ken scharf wrote:
W3JDR wrote:
There are also some transformer-input types that produce well-regulated DC
outputs using linear regulator technology, and a growing number of units
that use switching regulator technology to make the transfoprmer so small
that it's hard to tell from the outside that there's even a transformer in
it.. The latter are easily distinguishable because they put out quite a bit
of power, yet are smaller and much lighter in weight than traditional units
with equivalent power capability.

Joe
W3JDR
....
Such units actually have a bridge rectifier across the AC mains and
supply DC to a switching regulator running at 20-50khz. There is an
isolation transformer running at this frequency (it's core is just a
small bit of lightweight ferrite). The output of the isolation
transformer feeds another bridge rectifier and filter (at ~40khz not
much of a filter is needed, small caps and chokes!). There may then
be a linear or switching regulator. If the later, it may actually
regulate by changing the switching frequency/duty cycle of the
original switcher at the AC mains input. In this case the feedback
is by means of an opto-isolator for safety.

In small ones (not a whole lot of power; things like 5V at 1A like I
have in front of me), the switcher is commonly a flyback type, and the
regulation is provided by monitoring how far the primary flies back.
The secondary side is a single rectifier diode feeding a filter cap.
Knowing the turns ratio and the expected losses in parts like the
secondary rectifier, you can achieve pretty decent regulation that way.
They commonly run at much higher than 50kHz these days. There are
some small ICs that make the job very easy. One characteristic of the
switchers is that they commonly run on 100-240V, 50-60Hz input, though
not always. It's rarely the case that a mains-frequency transformer
model will operate properly over such a wide voltage range. Wide input
range switchers do make travel to countries with different mains
voltages more convenient.

Cheers,
Tom