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NTC and PTC thermistors respond differently to temperature. An NTC thermistor decreases in resistance as temperature rises, while a PTC thermistor increases in resistance over its operating range. NTC devices are commonly used for temperature sensing and inrush-current limiting, while PTC devices are used for overtemperature detection, overcurrent protection, and self-regulating heating. Choosing the right thermistor also depends on resistance-temperature data, tolerance, current and voltage ratings, thermal response, and operating conditions.
Figure 1. NTC vs. PTC Thermistor Symbols and Physical Appearance
NTC and PTC thermistors both change resistance with temperature, but they have fundamentally different resistance-temperature (R-T) characteristics.
Figure 2. NTC vs. Ceramic PTC Resistance-Temperature Characteristics
An NTC thermistor has a negative temperature coefficient, so its resistance decreases as temperature increases. This relationship is nonlinear, meaning the resistance does not change by the same amount for every 1°C increase.
Important parameters include R₂₅, Beta value, resistance tolerance, temperature coefficient, and the manufacturer’s R–T table. R₂₅ is the nominal resistance at 25°C, while the Beta value describes the thermistor’s resistance-temperature characteristic over a specified temperature range. Two 10 kΩ NTC thermistors can therefore have different resistance values at other temperatures if their Beta values or R–T characteristics differ.
For example, the TDK B57861E0103A010 has an R₂₅ of 10 kΩ ±2% and a B₂₅/₁₀₀ value of 3988 K ±1%. According to TDK’s official R–T table, its nominal resistance is 32.65 kΩ at 0°C, 10 kΩ at 25°C, 3.603 kΩ at 50°C, and 1.752 kΩ at 70°C. These values demonstrate why temperature-sensing applications should use the exact manufacturer R–T data for the selected part rather than choosing a thermistor based only on its 10 kΩ rating.
PTC devices do not share one universal R-T curve. Ceramic switching PTC thermistors, linear PTC sensors, ceramic PTC heaters, and polymer PPTC protectors have different materials, operating mechanisms, and selection requirements.
These devices have relatively low resistance below a transition region, then show a sharp resistance increase near the switching or Curie temperature. They are used for temperature switching, motor protection, and current limiting.
Linear PTC sensors increase resistance more gradually with temperature and are intended for temperature measurement or monitoring. Key specifications include reference resistance, tolerance, temperature coefficient, and R-T data.
Ceramic PTC heaters increase resistance as they heat, which reduces current and heating power. This provides self-regulating heating. Important parameters include rated voltage, resistance, heating power, and operating temperature.
PPTCs are resettable overcurrent protectors made from conductive polymer materials. Excess current heats the device and causes its resistance to rise sharply, limiting current. Key parameters include hold current, trip current, maximum voltage, resistance, and trip time.
Thermistors and related PTC devices should be selected according to their intended function. A temperature-sensing NTC, ceramic switching PTC, linear PTC sensor, ceramic heater, and polymer PPTC protector require different specifications.
| Device Type | Main Function | Important Selection Parameters |
| NTC temperature-sensing thermistor | Temperature measurement | R₂₅, resistance tolerance, Beta value or full R-T table, temperature coefficient, self-heating, thermal time constant, dissipation factor, operating temperature range |
| Ceramic switching PTC thermistor | Temperature switching, overtemperature protection, or current control | Nominal resistance, switching or sensing temperature, R-T curve, maximum voltage, allowable current, trip behavior, operating temperature range |
| Linear PTC temperature sensor | Temperature measurement and control | Reference resistance, resistance tolerance, temperature coefficient, R-T data, temperature accuracy, sensing current, self-heating, operating temperature range |
| Ceramic PTC heater | Self-regulating heating | Rated voltage, cold or minimum resistance, target or surface temperature, heating power, current, maximum voltage, thermal mounting conditions |
| Polymer PPTC protector | Resettable overcurrent protection | Hold current, trip current, maximum operating voltage, maximum fault or interrupt current, initial resistance, post-trip resistance, time-to-trip, ambient-temperature derating |
NTC thermistors are widely used for continuous temperature measurement because they provide high sensitivity over a useful temperature range. Their resistance changes nonlinearly with temperature, so accurate measurement depends on the thermistor tolerance, ADC accuracy, reference voltage, self-heating, and the resistance-to-temperature conversion method.
