 ## How do I limit the inrush current with an NTC thermistor?

At the moment when the power circuit is energized, the energy of the external power source is first transferred to the input filter capacitor. This moment generates a large surge current, and if it is not limited, it is easy to damage the peripheral electronic components such as the fuse and the subsequent rectifier diode. Therefore, in the circuit design, you need to consider how to limit the inrush current.
Limiting inrush current, why use NTC?
NTC (negative temperature coefficient) A thermistor is a thermistor that has a negative temperature coefficient with a decreasing exponential relationship with increasing temperature.

Figure 1 below shows the front end of a typical AC-DC power supply circuit. In the figure, Z1 is an NTC thermistor. This resistor acts as an instantaneous current limit protection when the power supply is energized.

The moment when the power circuit is energized, it can be seen as the process of charging the filter capacitor (C1, C2 in Figure 1 below). The magnitude of the surge current can be estimated by dividing the voltage by the equivalent series resistance of the filter capacitor. The larger the current value, the greater the destructive power to the surrounding circuits.

In order to solve this problem, the easiest way is to add an NTC thermistor (Z1 in Figure 1 below) to reduce the magnitude of the surge current. The surge resistance at the moment of power-on is equivalent to the voltage divided by the NTC thermistor. The sum of the equivalent series resistances of the filter capacitors. For example, if you use a 10Ω NTC thermistor at 25°C, assuming that the equivalent series resistance of the filter capacitor is 1Ω, the inrush current will be reduced to about one-tenth. It can be seen that the greater the resistance of the NTC thermistor, the better the effect of limiting the inrush current.

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