wrote:


" The resistor or NTC limits the surge current to a value that is tolerated indefinitely by fuses, diodes and switches used in the PSU - ie something like 20amps peak for a couple of milliseconds. "


I think that since the surge current occurs during the peak of the 120V
60Hz power waveform, I think the timing is more like several milliseconds
or a few 10's of milliseconds at most, but not just a couple of
milliseconds.


** The surge occurs soon as the PSU is switched on - the magnitude depends on the timing relative to the AC wave. My comments referred to the worst case surge, when the switch on is at or near an AC voltage peak.

When this happens and if the total series resistance is say 5 ohms and the main electro is 220uF, the time constant is 1mS.

Remember, the context was all about *SMALL* SMPSs.


..... Phil