Re: Power supply

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "David G. LeVine"
<dlevine144@...> wrote:

>
> Jon Elson wrote:
> > Jameco has some really cheap switching power supplies. This may
> > be at the upper range of the wattage they have. You could
> > probably get two 12 V 8 A supplies and put them in series. The
> > single-output supplies generally are fully floating, so can be
> > series-ed easily.
>
> Switching power supplies usually do not sink current well. Put a BIG
> computer grade electrolytic across the output to help absorb the spikes
> generated by switching.
>
> --
> David G. LeVine
> Nashua, NH 03060
>

Power supplies do not typically "sink" current. They have smaller LC
type filters on the outputs because they only have to filter the HF
switching. Most have pretty decent transient response times but a
moderate amount of external capacitance acts to lower the loop
response gain and provide a regulated output. Putting too much
capacitance in the output could cause the power supply to go into
current limiting on power up and refuse to turn on (sees the big ole
filter cap as a low impedance load). The current waveform of the PWM
to the motors is based on the primary PWM frequency and the inductance
of the motors.

For smaller loads (typical 3A motors times 3 or 4) we have used
switching power supplies with no external caps. Been running one in
production for over a year. Without an external output cap the
ability of the unit to reduce back EMF is diminished. Perhaps that
was the reference to "sinking" current.