harryagain wrote:

I noticed lots of Christmas tree lights still come with conventional
transformers. Not much else though.

Only a transformer gives proper isolation from the mains for safety.

SMPSs have a transformer. It's just smaller than is traditional as
they're running at a frequency a lot higher than 50Hz. The rest of the
circuitry in one is then the oscillator and a some feedback control
(hence being regulated by default compared to the old
12v-on-load-20v-when-not jobs.)

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.


[1] Apparently. I've not come across one in the few that I've had apart.
I found an article on converting ordinary fixed voltage wallwarts into
variable ones that showed some like this.

[2] Can't remember exactly.

--
Scott

Where are we going and why am I in this handbasket?