How do I Select the Right Inrush Current Limiter for My Application?

Inrush current limiters are designed with different characteristics like resistance versus temperature curve to accommodate numerous applications. Because of this, it is necessary to make some calculations based on your system requirements to select the best inrush current limiter for your needs.

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.

A time delay circuit, consisting of a relay and timer, can be employed to take the inrush current limiter out of the circuit as soon as the inrush current has passed through, allowing the inrush current limiter to begin to cool down much sooner. A great example of how this is done can be found at instructables.com, showing how to create this time delay when the inrush current limiter needs a little rest.

Large transformers have a huge current demand when they are initially turned on. This is because, until the magnetic field and inductive resistance builds, they are essentially short circuits. For example, you may have turned on some large tool or appliance and heard an initial large "HUMMMMMMM". That is the transformer say "Ow". The circuit breaker for that outlet might also go "Whoa, what are you doing!"

The transformer above (Avel Y236907 800VA 45V+45V Toroidal Transformer), for example, will try to draw over 100 Amps on the first cycle of 60 Hz Power.

To keep a large transformer from being damaged at turn-on (and to keep it from saying "ow"), or to keep a breaker from popping, you put in an inrush current limiter circuit. This Instructable will detail how to do that.

Calculates the short circuit fault current level of a 3-phase, core type transformer with a Dyn winding connection.