Linear silicon PTC thermistors can also provide continuous temperature measurement. Their resistance increases with temperature in a more nearly linear and predictable way over their specified range, which can simplify signal processing. In contrast, ceramic switching PTC thermistors show a much sharper resistance increase near their switching temperature and are better suited to temperature-limit detection and thermal protection.
Figure 3 shows a typical 10 kΩ NTC voltage-divider connected to a microcontroller ADC. The NTC resistance is calculated as:
The resistance can then be converted to temperature using the manufacturer’s R-T table, Beta equation, or Steinhart-Hart equation.
Figure 3. Typical 10 kΩ NTC Voltage-Divider Temperature Sensing Circuit
Measurement accuracy also depends on sensor placement, sensing current, thermal contact, package thermal mass, mounting method, and lead resistance. Excessive sensing current can cause self-heating and introduce temperature error.
| Sensing Requirement | NTC Thermistor | Linear Silicon PTC | Ceramic Switching PTC |
| Continuous temperature measurement | Widely used; high sensitivity | Suitable | Generally not the primary use |
| Temperature-limit detection | Possible | Possible | Well suited |
| Resistance behavior | Nonlinear decrease | More gradual increase | Sharp increase near switching temperature |
| Typical interface | Voltage divider and ADC | Voltage divider, ADC, or measurement circuit | Comparator or protection circuit |
| Typical use | Batteries, HVAC, appliances, electronics | Temperature monitoring and control | Motors, transformers, windings, thermal protection |
NTC and PTC thermistors perform almost opposite protection roles. An NTC is commonly used to limit startup inrush current, while a PTC is used to restrict current after an overload or overheating condition develops.
At power-up, the NTC is cold and relatively resistive, so it limits the initial current surge. As current heats the device, its resistance falls and normal current flows with less voltage drop. Selection should consider cold resistance, continuous-current rating, surge-energy capability, hot resistance, ambient temperature, and cool-down time.
For example, the TDK B57153S0100M051 is rated at 10 Ω at 25°C, 2 A maximum current, and −55°C to 170°C operating temperature. Its capacitive-load data also shows that inrush limiters should not be selected by resistance alone.
Ceramic PTC thermistors are commonly used for temperature-limit protection. Their resistance rises sharply near a specified switching temperature, helping reduce current or trigger a protection circuit when excessive temperature occurs. Important parameters include switching temperature, resistance at specified temperatures, minimum and maximum resistance limits, rated voltage, and operating temperature range.
A PPTC overcurrent protector increases in resistance when fault current causes sufficient self-heating. This higher-resistance state limits current until the fault is removed and the device cools.
PPTC selection should consider hold current, trip current, maximum voltage, time-to-trip curves, ambient-temperature derating, initial resistance, and reset conditions. Hold and trip current ratings depend on temperature, while reset usually requires reducing or removing current long enough for the device to cool and return toward its low-resistance state.
| Application | Usually Choose | Why | What to Verify |
| Battery temperature monitoring | NTC | Provides continuous temperature measurement over a defined range | R-T data, tolerance, accuracy, operating range |
| HVAC temperature sensing | NTC | Suitable for continuous air or surface temperature measurement | Calibration, response time, sensor placement |
| Motor winding overtemperature detection | PTC sensor | Detects when the winding reaches a defined temperature limit | Switching temperature, tolerance, insulation requirements |
| Transformer thermal protection | PTC sensor | Provides threshold-based overtemperature detection | Switching temperature, mounting, insulation class |
| Power-supply inrush limiting | NTC inrush limiter | Restricts startup current, then drops to lower resistance as it heats | Cold resistance, rated current, surge energy, recovery time |
| Resettable overcurrent protection | PTC/PPTC | Increases resistance during a fault to limit current | Hold current, trip current, maximum voltage, ambient derating |
| Self-regulating heating | Ceramic PTC heater | Increasing resistance helps limit current as temperature rises | Rated voltage, power, surface temperature, mounting |
NTC and PTC thermistors are not interchangeable. Select the device according to the required function, then verify its electrical and thermal ratings against the actual operating conditions.
Rodriguez and McCarthy (2022) discussed Analog Devices’ CN-0545 reference design using a 10 kΩ Type 44031 NTC thermistor with an AD7124-4/AD7124-8 24-bit sigma-delta ADC. The circuit used the ADC’s 2.5 V internal reference, PGA gain of 1, and a 10 kΩ, 0.01%, 10 ppm/°C sense resistor.
