Have you ever turned on a transformer and heard a loud humming sound? What about having a blown fuse or tripped circuit breaker? These signs can all indicate transformer inrush current.

Transformer inrush current describes a spike in current that occurs when you initially turn on your transformer. This spike can be up to 10 times higher than normal current. Why does inrush current occur? It can happen because large transformers demand a huge amount of current when energized. Until the inductive resistance and magnetic field builds, they essentially act as short circuits. //

What is a practical solution to this problem? One convenient way to limit inrush current in a transformer is by using an NTC thermistor. The photo below shows an NTC thermistor placed in the circuit board to provide optimal inrush protection (Figure 2).

Transformer inrush current limiter in circuit

Figure 2: The NTC thermistor is placed in series with the input line to limit inrush current in a transformer.
Transformer Inrush Current Calculation

Here at Ametherm, we have a calculation that we use to help our customers select the right NTC thermistor part number. We thought we would share this with you so you can do the math for yourself, if you feel so inclined.

The 7 step process below will walk you through the calculation we performed for a 40VA transformer. You can apply these calculations to your own transformer as well.