In article ,
Tim Watts writes:
On Monday 06 January 2014 09:24 Scott M wrote in uk.d-i-y:

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).

I haven't seen any regulated ones which use anything other than opto-
couplers for last ~20 years (and I have taken apart and modified quite
a lot to convert them from constant voltage to constant current).

The 12V lighting transformers don't have any active regulation, and
only check that the input power is within an acceptable range (which
is why they have a minimum load), and rely of the much better regulation
of the HF transformer to maintain output voltage over their design range.
Philosophically, these are a different design, being plain HF transformer,
rather than being the variable energy pump design of a regulated SMPSU.

--
Andrew Gabriel
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