The following is a circuit diagram of [Power Type NTC Thermistor Selection Principle] Power NTC Thermistor Selection Principle
In order to avoid the inrush current generated in the electronic circuit at the instant of power-on, a power type NTC thermistor is connected in series in the power supply circuit, which can effectively suppress the inrush current during the startup, and after the surge current suppression is completed, Through the continuous action of its current, the resistance of the power type thermistor will drop to a very small extent, the power consumed is negligible, and it will not affect the normal operating current, so the power type is used in the power circuit. The NTC thermistor is the easiest and most effective way to prevent the startup surge current from protecting the electronic equipment from damage.
Selection principle of power type NTC thermistor
1. Maximum operating current of the resistor> Operating current of the actual power circuit
2. The nominal resistance of the power resistor
R≥1.414*E/Im
E is the line voltage Im for the surge current for the conversion power supply, inverter power supply, switching power supply, UPS power supply, Im=100 times working current for the filament, heater, etc. Im=30 times working current
3. The larger the B value, the smaller the residual resistance and the smaller the temperature rise during operation.
4. Generally speaking, the larger the product of the time constant and the dissipation coefficient, the greater the heat capacity of the resistor and the stronger the ability of the resistor to suppress the inrush current.
 The figure below shows the comparison of the inrush current before and after using the MF72 thermistor. The dotted line is before using the thermistor, and the solid line is after using the thermistor.
The figure below shows the RT temperature characteristic curve of MF72-3D25
NTC Negative Temperature Coefficient Thermistor Terminology Zero power resistance value RT (Ω) RT refers to the resistance value measured at a specified temperature T using a measurement power that causes a change in resistance value relative to the total measurement error. The relationship between resistance value and temperature change is: RT = RN expB(1/T – 1/TN) RT: NTC thermistor resistance at temperature T ( K ). This relationship is an empirical formula that is only accurate over a limited range of rated temperature TN or rated resistance RN because the material constant B itself is also a function of temperature T. Rated zero power resistance value R25 (Ω) According to the national standard, the rated zero-power resistance value is the resistance value R25 measured by the NTC thermistor at the reference temperature of 25 °C. This resistance value is the nominal resistance value of the NTC thermistor. The value of the NTC thermistor is usually referred to as the value. Maximum steady state current The maximum continuous current applied to the thermistor is allowed at an ambient temperature of 25 °C. Approximate resistance value (Ω) at maximum current at 25°C The approximate resistance value at the maximum current at 25 ° C is the residual resistance of the thermistor, also known as the maximum residual resistance value, when the maximum continuous current allowed for the thermistor is applied at an ambient temperature of 25 ° C. Material constant (thermal index) B value ( K ) The B value is defined as:
RT1 : Zero power resistance value at temperature T1 ( K ). For commonly used NTC thermistors, the B value range is generally between 2000K and 6000K. Zero power resistance temperature coefficient (αT) The ratio of the relative change in the NTC thermistor's zero-power resistance value to the temperature change that caused the change at the specified temperature.
αT : Temperature coefficient of zero power resistance at temperature T ( K ). Dissipation coefficient (δ) At the specified ambient temperature, the NTC thermistor dissipation factor is the ratio of the power variation dissipated in the resistor to the corresponding temperature change in the resistor.
δ: NTC thermistor dissipation factor, ( mW / K ). Thermal time constant (τ) Under zero power conditions, when the temperature is abrupt, the temperature of the thermistor changes the time required to start 63.2% of the temperature difference. The thermal time constant is proportional to the heat capacity of the NTC thermistor, and its dissipation coefficient. In inverse proportion.
Ï„: thermal time constant (S). Rated power Pn The power that the thermistor allows for long-term continuous operation under specified technical conditions. At this power, the temperature of the resistor itself does not exceed its maximum operating temperature. Maximum working temperature Tmax Under the specified technical conditions, the thermistor can work at the highest temperature allowed for long-term continuous operation. which is:
T0 - ambient temperature. Measuring power Pm When the thermistor is at a specified ambient temperature, the resistance change caused by the heating of the resistive current is negligible compared to the total measurement error.
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