Analysis on Suppression Scheme of NTC Thermistor Inrush Current.Smart PIR Sensor price

　　The power supply part of electronic products simplifies the circuit, and C1 is a filter capacitor connected in parallel with the load. At the moment of power-on, the capacitor voltage cannot change abruptly, so a large charging current will be generated. According to the first-order linear inhomogeneous equation established by the zero-state response model of the first-order circuit, the initial value of the current can be obtained, which is equivalent to the current value obtained by short-circuiting the filter capacitor. This current is what we often call the input surge current, which is generated when the filter capacitor is initially charged, and its magnitude depends on the amplitude of the input voltage at the time of power-on and the voltage formed by the bridge rectifier and the electrolytic capacitor. The total resistance of the loop.

　　Assuming that the input voltage V1 is 220Vac, the internal resistance of the entire power grid (including the rectifier bridge and filter capacitor) Rs=1Ω, if the power is turned on when the input waveform of the power supply reaches 90 degrees of phase, the peak value of the inrush current at the moment of power-on will reach I=220 ×1.414/1=311(A). Although this inrush current is very short, if it is not suppressed, it will shorten the life of the input capacitor and the rectifier bridge, and may also cause the voltage of the input power supply to drop, causing other power equipment using the same input power supply to lose power instantaneously. interfere with the normal operation of the equipment.

　　Suppression of surge current

　　There are many ways to suppress inrush current. Generally, resistance current limiting is used in small and medium power power supplies to suppress power-on inrush current. Taking this as an example, let's analyze how to use an NTC thermistor to suppress inrush current.

　　NTC thermistor, that is, a negative temperature coefficient thermistor, is 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.

　　R1~R4 are the positions where the thermistor surge suppressor is usually placed. For dual-voltage input products that are compatible with 110Vac and 220Vac input at the same time, two NTC thermistors should be placed at the R1 and R2 positions at the same time, so that the inrush current when the 110Vac input cable is connected and the 220Vac input cable is disconnected. Consistent, an NTC thermistor can also be placed at R3 or R4 alone. For single voltage products with only 220Vac input, just put 1 NTC thermistor at R3 or R1 position.

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