1. Maximum rated voltage and filter capacitance
The size of the filter capacitor determines how large the NTC should be. For a certain size of NTC thermistor, there is a strict requirement on the size of the filter capacitor allowed to access, and this value is also related to the maximum voltage rating. In power supply applications, startup surges are caused by capacitor charging, so the ability of an NTC thermistor to withstand surges is usually evaluated by the capacity of the capacitor to be plugged at a given voltage value.
For a specific NTC thermite, the maximum energy it can withstand has been determined. According to the energy consumption formula E=1/2¡ÁCV2 of the resistance in the first-order circuit, it can be seen that the capacitor value allowed to access is inversely proportional to the square of the rated voltage. In simple terms, the larger the input voltage, the smaller the maximum capacitor value allowed to access, and vice versa.
2. The maximum starting current allowed by the product and the working current loaded on the NTC thermistor for a long time
The resistance of an NTC thermistor is determined by the maximum allowable starting current of an electronic product. Assuming the rated power input is 220Vac, the internal resistance is 1 ¦Ø, and the maximum allowable starting current is 60A, the minimum resistance of the selected NTC in the initial state is Rmin=(220¡Á1.414/60)-1=4.2(¦Ø). At this point, there will be one or more NTC thermistors that meet the conditions, and then choose according to the following method.
The current applied to the NTC thermistor shall not exceed the current specified in the specification when the product is in normal operation. According to this principle, a suitable resistance value can be selected from multiple resistors with a resistance value greater than 4.2 ¦Ø.
3, NTC thermistor working environment
Because the NTC thermistor is greatly affected by the environmental temperature, the general product specifications only give the resistance value at room temperature (25¡æ), if the product application conditions are not at room temperature, or because of the design or structure of the product itself, resulting in the ambient temperature of the NTC thermistor is not at room temperature, You must calculate the resistance value of the NTC in the initial state before you can choose the above steps. When the ambient temperature is too high or too low, derate the power consumption curve provided by the manufacturer.
Matters needing attention:
In product selection, the product series should be selected according to the maximum rated voltage and filter capacitor value, and the resistance value of the NTC thermistor should be selected according to the maximum allowable starting current value and the working current