A thermistor is a semiconductor device that is highly sensitive to temperature changes in resistance, mainly divided into two categories: negative temperature coefficient (NTC) and positive temperature coefficient (PTC). The core parameters are the key basis for selection and application, and the following is a logically clear original summary:
Core parameters (sorted by importance and generality):
Resistance Value - R:
Definition: The resistance value measured at a specific reference temperature (usually 25 ° C).
Importance: This is the most fundamental and core parameter that determines the benchmark point of circuit design (such as the value in a voltage divider circuit).
Unit: Ohm (Ω), with a wide range of commonly used values (from a few Ω to several M Ω).
Note: The resistance values of NTC and PTC at this temperature point have the same meaning, but their trends with temperature change are opposite.
B value (Beta value/B constant/material constant):
Definition: A key parameter that describes the sensitivity of a thermistor material's resistance value to temperature changes. It represents a constant of the resistance temperature relationship between two specific temperature points (T1 and T2, typically 25 ° C/85 ° C or 25 ° C/50 ° C).
Importance: NTC thermistors are particularly critical as they determine the rate at which the resistance decreases with temperature (the larger the B value, the more sensitive it is to temperature changes). It is a key parameter for calculating the resistance value at a specific temperature.
Calculation formula (NTC): B=[ln (R1/R2)]/[(1/T1) - (1/T2)] (R1 and R2 are the resistance values at temperatures T1 and T2, respectively, in Kelvin K).
Unit: Kelvin (K), with a typical value range of approximately 2000K-5000K.
Note: The B value is not an absolute constant and may vary slightly with temperature range. PTC also has the concept of B-value, but its application is not as widespread as NTC.
Temperature Coefficient - α:
Definition: Refers to the relative rate of change in resistance value with temperature at a specific temperature.
Importance: It intuitively reflects the sensitivity of thermistor to temperature.
Calculation formula (NTC): α=- (B/T ²) 100% (T is the specified temperature, unit K). The result is negative, indicating that the NTC resistance decreases with increasing temperature.
Calculation formula (PTC): α=+(B/T ²) 100% (in the region below Curie temperature). The result is positive, indicating that the PTC resistance increases with temperature (rapidly increasing in the switching region).
Unit:%/° C or%/K.
Note: There is a strong correlation between alpha and B values, which vary with temperature and are typically given at specific temperatures (such as 25 ° C).
Dissipation constant - δ:
Definition: The temperature rise (° C) caused by the dissipation of 1 milliwatt of power (mW) by a thermistor itself.
Importance: A key parameter for measuring the strength of the self heating effect of a thermistor. The smaller the δ, the more significant the self heating effect (i.e. even small measured currents can cause significant temperature rise). This is crucial for precision temperature measurement applications, requiring the selection of devices with sufficiently large delta or the use of extremely small measurement currents (constant current sources) to reduce self heating errors.
Unit: mW/° C. Typical value range is from a few tenths to tens of mW/° C.
Test conditions: Usually measured in still air at a specific temperature (such as 25 ° C). The medium (air, oil), packaging, and installation method all affect the actual value.
Thermal Time Constant - τ:
Definition: Under zero power conditions, the time required for the temperature change of a thermistor to reach 63.2% of the total change when there is a step change in ambient temperature.
Importance: The core parameter that characterizes the response speed of a thermistor. The smaller the τ, the faster the response to changes in environmental temperature.
Unit: seconds (s). The range ranges from millisecond level (bead shaped) to minute level (large package).
Influencing factors: heat capacity, dissipation coefficient, packaging size, thermal contact medium.
Maximum Power Rating - Pmax:
Definition: The maximum power that a thermistor can continuously withstand without permanent damage or parameter drift at a specified ambient temperature.
Importance: It concerns the reliability and lifespan of the device. The steady-state or pulse power in applications (especially when used as surge suppression or heating elements) must be lower than this value, and consideration should be given to derating usage (the higher the ambient temperature, the lower the power that can be sustained).
Unit: Watt (W) or Milliwatt (mW).
Note: Pulse power tolerance is usually higher than continuous power.
Operating Temperature Range:
Definition: The ambient temperature range within which a thermistor can operate normally (maintaining its specified characteristics without permanent damage).
Importance: Defined the applicable environmental limits of the device.
Unit: ° C.
Note: For NTC, the high temperature limit is usually related to material stability and low resistance values; The low temperature end is related to high resistance values and material phase transitions. For PTC, the high-temperature limit is related to the Curie point and stability of the material.
For additional core parameters of NTC thermistor:
Resistance Temperature Curve/Table:
Although the B value can be approximately calculated, the most accurate way is to directly use the detailed resistance temperature correspondence table or curve provided by the manufacturer (usually based on the Steinhart Hart equation). This is essential for high-precision temperature measurement applications.
For additional core parameters of PTC thermistor:
Curie Temperature/Switching Temperature - Tc:
Definition: The critical temperature point at which a PTC material undergoes a ferroelectric phase transition (with a sharp increase in resistance). It is the core characteristic parameter of PTC as a temperature switch or overcurrent protection.
Importance: It determines the "action" temperature point of PTC.
Unit: ° C.
Minimum resistance (Rmin):
Definition: The lowest resistance value that PTC can achieve below the Curie temperature (usually at room temperature or slightly above room temperature).
Importance: Affects power consumption and voltage drop under normal working conditions.
Trip Current/Switching Current:
Definition: The minimum steady-state current that can cause a step increase in PTC resistance value (entering a high resistance state) within a specific time period (such as a few seconds to a few minutes) at a specified ambient temperature and voltage.
Importance: Key selection parameters for overcurrent protection applications.
Maximum current/Hold current:
Definition: The maximum steady-state current at which a PTC can maintain a low resistance state without any action (jump) at a specified ambient temperature.
Importance: The maximum allowable current for the normal operation of the circuit.
Maximum Voltage - Vmax:
Definition: The maximum voltage that PTC can withstand without breakdown or flashover at a specified ambient temperature (usually referring to the high resistance state after operation).
Importance: Overvoltage protection capability and upper limit of applied voltage.
General structural parameters:
Package form: such as SMD, Dipped, Axial, Glass Package Bead, Probe, etc., which affects size, thermal response, power tolerance, and installation method.
Tolerance: usually refers to the allowable deviation of the resistance value at 25 ° C (such as ± 1%, ± 3%, ± 5%, ± 10%). For precision temperature measurement NTC, it is also possible to specify the tolerance of B value or the accuracy level throughout the entire temperature range.
Summarize key points:
NTC core: nominal resistance value (R25), B value, dissipation coefficient (δ), thermal time constant (τ), accuracy (for temperature measurement). `The resistance temperature characteristic (curve or table) is the basis for application.
PTC core: Curie temperature (Tc), nominal resistance value (R25 or Rmin), jump current, holding current, maximum voltage, maximum power. `The resistance temperature characteristic (especially the switching characteristic) is the core.
General important parameters: maximum power, operating temperature range (related to reliability), dissipation coefficient (related to temperature measurement accuracy), thermal time constant (related to response speed).
When choosing a thermistor, it is necessary to consider the specific application requirements (temperature measurement accuracy, response speed, power control, overcurrent protection threshold, environmental conditions, etc.), and comprehensively consider the above core parameters and their interrelationships in order to select the most suitable device.