If the AC wave is going through its zero value, the current drawn will be very high and exceed the saturation current (Figure 1). In this situation, transformer inrush current protection becomes necessary to keep the transformer functioning properly.

Transformer inrush current wave

Figure 1: A transformer draws inrush current that can exceed saturation current affecting the magnetic property of the core.

Solution to Transformer Inrush Current

What is a practical solution to this problem? One convenient way to limit inrush current in a transformer is by using an NTC thermistor.