Selection of NTC thermistors.Infrared IC wholesale

　　Maximum rated voltage and filter capacitor value

　　The size of the filter capacitor determines what size NTC should be used. For a certain size of NTC thermistor, the size of the filter capacitor that is allowed to be connected is strictly required, and this value is also related to the maximum rated voltage. In power supply applications, the power-on surge is caused by the charging of the capacitor, so the capacity of the NTC thermistor to withstand surge current is usually evaluated by the allowable access capacitance under a given voltage value. For a specific NTC thermistor, the maximum energy it can withstand has been determined. According to the energy consumption formula E=1/2×CV2 of the resistance in the first-order circuit, it can be seen that the allowable access capacitance The value is inversely proportional to the square of the rated voltage. Simply put, the larger the input voltage, the smaller the maximum capacitance allowed to be connected, and vice versa.

　　The specifications of NTC thermistor products generally define the maximum capacitance value that is allowed to be connected under 220Vac. Assuming that the maximum rated voltage of an application condition is 420Vac, and the filter capacitor value is 200μF, according to the above energy formula, the equivalent capacitance value under 220Vac can be converted to 200×4202/2202=729μF, so 220Vac must be selected when selecting the model Models with a capacitance value greater than 729μF are allowed to be connected.

　　The maximum starting current value allowed by the product and the working current loaded on the NTC thermistor for a long time

　　The maximum starting current value allowed by electronic products determines the resistance value of the NTC thermistor. Assuming that the rated input of the power supply is 220Vac, the internal resistance is 1Ω, and the maximum allowable starting current is 60A, then the minimum resistance value of the selected NTC in the initial state is Rmin=(220×1.414/60)-1=4.2(Ω). So far, there are generally one or more NTC thermistors that meet the conditions. At this time, choose the following methods.

　　When the product is working normally, the current loaded on the NTC thermistor for a long time should not be greater than the current specified in the specification. According to this principle, a suitable resistance value can be selected from multiple resistors whose resistance value is greater than 4.2Ω. Of course this refers to normal temperature. If the working ambient temperature is not normal temperature, the derating design of the NTC thermistor needs to be carried out according to the principles mentioned below.

　　The working environment of NTC thermistor

　　Since NTC thermistors are greatly affected by the ambient temperature, generally only the resistance value at room temperature (25°C) is given in the product specification. If the application conditions of the product are not at room temperature, or due to the design or structure of the product itself, When the ambient temperature of the NTC thermistor is not normal temperature, the resistance value of the NTC in the initial state must be calculated before the selection of the above steps.

　　When the ambient temperature is too high or too low, the derating design must be carried out according to the power consumption reduction curve provided by the manufacturer. The power consumption curve generally has two forms.

　　In fact, many manufacturers have defined the ambient temperature category for their products. In practical applications, the ambient temperature of the NTC thermistor should not exceed the upper/lower limit temperature specified by the manufacturer. At the same time, care should be taken not to make it work in a humid environment, because a too humid environment will accelerate the aging of the NTC thermistor.

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