How to choose NTC thermistor.motion detector Vendor

　　NTC thermistors, namely negative temperature coefficient thermistors, are characterized by a non-linear decrease in the resistance value as the temperature increases. NTC is generally divided into temperature measurement thermistor and power type thermistor in application. The NTC thermistor used to suppress surge refers to the power type thermistor. NTC thermistor is a power type thermistor specially designed for surge suppression. NTC thermistors need to pay attention when selecting:

　　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 can 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 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 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 the electronic product 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, and then 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.

　　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 product application conditions 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 reduction curve provided by the manufacturer.

　　summary:

　　In product selection, the product series should be selected according to the maximum rated voltage and filter capacitance value, and the resistance value of the NTC thermistor should be selected according to the maximum allowable starting current value of the product and the working current loaded on the NTC thermistor for a long time. , At the same time, the temperature of the working environment should be considered, and the derating design should be appropriately carried out.

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