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soft starters:




"Soft Starters" aka "Inrush Limiters" are used to prevent accessories with high power-on current surges from
causing problems like blowing fuses and tripping off other overcurrent safeties (or UPS's) when turned on.

These can include motors with highly inductive startup loads, as well as electronic units such as power
supplies and inverters, that have large filter capacitors thar get rapidly charged on power-up.

I recently installed one in an Astron 50A linear supply because it was tripping in my UPS momentarily every
time I turned it on. Flip the switch, and hear THUMP as it charged its caps, causing a brief magnetic shake
of the case, and the UPS goes beep beep beep! After I installed the soft start, it just makes a quieter
and click, then it's ready to go.

They work by starting the device connected to line power through a small resistance (usually a resistor or
thermistor) for the first second or so it's on. This limits the current to a reasonable amount, but gives it
enough energy to charge up the magnetic or electric field that's presenting a near-short to the line power.
After the field has mostly charged (usually less than a second later), a relay toggles and shorts across the
resistance, which allows the device to operate at full power.


soft starters for $10-15 (on eBay)


inrush limiter:


soft starter schematic:



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last updated 08/06/2023 at 21:29:25