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
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
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
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