In article , Dave Plowman (News)
writes
In article ,
Tim Watts wrote:
There is only one caveat to this. SMPSs generally use opto-isolators to
provide the feedback from the LV side to the HV frequency generator
side. This and the transformer means they are totally isolated from the
mains. However, some of the very cheap ones skip the opto-isolators[1]
and use FETs or TRIACs or something[2] instead which ruins the mains
isolation, especially should they break down.

That's the bit that worries me - I would not like to have any PSU with
any sort of electrical coupling between the ELV and LV(mains) side, bar
perhaps an earth connection (that's rare now, but used to be not
unheard of to bond to earth to the -ve rail in ye olden days).

If a 'normal' main transformer fails, it is possible to get a short
between the mains and LV windings.

Never seen it happen, they are built to pretty stringent standards,
either split bobbin (LF only) or reinforced insulation which, in the
case of thin layers, is 3 layers of heat resistant insulation, each of
which is proof to 1500Vac. Overall proof test is 3750Vac.

Thermal fuse required too (on LF).

Of course there are some designs which
pretty well prevent this happening - but not likely to be used in a cheap
wall wart.

They wont get approval if they don't.

You might be saying that cheap Chinese stuff wont meet approvals but
that's a fake goods issue for Trading Standards.
--
fred
it's a ba-na-na . . . .