Re: POWER SUPPLIES

Okay, here is the deal with switching power supplies. The incoming
AC is rectified, usually doubled and changed to DC (300 VDC + )
then "chopped" at a much higher frequency that 60HZ, typically from
20,000 to 100,000. This higher frequency does a bunch for getting
the size of all the components down, like the magnetics, the output
filters, etc. The result is a supply that weighs 3 lbs instead of
40. Just like servos, regulated power supplies have a control loop
that must react to a change in load. The output filter caps have to
supply instantaneous current (like what motors want) until the
control loop of the supply catches up. If you look at the output
filters on a switcher they are tiny (because of the high frequency)so
they store very little energy. By putting a large cap across the
output you a storing energy (like a flywheel) so your motors can suck
what they want in big gulps. Some switchers are not designed to
operate without a load and can do some funky things if you operate
them in that mode; HOWEVER if you can find a switcher that has
several outputs, normally only the critical (+5 usually) voltage is
regulated so you can just lightly load that one output and the higher
voltage outputs are essentially unregulated (sorta).


A linear regulated supply might have some issues with reverse current
(where the back emf from the motors causes the voltage to be greater
than the supply output) causing the linear pass elements (power
transistors) to conduct backwards...not a good thing. I always use
an isolation diode on the output of linear regulated supplies when
driving highly inductive loads or charging batteries. The diode
needs to be able to handle the peak current of the combined load.

The nice thing about regulated supplies is that they normally have
current limiting so if you overload the output it reduces the current
and keeps the blue smoke from leaking out.

Motors really don't care where the voltage comes from. They are
current devices and they want lot's of it all at once. Caps store
energy and will provide huge amounts of current the larger they are.

And the beat goes on....