Re: Computer power supplies

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Darrell Daniels
<ddaniel1@s...> wrote:

> Do the high wattage (~400W) ATX computer power supplies have any
> application for CNC? The amperage seems high enough but I would

think

> the voltage is to low, anyone use them for anything? At the least I
> think I should salvage the filter caps and the transformers.
> Darrell
>
>
> >
> >

Computer power supplies typically have their regulated high wattage
output on their "logic" voltage output. In newer computers that is
the 3V output. There will also be a +5 -5 and +12VDC at lower
currents.

For anything but the logic side of things (driving 12VDC relays
and/or a +5 source) they are pretty useless for any type of
performance CNC applications. You need to be looking for supplies
that will output 28 to 70 VDC at several amps (depends on your motors
and the motor drive electronics).

You won't be able to salvage much from the computer supply since they
are flyback type switching power supplies and the transfomer will fit
in your pocket. The transistors will be of the high voltage type for
operation off the line at moderate currents. Remember that a 120VAC
input gets doubled to over 300VDC and that is what is switched, so
400 watts (assuming 70% efficiency) would mean the primary current is
less than 2A (RMS). There will be some 30A or so rectifiers but they
will be dual diode units and designed for low voltage, low forward
drop and fast switching (not 120Hz).

All in all you would be better off to toss the supplies...well maybe
steal the 12VDC fans, and go about looking for parts to build a real
supply.

While the typical stepper or servo design does not need the motor
supply to be very sophisticated (just a big iron transformer, a
bridge rectifier and large filter cap) the voltages fall outside the
normal range of most off-the-shelf supplies. Once you get above
48VDC (common telephone voltage) finding one already built at a price
that won't cause heartburn will be a chore.

There are possibilities of hooking transformers in series or using a
transformer will multiple windings to get the voltage you want.
First thing to do is decide what motors you will be using and how
fast you need to spin them to achieve the max speeds you want with
the proposed gearing THEN figure the voltage/current you need to get
there.