- Thermistor Definition: A thermistor (or thermal resistor) is defined as a resistor whose electrical resistance varies significantly with changes in temperature.
- Uses of Thermistors: Thermistors are used in digital thermometers, automotive applications, household appliances, and circuit protection.
- Working Principle: Thermistors work by changing their resistance with temperature; this resistance is measured to determine the temperature.
- NTC and PTC Thermistors: NTC thermistors decrease resistance with increasing temperature, while PTC thermistors increase resistance as temperature rises.
- Thermistor Construction: Thermistors are made from semiconductor powders, shaped into beads, disks, or washers, and coated with glass for stability.
What is a Thermistor?
Thermistors act as a passive component in a circuit. They can provide high temperature sensitivity in a small, inexpensive and rugged sensor. Accuracy depends on calibration, readout electronics, thermal coupling and self-heating.
Their useful temperature range is usually narrower than that of platinum resistance thermometers or thermocouples, and it depends on the material and package.
Thermistors suit measurements that need high sensitivity over a limited range. The circuit symbol for a thermistor is shown below:
Uses of Thermistors
Thermistors have a variety of applications. They are widely used as a way to measure temperature as a thermistor thermometer in many different liquid and ambient air environments. Some of the most common uses of thermistors include:
- Digital thermometers and thermostats
- Automotive oil, coolant and air-temperature sensing
- Temperature sensing in microwaves, refrigerators and ovens
- Inrush-current limiting and temperature-dependent surge protection circuits
- Temperature monitoring for rechargeable batteries
- Test systems that estimate the thermal conductivity of electrical materials
- Temperature experiments in basic circuits, including a beginner Arduino starter kit
- Temperature compensation for another component or circuit
- Resistance measurement in wheatstone bridge circuits
How Does a Thermistor Work?
A thermistor’s resistance depends on its temperature. An ohmmeter can check resistance when the sensor is disconnected, but an operating circuit usually infers resistance from a known excitation and a measured voltage or current.
The readout converts resistance to temperature using the sensor’s calibration curve or coefficients. The measuring current must be low enough to keep self-heating within the allowed error.
The material and construction determine how resistance changes with temperature. The relationship is non-linear and varies among sensor models. A typical NTC thermistor graph is shown below:
A graph can provide an approximate temperature for a measured resistance. Accurate work uses the calibration data for the specific sensor and accounts for readout uncertainty.
Read across from resistance to the curve and then down to the temperature axis. Interpolation accuracy is limited by the graph scale and by differences between the actual thermistor and the nominal curve.
Thermistor Types
There are two types of thermistors:
- Negative Temperature Coefficient (NTC) Thermistor
- Positive Temperature Coefficient (PTC) Thermistor
NTC Thermistor
In an NTC thermistor, resistance decreases as temperature increases. Resistance rises again as temperature falls. This inverse relationship makes NTC thermistors the most common type.
A common single-beta approximation for the resistance-temperature relationship is:
Where:
- RT is the resistance at temperature T (K)
- R0 is the resistance at temperature T0 (K)
- T0 is the reference temperature (normally 25oC)
- β is a material and device constant stated in kelvins; use the value given for the thermistor and temperature interval.
A larger beta value generally means a larger fractional resistance change at a given temperature. It does not by itself guarantee accuracy. Accuracy also depends on beta tolerance, resistance tolerance, calibration, self-heating and the validity of the model over the selected range.
The local resistance temperature coefficient describes the fractional change in resistance per kelvin and therefore indicates sensitivity at that temperature.
The negative sign of αT indicates that an NTC thermistor’s resistance decreases as temperature increases.
If β = 4000 K and T = 298 K, then αT is about –0.045/oK, or –4.5% per kelvin. This high sensitivity can resolve small temperature changes when the readout and calibration are suitable.
Positive-temperature-coefficient thermistors use different materials and can show a gradual or sharp rise in resistance over their operating region.
A thermistor is non-linear, but a linear approximation can be useful over a sufficiently narrow range. Wider-range conversion normally uses a beta equation, Steinhart-Hart coefficients or a lookup table.
PTC Thermistor
A PTC thermistor has the reverse relationship between temperature and resistance. When temperature increases, the resistance increases.
