Thermistor (Thermistor) is a semiconductor device that is highly sensitive to temperature in terms of resistance value. Its resistance value shows significant nonlinear changes with temperature, and is mainly used in fields such as temperature detection, temperature compensation, overcurrent protection, and temperature control. Thermistors are divided into two categories based on their temperature coefficients: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). The following is a detailed analysis of its working principle, core characteristics, and applications:
1、 Basic Definition and Classification
Positive Temperature Coefficient Thermistor (PTC)
Characteristic: When the temperature rises, the resistance value increases sharply (such as after exceeding the Curie point).
Material: BaTiO3 based ceramics doped with rare earth elements.
Typical applications: self recovering fuses, motor starters, constant temperature heating.
Negative Temperature Coefficient Thermistor (NTC)
Characteristic: As the temperature increases, the resistance gradually decreases, and the change is continuous and reversible.
Material: Sintered transition metal oxides (such as manganese, cobalt, nickel oxides).
Typical applications: temperature sensors, surge current suppression, temperature compensation circuits.
2、 Core Features and Technical Parameters
Temperature resistance relationship
NTC: Approximate exponential relationship, formula is \ (R_T=R0 \ cdot e ^ {B (1/T 1/T0)} \), where:
\(R_T \): Resistance value at temperature T (unit: Ω)
\(R_0 \): nominal resistance value at reference temperature T ₀ (usually 25 ℃)
\(B): Material constant (unit: K), reflecting sensitivity (typical value 2000~5000K).
PTC: The resistance value in the low-temperature region is flat, and after exceeding the Curie point, the resistance value increases sharply (steep change).
Key parameters
Nominal resistance value (R25): The resistance value at 25 ℃, such as NTC 10k Ω, PTC 100 Ω
B value: Sensitivity index of NTC thermistor (such as B25/85=3950K).
Rated power: The maximum power consumption for continuous operation (such as NTC 100mW, PTC 1W).
Response time: Thermal time constant (τ), representing the time required for a temperature change of 63.2% (e.g. NTC τ=3 seconds).
Working temperature range: NTC typically ranges from 50 ℃ to+150 ℃, while PTC can reach up to 40 ℃ to+200 ℃.
3、 Typical application scenarios
Temperature detection and monitoring
Home appliances: Temperature feedback control for air conditioners and refrigerators (NTC 10k Ω).
Automotive electronics: coolant temperature sensor, battery pack temperature monitoring (automotive grade NTC, such as AECQ200 certification).
Surge current suppression
Switching power supply: NTC is connected in series at the AC input terminal, and the cold state high resistance limits the startup surge current. After heating up, the resistance decreases to reduce losses.
Overcurrent protection
PTC self recovery fuse: When the current is overloaded, the PTC resistance suddenly increases and cuts off the circuit; Automatically recover after the fault is resolved (such as USB port protection).
Temperature compensation
Precision circuit: Compensate for temperature drift of other components (such as crystal oscillators) (NTC series/parallel adjustment).
Constant temperature heating
PTC heater: Used for warm air heaters and humidifiers, it automatically limits current and maintains a constant temperature when the temperature reaches the Curie point.
4、 Selection and Design Points
Type selection
NTC: Suitable for continuous temperature measurement and surge suppression.
PTC: Suitable for overcurrent protection and self recovery switches.
Parameter matching
Temperature range: Select according to the application environment (e.g. automotive grade NTC needs to support 40 ℃~+125 ℃).
B value and accuracy: High precision temperature measurement requires NTC with stable B value (such as ± 1%), and medical equipment should choose ± 0.5%.
Packaging form:
Surface mount type (such as 0402, 0603): Compact design, used for consumer electronics.
Epoxy encapsulation type: moisture resistant, suitable for industrial environments.
Metal shell type: resistant to mechanical impact, used in automobiles and aerospace.
Circuit Design
Voltage divider circuit: NTC is connected in series with a fixed resistor, and the temperature is inferred by reading the voltage through ADC.
Formula: \ (V_ {out}=V_ {cc} \ cdot \ frac {R_ {NTC}} {R_ {Fixed}+R_ {NTC}}} \)
Linearization processing: NTC output nonlinearity can be corrected through table lookup, Steinhart Hart equation, or hardware compensation circuit (such as operational amplifier feedback network).
Self recovery fuse design: The holding current (I2 hold) of PTC should be greater than the normal operating current of the circuit, and the triggering current (I2 trip) should be less than the maximum withstand current of the circuit.
5、 Thermistor vs. Other Temperature Sensors
|Features | NTC Thermistor | RTD (Platinum Resistance) | Thermocouple | Integrated Temperature Sensor (IC)|
|Temperature measurement range | 50 ℃~+150 ℃ | 200 ℃~+850 ℃ | 200 ℃~+1800 ℃ | 55 ℃~+150 ℃|
|Accuracy | ± 0.1 ℃~± 1 ℃ | ± 0.1 ℃~± 0.5 ℃ | ± 1 ℃~± 5 ℃ | ± 0.5 ℃~± 2 ℃|
|Sensitivity | High (Nonlinear) | Medium (Linear) | Low (Cold End Compensation Required) | Medium (Linear)|
|Response speed | Fast (millisecond level) | Slow (second level) | Fast (millisecond level) | Slow (second level)|
|Cost | Low | High | Medium | Medium|
|Typical applications | Home appliances, automotive temperature detection | Industrial precision temperature measurement | High temperature furnace, engine monitoring | Digital temperature display, system monitoring|
6、 Representative model and brand
NTC thermistor
Murata NXFT series: automotive grade, accuracy ± 1%, B-value 3435K, supports 40 ℃~+150 ℃.
Vishay NTCLE series: epoxy encapsulation, accuracy ± 0.5%, suitable for medical devices.
PTC thermistor
TDK B5900 series: self recovering fuse, holding current of 0.1A~5A, withstand voltage of 60V
Littelfuse 1812L series: Surface mount PTC, used for USB overcurrent protection.
7、 Common Problems and Solutions
Large nonlinear error
Solution: Use Steinhart Hart equation for calibration, or choose a wide temperature range linearized NTC (such as EPCOS B57891 series).
Poor long-term stability
Solution: Choose glass packaging or high stability materials (such as manganese oxide based NTC) to avoid high-temperature aging.
Insufficient response speed
Solution: Choose small patch packaging (such as 0402) to reduce thermal mass, or force airflow to accelerate heat exchange.
Thermistors have become the core components of temperature sensing and protection circuits due to their high sensitivity, fast response, and low cost advantages. NTC is suitable for precise temperature measurement and surge suppression, while PTC is irreplaceable in overcurrent protection and constant temperature control. When selecting, it is necessary to comprehensively consider temperature range, accuracy, packaging form, and environmental adaptability, and overcome nonlinear limitations through reasonable circuit design to achieve optimal system performance.