"Soft-Start" Circuit For Toroidal Transformers

Toroidal transformers larger than 300VA present a large inrush current (some times in the order of 115 amps!) for a few milliseconds at power on. This can lead to nuisance blowing of fuses, or tripping of circuit breakers. A "soft-start" circuit helps limit that inrush current for a few cycles of the mains voltage, allowing enough time for the transformer/PSU to settle down, then allowing the full line voltage and current to the transformer. E-L core transformers also present an inrush current, however not as much as the toroid counter-parts (of similar VA rating). Even some high current switch-mode power supplies can present large inrush currents, as well.

This circuit comes from an article on the ESP website by Rod Elliott: https://sound-au.com/articles/soft-st...

This circuit is based on Figure 6A. However, for some reason in my circuit I used a 470uF/63V capacitor for C2 instead of the 100uF. This still works, however presents a slightly longer turn on delay and longer reset time (in order of 500ms, which is borderline too long).

The PCB for this project can be obtained here: https://www.pcbway.com/project/sharep...

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