Inrush limiter for toroidal transformer

September 10, 2016


When a toroidal transformer is first switched it can have up to 60 times inrush to running current.

The circuit protects against inrush current by using a series resistor.

A relay controlled by a simple timer shorts the resistor when the toroid has built up its magnetic inductance.

The resistor is an NTC type. This protects the resistor from being destroyed if the relay suffers mechanical failure - since the NTC resistance will fall as it heats under load.

During normal conditions the NTC is remains shorted and stays cool. This means the circuit resists inrush even if power is rapidly cycled off and on.

The timing has been slowed for testing.