How to choose ntc thermistor

Where:

· R: Nominal resistance

· E: Line voltage

· Im: Inrush current

For conversion power supply, inverter power supply, switching power supply, UPS power supply, Im=100 times the operating current
For filament, heater and other circuits Im=30 times the operating current

Example Calculation:

For a power supply with a line voltage of 220V and an inrush current of 60A:

Factor 3: NTC thermistor B value and Thermal Characteristics

The B value of thermistor indicates its sensitivity to temperature changes. A higher B value means a greater change in resistance with temperature, which can be beneficial for certain applications.

Thermal Characteristics:

· Thermal Dissipation Coefficient (δ): Indicates how much power the thermistors ntc can dissipate per degree of temperature rise.

· Time Constant (τ): Represents the ntc thermistor's thermal response time.

The product of the time constant and the thermal dissipation coefficient gives an indication of the NTC thermistor's thermal capacity and its ability to suppress inrush currents.

Additional Considerations

Maximum Rated Voltage and Filter Capacitor Value

The maximum rated voltage and the value of the filter capacitor are crucial for selecting the appropriate NTC thermistor. The size of the filter capacitor determines the size of the NTC thermistor needed. In power applications, the inrush current results from the capacitor charging, and the NTC thermistor must handle this current.

Allowable Startup Current and Long-Term Load

The maximum startup current and the continuous current load on the NTC thermistor must be within the specified limits. For example, if an electronic device allows a maximum startup current of 60A and the thermistor has a minimum resistance of 4.2 ohms at startup, it must be selected to handle these conditions.

Environmental Factors

The operating environment significantly affects the performance of NTC thermistors. Factors such as ambient temperature, humidity, and mechanical stress must be considered. For instance, high humidity can accelerate the aging process of the thermistor, reducing its effectiveness over time.

Ensure the NTC thermistor operates within the specified temperature , humidity, and mechanical stress range to avoid accelerated aging or failure.

Conclusion

Selecting the right NTC thermistor involves careful consideration of the maximum working current, nominal resistance value, ntc thermistor beta value and thermal characteristics. Understanding these factors ensures optimal performance and longevity of the NTC thermistor in various applications. By considering additional factors like maximum rated voltage, filter capacitor value, allowable startup current, and environmental conditions, engineers can make informed decisions to enhance the reliability and efficiency of their electronic designs.