Select NTC thermistor requirements

The size of the filter capacitor determines how large the NTC should be. For a certain size of NTC thermistor, the size of the filter capacitor that is allowed to access is strictly required, and this value is also related to the rated voltage. In power applications, the power-on surge is due to capacitive charging, so the ability of the NTC thermistor to withstand inrush current is typically evaluated with the allowable capacitance at a given voltage value.

For a specific NTC thermistor, the amount that can be withstood has been determined. According to the amount of resistance in the first-order circuit, the formula E=1/2×CV2 can be seen as the allowable capacitance value of the access. It is inversely proportional to the square of the rated voltage. Simply put, the larger the input voltage, the smaller the value of the large capacitor that is allowed to be accessed, and vice versa.

Secondly, the allowable starting current value of the product and the operating current applied to the NTC thermistor for a long time. The allowable starting current value of the electronic product determines the resistance of the NTC thermistor. Assuming that the power supply rated input is 220Vac, the internal resistance is 1Ω, and the allowable large starting current is 60A, then the small resistance of the selected NTC in the initial state is Rmin=(220×1.414/60)-1=4.2(Ω). At this point, there are generally one or more NTC thermistors that meet the conditions, and then select them as follows.

When the product is in normal operation, the current applied to the NTC thermistor for a long time should not exceed the current specified in the specification. According to this principle, a suitable resistance can be selected from a plurality of resistors having a resistance greater than 4.2 Ω. Of course this refers to the case of normal temperature.

Then the working environment of NTC thermistor is very affected by the ambient temperature. Generally, only the resistance value at room temperature (25 °C) is given in the product specification. If the application condition is not at normal temperature, or Due to the design or structure of the product itself, when the ambient temperature around the NTC thermistor is not normal temperature, the resistance of the NTC in the initial state must be calculated before the above steps can be selected.