Just a silly question from an ME.
Why are ordinary DC power supplies so complicated? What's going on in
there that seems to take so many components and makes them so big and
expensive? It seems to me that you would just need to transform the
voltage down, rectify it, and filter it to DC. What does all the rest
of that stuff do? Thanks for any replies.
That process would be open-loop. Your voltage at the output of the
transformer would vary depending on the input source voltage (which
should be able to tolerate some +/-10% to +/-15% variation, I suspect)
and on loading. Rectifying and filtering would yield something. But
what? And how would the voltage vary versus various time-varying load
situations?
Closed-loop control helps to hold a precise voltage over a wide range
of currents, provide protection against all-too-easily applied short
circuits, guard against over-voltage to protect the attached circuit,
deal with large capacitive or especially inductive loads like a motor,
accept a relatively large range of line voltages, etc. It may also
need to start up the entire system with all those protections without
accidentally tripping them (for example, protecting against
over-current/short circuit while also properly powering up your own
internal system, when current loading is momentarily high as you
provide the initial charging of capacitors.)
And, of course, there is _efficiency_. If you plan for the worst case
(say, a -15% low input source voltage), then when you have it hooked
up to a line with +10% high source your output would nominally be too
high and you will need to burn off the excess, unless you apply a
switching system.
It does depend on what you are powering, though. If you are powering
a train set and have a human to adjust a knob to get the speed they
want, it can be a very simple system and not much different from what
you point out above.
But I'm a hobbyist and not a professional designer in electronics.
Jon