[sdiy] blowing fuses

Richie Burnett rburnett at richieburnett.co.uk
Sun Feb 16 21:44:27 CET 2020


> How does this magnetized transformer problem work? Doesn't that magnetism dissipate after a short time?

If the AC mains voltage happens to be crossing through zero at the instant when the transformer is connected to the mains line, then the transformer core sees asymmetric magnetisation and soon saturates.  This means it loses most of its inductance and the primary current is limited only by the winding resistance.  The larger the transformer the bigger the current surge due to this effect because the windings are thicker and have lower resistance. Low resistance makes the current surge larger and last longer before the transformer core moves out of saturation and returns to normal operation.

Some additional resistance in series with the primary of the transformer reduces the magnitude of the current surge and also makes it go away quicker. This resistance can be bypassed to minimise losses once the Inrush event has passed. An NTC thermistor in series with the transformer primary is another common method to limit Inrush. 

-Richie,




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