How to Judge the Quality of a Thermistor? How to Choose the Right Thermistor for Your Needs?

Judging the performance of a thermistor and selecting a suitable product require comprehensive consideration of both technical parameters and application scenarios. Here is a detailed guide:

I. How to Judge the Quality of a Thermistor?

Key performance parameters are the core for evaluation:

1. Nominal Resistance Value (R25):

• Definition: The resistance value at a specific reference temperature (usually 25°C).
• Quality Judgment: The nominal value itself isn't inherently good or bad; the key is whether it meets the design requirements of the application circuit (e.g., voltage divider, current limiting). Consistency (the spread of resistance values within the same batch) is a crucial indicator of manufacturing quality – smaller dispersion is better.
• Note: NTC and PTC have vastly different resistance ranges at 25°C (NTC: ohms to megohms, PTC: typically ohms to hundreds of ohms).

2. B Value (Beta Value):

• Definition: A parameter describing the sensitivity of the thermistor's resistance change with temperature. Usually refers to the B value between two specific temperatures (e.g., B25/50, B25/85).
• Calculation Formula: B = (T1 * T2) / (T2 - T1) * ln(R1/R2)
• Quality Judgment:
  • NTC: A higher B value indicates greater temperature sensitivity and a steeper resistance change with temperature. High B values offer higher resolution in temperature measurement but worse linearity over wide temperature ranges. Consistency (B value dispersion within a batch) is critical.
  • PTC: The B value (though temperature coefficient α is more common) describes the rate of resistance increase below the Curie point. For switching applications, the steepness of the resistance jump near the Curie point (α value) is key.
  • Note: Different manufacturers may define B values using different temperature pairs (T1/T2); ensure consistency when comparing.

3. Accuracy (Tolerance):

• Definition: The allowable deviation range between the actual value and the nominal value. Usually categorized as:
  • Resistance Value Accuracy: Allowable deviation of actual resistance from nominal resistance at 25°C (e.g., ±1%, ±3%, ±5%).
  • B Value Accuracy: Allowable deviation of actual B value from nominal B value (e.g., ±0.5%, ±1%, ±2%).
  • Quality Judgment: Higher accuracy indicates better performance, usually at a higher cost. High-precision applications (e.g., precision temperature measurement, compensation circuits) require high-accuracy products (e.g., ±1% R25, ±0.5% B value). Lower accuracy products can be used in less demanding applications (e.g., overcurrent protection, rough temperature indication).

4. Temperature Coefficient (α):

• Definition: The relative rate of resistance change with temperature (usually near the reference temperature of 25°C). For NTC, α = - (B / T²) (%/°C); for PTC, there is a small positive α below the Curie point, which increases dramatically near it.
• Quality Judgment: A high |α| value (negative for NTC, positive for PTC near the switch point) is an advantage in applications requiring fast response or high sensitivity. However, this also means a narrower effective operating range and worse linearity.

5. Thermal Time Constant (τ):

• Definition: Under zero-power conditions, the time required for the thermistor's temperature to change by 63.2% of the total difference when the ambient temperature undergoes a step change.
• Quality Judgment: A smaller time constant means faster response to ambient temperature changes. This is crucial for applications requiring fast temperature measurement or reaction (e.g., overtemperature protection, airflow detection). The time constant is influenced by package size, material heat capacity, and thermal conductivity. Small, unencapsulated bead NTCs respond fastest.

6. Dissipation Constant (δ):

• Definition: The power required to raise the thermistor's temperature by 1°C above the ambient temperature due to its own power dissipation (unit: mW/°C).
• Quality Judgment: A higher dissipation constant means less self-heating effect (i.e., smaller temperature rise for the same current). This is very important for accurate temperature measurement, as low self-heating means smaller measurement errors. Thermistors with low dissipation constants (small size, thermally insulated package) are more prone to significant self-heating errors from measurement current.

7. Maximum Power Rating (Pmax):

• Definition: The maximum power at which the thermistor can operate stably long-term at a specified ambient temperature without damage or permanent parameter drift.
• Quality Judgment: Must meet the maximum power dissipation requirement of the application with sufficient margin (usually derated). Resistors with higher power handling capability are more reliable.

8. Operating Temperature Range:

• Definition: The ambient temperature interval within which the thermistor can operate normally while parameters stay within specified accuracy limits.
• Quality Judgment: A wider range means greater applicability. Ensure the highest and lowest ambient temperatures in the application fall within this range.

9. Stability and Reliability:

• Definition: The ability to maintain stable resistance and B values during long-term use or after experiencing temperature cycling and high/low-temperature storage.
• Quality Judgment: High stability is critical for precision applications. Glass-encapsulated or specially treated NTCs generally have better long-term stability than epoxy-encapsulated ones. The switching endurance (number of switch cycles it can withstand without failure) is a key reliability indicator for PTCs.

