Re: Power Supplies

--- In CAD_CAM_EDM_DRO@egroups.com, "Steve Kelley" <info@p...> wrote:

> Greetings.....
>
> About switching power supplies.....Anne's experience may
> have been avoided if a load was placed on the +5v output.
> A word of caution when using " switchers " that have multiple
> output voltages! Typically , the regulation of the supply
> is provided via feedback from the " lowest " output voltage.
> Plus...the peak primary current , in current-mode control.
> Not all topologies of switchers are satisfied to operate in
> an unloaded condition. Skipping past the thesis , you can use
> a couple of approaches : First-beware of " vintage " switchers,
> Second-check datasheet or with the manufacturer to verify that your
> switcher can indeed operate " unloaded ". Third-apply a minimum
> load to the lowest voltage output ( especially if unused ). A
> minimum load is usually 10% of the rated output current. If all
> else still looks dim , save yourself and pick-up a surplus switcher
> at Jameco <www.jameco.com> ....they are kind enough to label their
> switching goodies with the loading requirements. Learn more about
> switchers at < www.smpstech.com >
>
> Regards......
> Steve
> --- In CAD_CAM_EDM_DRO@egroups.com, Anne Ogborn <anniepoo@n...>

wrote:

> > Jon Elson wrote:
> > >
> > > William Scalione wrote:
> > >
> > > > If anyone is interested in some 24V @ 62A switching power
> supplies for
> > > >
> > > > drives
> > > > I have some $25.00 each new in the box. Astec VS3 models They
> also
> > > > have
> > > > a
> > > > 5V @ 60A output which should be enough to power your TTL (or
> weld
> > > > with it).
> > >
> > > Wow, that's almost 1800 W DC. What is the input? 240 V? 3
> phase?
> > >
> > > Jon


> I have a power supply, sounds like this. Mine's marked +- 12@8A &
> > +5@75A, but looking inside it we can't believe it'll supply that.
> > We hooked the +12 up to a 3A draw, it worked for an hour, then
> > failed.

Steve is right about the +5 needing some level of load. Since in
most systems the logic supply (+5) is the most critical it is the one
that is regulated. Most switchers do their regulation from the
primary side and the other voltages are derived from additional
windings on the transformer. In the design the regulation feedback
transfer function has as one of its components a load resistance
factor. All that means is that to keep things nice and stable in the
PWM high voltage side of the circuit there needs to be something to
dump some energy into. Newer designs based on tuned magnetics and
modern control circuits have all but eliminated the need for pre-
loads. Also be careful using regulated supplies to drive inductive
loads like motors. It is possible for back EMF to do some nasty
things to semiconductors. The cure is to provide a place for the
excess energy to go like a freewheeling diode or snubber circuit. A
heafty diode in series with the output to the motors and a place to
dump the excess energy in the cathode side of the diode works. I am
building my system to run off one supply but each motor has an
isolation diode and a snubber circuit to keep one motor from
effecting another.