[Paul Dorman]

On 12/20/2020 8:01 PM, wrote:
Paul wrote:
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But I would assume the bulb lighting up initially, is due
to the initial in-rush current, that charges up the electrolytic
filtering caps, on the outputs of the rectifiers?


** The largest part of the " inrush surge " is down to the iron transformer core *magnetising* and losing nearly all of its inductance at switch own.
Can take 20 cycles of AC power for the core to unmagnetise too.

SMPSs usually surge hard at a switch on too, yours must be an exception.


The iron transformer is magnetizing every cycle, which
would be 60 times a second, for 60Hz AC.

If the secondary was open, and not connected to anything,
then the back EMF would be high enough, to prevent the light
bulb from turning on, due to the low current draw.

When the secondary is wired to the rectifiers, there is
now a load, and the large filtering caps need to be charged up,
which I believe is the reason the light bulb turns on briefly,
for about one second.