II. How to Choose the Right Thermistor for Your Needs?

The selection process involves matching performance parameters to application requirements:

1. Identify the Application Type: This is the foundation.

• Temperature Measurement: NTC is preferred. Focus on accuracy (R and B value), stability, operating temperature range, self-heating effect (dissipation constant), response speed (time constant), linearity (or whether linearization compensation is needed), and package type (probe, SMD, glass-encapsulated).
• Temperature Compensation: NTC is commonly used (compensating for drift in transistors, crystals, etc.). Ensure the NTC's temperature characteristics match the drift characteristics of the compensated component, and prioritize stability and accuracy.
• Inrush Current Limiting: NTC is preferred. Key parameters are the Nominal Resistance Value (determines initial limiting effect), Maximum Steady-State Current/Power (determines handling capacity during normal operation), Maximum Surge Current Withstand (I²t value or peak current for specific waveforms), and Recovery Time (time to cool down to low-resistance state after power-off, affecting frequent switching applications).
• Overtemperature/Overcurrent Protection: PTC (resettable fuses) are commonly used.
  • Overtemperature Protection: Choose a PTC with a Curie point slightly above the upper limit of normal operating temperature. Focus on trip temperature, trip time, reset temperature, rated voltage/current.
  • Overcurrent Protection: Choose a PTC with a hold current slightly above the circuit's normal operating current and a trip current below the level that could cause damage. Key parameters include hold current, trip current, max voltage, max current, trip time, resistance.
  • Liquid Level/Flow Detection: NTC is commonly used, utilizing its self-heating effect. Key parameters are dissipation constant, thermal time constant (response speed), power handling capability, and package (must resist media corrosion).

2. Determine Key Parameter Requirements: Quantify needs based on the application scenario.

• Measurement Range: Minimum and maximum temperatures to be measured.
• Measurement Accuracy Requirement: What temperature error range is acceptable? This determines the required resistance and B value accuracy grade.
• Response Speed Requirement: How quickly must a temperature change be detected? This determines the required time constant, influencing package choice.
• Circuit Interface: Role of the thermistor in the circuit (voltage divider? series current limiter?). This determines the required nominal resistance range and drive current/voltage, affecting self-heating error calculation.
• Environmental Conditions: Humidity, chemical corrosion, mechanical stress, need for insulation? This directly affects package choice (e.g., epoxy, glass, stainless steel sheath, silicone-coated, SMD).
• Power Consumption Limits: How much drive current can the circuit provide? How much self-heating temperature rise is allowed? This determines the acceptable dissipation constant and drive current level.
• Reliability Requirements: Need long-term high stability? Must withstand frequent switching? Need high voltage/current withstand capability?
• Size Constraints: PCB space? Mounting space?

3. Choose NTC or PTC: Based on Step 1 (application type), this is usually determined.

4. Filter Specific Models:

• Consult Manufacturer Datasheets: This is the most direct and effective way. Major manufacturers include Vishay, TDK (EPCOS), Murata, Semitec, Littelfuse,TR Ceramic, etc.
• Match Parameters: Based on the key requirements identified in Step 2, search datasheets for models meeting criteria for nominal resistance, B value, accuracy grade, operating temperature range, package size, dissipation constant, time constant, max power, etc.
• Package Type:
  • Surface Mount Device (SMD): Small size, suitable for high-density SMT, low cost. Medium response speed, medium dissipation constant, lower power handling. Common sizes: 0201, 0402, 0603, 0805, etc.
  • Glass-Encapsulated: Very fast response (small time constant), good stability, high-temperature resistant. Small but fragile. Often used as the core in precision temperature probes.
  • Epoxy-Coated: Low cost, some protection. Average response speed, stability, and temperature resistance.
  • Axial/Radial Leaded: Relatively higher power handling, easy for hand soldering or through-hole mounting.
  • Metal/Plastic Encased Probe: Easy to mount and secure, provides insulation, waterproofing, corrosion resistance, mechanical protection. Slower response speed (depends on housing/filling). Suitable for industrial, appliance applications needing reliable mounting.
  • Surface Mount Power Type: Designed for high-power inrush limiting, larger size, strong power handling.

5. Consider Cost and Availability: Select a cost-effective model with stable supply and acceptable lead times that meets performance requirements. High-accuracy, special package, fast-response models are usually more expensive.

6. Perform Test Validation if Necessary: For critical applications, especially involving accuracy, response speed, or reliability, test samples under actual or simulated operating conditions.

Summary of Selection Steps

1. Define Needs: What is the application? Measuring what? Protecting what? Compensating for what?
2. Determine Type: NTC (Measure/Compensate/Limit) or PTC (Protect)?
3. Quantify Parameters: Temperature range? Accuracy? Response speed? Power? Size? Environment?
4. Check Datasheets: Filter candidate models based on needs, compare parameter tables.
5. Review Package: Select suitable package based on environment, mounting, response.
6. Compare Cost: Choose an economical model that meets requirements.
7. Validate: Test sample performance in actual or simulated conditions for critical applications.

By systematically analyzing performance parameters and combining them with specific application requirements, you can effectively judge thermistor quality and select the most suitable one for your project. Remember, there is no "best" thermistor, only the thermistor "most suitable" for a particular application. During the selection process, detailed datasheets are your most reliable reference.