thermistors monitor engine coolant temperature, cabin air temperature, and battery temperature in electric vehicles. In your home, they’re inside digital thermostats, refrigerators, and ovens. HVAC systems rely on them to regulate heating and cooling.
In medicine, thermistors are built into patient monitoring equipment, diagnostic instruments, and life science devices. Their small size and fast response time make them well suited for environments where precise, real-time temperature readings matter. They’re also used in food processing and handling, where temperature control is critical for safety.
PTC thermistors, with their dramatic resistance spike at a set temperature, are commonly used as resettable fuses. If a circuit overheats, the PTC thermistor’s resistance jumps high enough to cut off current flow, protecting the circuit. Once things cool down, resistance drops back and current flows again.
How Thermistors Compare to Other Sensors
Three main technologies compete for temperature sensing jobs: thermistors, RTDs (resistance temperature detectors), and thermocouples. Each has tradeoffs that matter depending on the application.
- Temperature range: Thermistors cover roughly –40°C to 250°C. RTDs handle –240°C to 650°C, and thermocouples span an enormous –210°C to 1,760°C. If you need to measure the inside of a furnace, a thermistor won’t work.
- Sensitivity and speed: Thermistors respond faster than either alternative and detect smaller temperature changes more easily. This makes them ideal for applications where quick, fine-grained readings matter.
- Cost: Thermistors are inexpensive. Thermocouples are the cheapest option overall, while RTDs are the most expensive.
- Accuracy: RTDs are the most accurate. Thermistors and thermocouples both fall into a medium accuracy range, though thermistors’ high sensitivity can partially compensate in narrow temperature bands.
- Durability: Thermistors are more fragile than the other two options, which can matter in harsh industrial environments.
Self-Heating and Response Time
Because thermistors need electrical current flowing through them to take a measurement, the current itself generates a small amount of heat inside the sensor. This is called self-heating, and it can throw off readings if not managed properly. The amount of power needed to raise a thermistor’s temperature by 1°C above its surroundings is called the dissipation constant, usually measured in milliwatts per degree. If you need accuracy within 0.1°C, for example, you’d need to keep the power dissipated in the thermistor below about 0.15 milliwatts in a well-coupled environment. In practice, this means using low measurement currents.
Response time is measured by the thermal time constant: the number of seconds it takes for a thermistor to register 63.2% of a sudden temperature change. A bare bead thermistor in moving air responds in seconds. Encapsulated thermistors in protective housings are slower, since the housing material adds a thermal buffer between the environment and the sensing element. The size of the thermistor, how it’s mounted, and the medium it’s measuring (air, liquid, or a solid surface) all affect how fast it reacts.
What Makes Thermistors Nonlinear
The biggest engineering challenge with thermistors is their curved resistance response. Unlike an RTD, where resistance changes in a relatively straight line as temperature rises, a thermistor’s resistance changes exponentially. At lower temperatures, a one-degree change produces a large resistance shift. At higher temperatures, the same one-degree change produces a smaller shift.
To convert a thermistor’s resistance into an accurate temperature, circuits use either lookup tables (pre-calculated resistance values for each temperature) or mathematical equations. The most widely used is the Steinhart-Hart equation, which uses three coefficients specific to each thermistor to map resistance to temperature with high precision across a wide range. A simpler version, the B-parameter equation, works well over narrower ranges and requires only two calibration points. These calculations happen inside the microcontroller or chip that reads the thermistor, so the end user simply sees a temperature value on a display.