Re: power supplies

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "loren_allison"
<loren_allison@y...> wrote:

> Can anyone tell me the difference between a switching power supply
> and a linear power supply. I'm looking at a driver that states nOT

to

> use a switching power supply. Thanks,

Loren: One of the laws of magnetics is that the higher the
frequency, the smaller the magnetics can be to act as transformers
and filters. A frequency of 60 hz from the AC mains requires large
bulky transfomers and big filter caps to get smooth DC. You also
need lots of turns of wire in the primary and secondary.

In a switching PS the AC is first rectified, sometimes doubled to
about 360VDC, then "chopped" by high voltage semiconductors so that
now the "AC" (actually modified square waves) are much higher in
frequency. A frequency of 50KZ will allow us to use magnetic
components that are a fraction of the size of 60HZ componets. In
electronics, reduction in size and weight means a reduction in cost.
As mentioned by another poster the higher frequency allows us to use
smaller output filters to get DC into a resistive load. That issue
can be a problem with driving motors but can be resolved by adding a
larger filter cap across the power supply output. If you want to
have added isolation you can put a large silicon diode in series with
the + output and before the large filter cap. That would prevent any
excessive back emf from getting to the power supply. The only other
issue could be a noise issue from the switching frequency being in
the same enclosure as your control electronics. If the switcher is
not totally enclosed, than it does not belong inside the same cabinet
as your logic level components (and that can include some motor drive
circuits)

Most of the modern motor control drives are of the PWM (pulse width
modulated) type which are in essence constant current switching
supplies rather than constant voltage.

A lot of commercial packaged stepper and servo motor drives have an
integral switching power supply so the concept is not without merit.
The main problem lies in the fact that normally regulated power
supplies (switching or standard linear) are more expensive to build
than simple transformer and bridge/filter setups. That coupled with
the fact that there are seldom switchers rated over 24VDC output
available makes the decision to use the 60HZ stuff obvious. If you
can find a surplus switcher that meets the voltage and current
requirments of you design and it's cheaper than building a
conventional 50/60HZ hunk-o-iron design, then using the added diode
and filter will make it an option.

I'll bet this is more than you wanted to know about the difference
between linear and switching power supplies but sometimes too much
detail is better than not enough.

If this is your first CNC project with this motor drive circuit I
would follow the vendor's suggestions.