re:stepper pwr suplies wasRe: motor/leadscrew/ps recomendations

OK, i've been designing and repairing switching
supplies for many years. Connecting two together
is a black art, there are a lot of things that
can go wrong.

Most common switchers like the chrome box psu
in your home computer expect a steady amperage,
and indeed a minimum amperage. If this changes a
bit, the switcher adjusts and regulates voltage.
If it changes a lot, like a big sudden current
draw, the switcher most likely goes into current
protect and shuts down its output. A safety for
when you short a wire or something.

Because the switcher can constantly adjust for
SMALL load variations, the manufacturers use
very small filter caps. Big caps are expensive!

Putting a chopper motor driver on the output
of most switchers spells disaster. It draws
nothing but spikes! Your poor switcher was
probably going into current protect every time
your chopper drive chopped! :o)

Putting very big caps on the switcher output
is obvious, this helps a bit. But the caps
do not *decouple* the supply well enough, so
as your chopper chops, there are still some
significant current surges drawn from the
switcher. You put the resistor BETWEEN the
switcher and your big caps, this provides
an impedance to the current surges, so they
are more likely to be drawn from the big caps
and not the switcher. You could use an inductor,
but this gets more complex and I know from
experience that if you don't mind losing the
0.5v (and gaining some extra heat!) this will
probably be enough to solve it.

Obviously the bigger your caps the better,
the more volts across the resistor the better,
(but that costs volts and makes heat).

Running one switcher from the output of another
is scary! But it should work if you want to
fiddle. :o)
-Roman

--- In CAD_CAM_EDM_DRO@egroups.com, richard@s... wrote:
> I don't understand the use of the resistor for the .5v drop.
>
> ~+.5v
> +24v DC---------------[R1, .01ohm]------------[load]
> |
> |
> -[10k cap]-
> |
> |
> GRD ----------------------------------[GRND]
>
> I thought a cap was to stabilize the circuit. If I remember my
> electronics right...
>
> If I put a large quick drain on this circuit, the current through R1
> will go up, increasing the voltage drop, CHARGING the cap. If I stop
> drawing current, the voltage drop on R1 will decrease, DISCHARGING
> the cap.
>
> This would not stabilize the circuit at all.
>
> If R1 was not there, a spike load on the circuit would drop the
> voltage, discharging the cap, stabilizing the output?
>
>
> Roman, what is the resistor for?
>
>
> --- In CAD_CAM_EDM_DRO@egroups.com, "Roman Black" <fastvid@e...>
> wrote:
>
> > From the output of the switcher use a small
> > series resistor, say 0.5v drop at max current,
> > and then add some big filter caps of a high
> > quality low impedance type like used in audio
> > amps. Most stores sell these. also there should
> > be about a 1/3 of your total capacitance value
> > as close to the driver bridge as possible,
> > as usually these are some distance from the
> > power supply and main caps.
> > -Roman