At 25°C, a sinc⁴ filter in full-power mode at 50 SPS produced 44.2 µV RMS noise and 14-bit noise-free resolution, equivalent to about 0.01°C measurement variation. With a post filter in low-power mode at 25 SPS, the system produced 72.9 µV RMS noise and 13.4-bit resolution, or about 0.02°C variation. These values describe resolution/noise variation, not total temperature accuracy. The Type 44031 thermistor itself is specified for ±0.1°C accuracy from 0°C to 70°C and ±1°C over its wider specified range.
Source: Rodriguez and McCarthy (2022), Thermistor-Based Temperature Sensing System—Part 2: System Optimization and Evaluation.
Liang and Tsai (2005) evaluated a ceramic BCST PTC thermistor in one telecom overcurrent-protection design based on ITU-T K.30 test circuits. The tested device had 14.3 Ω resistance at 25°C and a 357.5 V RMS maximum operating voltage.
In the rated-condition test, the PTC was required to carry 110 mA for 1 hour from a 50 V DC supply, but stable operation was not maintained above 45°C ambient temperature. In the simulated trip test, a 220 V AC power contact with 1 A RMS transition current was applied. The measured response time remained below 4 seconds from −10°C to 80°C. These results apply to the specific 2005 ceramic PTC telecom protection design and should not be treated as universal PTC specifications.
Source: Liang and Tsai (2005), Evaluation of a Novel PTC Thermistor for Telecom Overcurrent Protection, DOI 10.1016/j.sna.2005.04.005.
Too much sensing current can heat an NTC above the actual temperature being measured, creating a positive measurement error. Reduce the sensing current or use intermittent excitation where appropriate, then confirm that thermistor power dissipation remains within the manufacturer’s limits.
A thermistor mounted too far from the target, near another heat source, or in unwanted airflow may report a temperature that does not represent the intended measurement point. Move the sensor closer to the actual thermal source and ensure good thermal contact with the surface, liquid, or air being monitored.
Slow response or unexpected temperature offset may come from the thermistor package, long leads, encapsulation, or mounting method. Choose a package suited to the required response time, reduce unnecessary lead effects, and test the sensor using mounting conditions similar to the final design.
If measured temperatures are consistently incorrect or the error increases across the temperature range, the controller may be using the wrong resistance-temperature data. Confirm the exact thermistor part number and update the software with the correct manufacturer R-T table, Beta equation, or Steinhart-Hart coefficients. Verify the corrected reading against a known temperature reference.
An undersized NTC inrush limiter may run too hot, fail early, or provide little current limiting during rapid restarts because it has not cooled sufficiently. Check the continuous-current rating, surge-energy capability, cold resistance, operating temperature, and required recovery time against the actual startup cycle.
A protection PTC may trip too early during normal operation or too late during a fault if current ratings and temperature effects are not matched to the circuit. Check the rated current, trip or switching current, maximum voltage, ambient-temperature derating, and manufacturer trip curves under worst-case operating conditions.
Repeated overheating, resistance drift, or device failure can indicate that voltage, current, power, or temperature limits are being exceeded. Review the design using worst-case supply voltage, ambient temperature, startup current, fault conditions, tolerances, and power dissipation, then compare these values with the applicable datasheet limits.
Use an NTC thermistor for continuous temperature measurement or startup inrush-current limiting. Use a ceramic switching PTC for temperature-limit detection, a PPTC device for resettable overcurrent protection, and a ceramic PTC heater for self-regulating heating. These device families are not interchangeable, so verify the exact R–T curve, current, voltage, temperature, energy, and thermal ratings in the manufacturer’s datasheet.
• TDK Electronics. NTC Thermistors: General Technical Information. 2018.
• TDK Electronics. PTC Thermistors: General Technical Information. 2016.
• Texas Instruments. Monitoring NTC Thermistor Circuit with Single-Ended ADC. SBAA338A, 2022.
• TDK Electronics. NTC Inrush Current Limiters: General Technical Information. 2015.
• TDK Electronics. B57153S0100M051 NTC Inrush Current Limiter Datasheet.
• Littelfuse. PolySwitch Resettable PPTC TR Series Datasheet. 2025.
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