When temperature decreases, resistance also decreases over the positive-coefficient operating region. The relationship is direct but usually non-linear.
PTC thermistors are used for temperature sensing, overtemperature detection, current limiting and resettable protection. They do not interrupt a circuit in the same way as a fuse.
When current passes through the device, resistive loss produces heat. Its temperature rises when generated heat exceeds heat transferred to the surroundings.
In a suitable PTC device, heating raises resistance and reduces current to a lower steady value. The source voltage, ambient temperature, thermal mounting and trip or hold ratings determine whether this protective action is reliable.
Thermistor Characteristics
The two-point beta form of the NTC resistance-temperature approximation is:
Where:
- R1 = resistance of the thermistor at absolute temperature T1[oK]
- R2 = resistance of the thermistor at temperature T2 [oK]
- β = constant for the thermistor transducer, not for an unrelated device such as an oscillator transducer
The equation shows a non-linear relationship. An NTC thermistor may have a coefficient near –0.05/oC around room temperature, but the value changes with temperature and beta.
Thermistor Construction
Many ceramic thermistors start with semiconductor metal-oxide powders mixed with a binder. Materials and manufacturing methods vary by thermistor type.
For a bead device, material can be formed around lead wires and sintered to create the resistive ceramic.
Sintering densifies the material and establishes its electrical characteristics and connection to the leads.
A glass or polymer coating can protect the element from moisture and mechanical damage. Environmental resistance depends on the complete package and its rating.
Thermistors are available as beads, discs, chips, probes and other packages. Small beads support fast response and point sensing, but dimensions vary by product.
Disc and washer forms can be pressed and sintered into flat shapes. Their size, resistance, power capability and thermal response depend on the intended application.
Commercial thermistors span many sizes and nominal resistances. A value such as 10 kΩ is normally specified at a reference temperature, often 25oC, but the datasheet defines the reference and tolerance.
Package choices include bead, rod, disc, chip and encapsulated probe types. Common advantages are small size, high sensitivity, fast response and relatively low cost.
A small sensing element can have a short time constant, although a sheath, mounting method and surrounding medium also affect response. Limited heat dissipation increases measurement error from self-heating and can damage an overloaded device.
Use an excitation current low enough to meet the allowed self-heating error. Lower current reduces the output signal, so the readout must provide adequate resolution and noise performance.
Thermistor vs Thermocouple
The main differences between a thermistor and a thermocouple are:
Thermistors:
- A relatively narrow sensing range; many measurement thermistors operate near –50 to +100oC, but the product rating controls
- Sensing parameter: electrical resistance
- Non-linear relationship between resistance and temperature
- NTC resistance decreases approximately exponentially with increasing absolute temperature over a limited range
- High sensitivity to small temperature changes, with accuracy over a wide span limited by calibration and model quality rather than a universal 50oC boundary
- Requires electrical excitation, resistance conversion and often linearisation; amplification depends on signal level and readout
- Accuracy depends on sensor tolerance, calibration, self-heating, readout and thermal installation rather than a fixed 1oC limit
Thermocouples:
- Cover a wider temperature range; NIST calibrates common types from –196 to 2100oC, with the valid range set by thermocouple type and construction
- Accuracy depends on thermocouple type, wire condition, calibration, reference-junction compensation and readout
- Sensing parameter: thermoelectric voltage associated with the measuring and reference junction temperatures
- Output voltage is relatively low and normally needs low-noise measurement and reference-junction compensation
- Voltage-temperature response is non-linear and uses standard reference functions, tables or calibration data
Thermistor vs RTD
Resistance Temperature Detectors, also known as RTD sensors, and thermistors both infer temperature from a change in electrical resistance.
Thermistors commonly use semiconductor ceramics or polymers, while RTDs use metals such as platinum. Thermistors usually offer greater sensitivity over a narrower range. Platinum RTDs cover a wider range. Their standardised resistance-temperature relation is also more nearly linear.
Cost, uncertainty and response time depend on the sensor construction, probe, calibration, readout and installation. Neither technology is universally more accurate or faster. RTDs can measure over a wider range, while small thermistors can respond quickly and resolve small changes.
Choose between them by required range, uncertainty, sensitivity, interchangeability, stability, response time, self-heating, environment and total system cost.
