[Amps] soft start circuits

[David Lisney]

Hi, I have used soft start in a big 13.8v power supply, it uses much the same VA rating transformer that a medium sized linear would use (about 2500-3000VA).  In my example mains (240v) is initially applied via a large 50ohm metal clad resistor, when the rails are about half way up a solid state relay conducts operating a large contactor, thereby applying full mains directly to the transformer.

The whole control is by just two push buttons, one to "start" and one to "stop"... one minor bug is that the lightly loaded solid state relay has once operated by itself, probably due to a mains transient, and the power supply has powered up by itself, hardly a major concern in a fully enclosed low voltage power supply but not something you'd want to trust entirely. This glitch has only occurred twice in maybe 10 years but still not acceptable for a high voltage supply.

Maybe I will revise the design to "ramp up" using sequentially operating solid state relays, the contactor does make an irritating hum!

73 de Dave G0FVT and Happy New Year