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Benchtop power supply for steppers - cap sizing — Parallax Forums

Benchtop power supply for steppers - cap sizing

ArchiverArchiver Posts: 46,084
edited 2003-05-12 14:28 in General Discussion
Stepper motors need big caps when used in high power applications.
Since many hobbiests are not running metal cutting tools, the need to
get high performace is not there.

Steppers are simle motors, or simple generators, depending on what
way you are connecting them.

neat trick. get 2 identicle steppers. if 6 wire, isolate the center
tap of each coil and tape on end of the coil, leaving one end and a
center tap for each coil. connect 2 motors together matching coils.
turn the shaft of one motor, the other motor will move exactly the
same distance. back to the math....

The calculation would be for the size of the filter capacitor.
Use the equation of (C= (80,000 * I) / V)). C is the value in
microfarads, I is the current and V is the voltage.

a 1 amp stepper, at 12 volts would need a 6,600 uF Cap.

A 12 VAC transformer would output a sine wave with peaks of 1.414
times the average (rms) voltage.

when you take the AC and pass it through a full wave bridge
rectifier, you get DC, you also lose a little voltage, around 1 to
1.5 volts. Then you put in your capacitor being DARN careful to get
the polarity correct, (big caps squirt lots of juice). The cap
allows the DC peaks to be stored, so will charge up to 1.414 times
the RMS value.

so your 12 VAC in becomes 11V DC after the rectifier, then 15.5 volts
after the cap.

Pretty simple. 3 parts is all.

The motor windings connect to the power supply thru a mosfet or
darlington or something, but there is no reason not to take the DC
from that power supply and filter it and regulate it for your 5
volt. that way, you are on the same common.

Since the motors will generate a electromotive force (generate
electricity) when slowing, the huge cap absorbes that energy. if you
use the same DC, take your low voltage DC between the rectifier and
cap. add another cap, (or three) then put in your voltage regulator.

Dave








--- In basicstamps@yahoogroups.com, John Walton <john@l...> wrote:
>
> Haa Haa, Dave thinks I know what I'm doing! Thanks for the info (I
printed
> it and added it to the "some day when I'm smarter" pile. I do
really need
> a good power supply. I'm tinkering enough now that I should have
one.
>
> Thanks again,
>
> -John
>
>
>
>
> >Hi John,
> >
> >you should be able to use just a transformer, rectifier and cap
(big
> >cap) to power steppers. no need for regulated power supplies.
> >
> >be careful as the cap size needs to be BIG. figure 20k uF or so.
> >and also when you go from AC to DC you will pick up the peaks of
the
> >AC, not the RMS, so multiple your AC RMS value by 1.414 to
determine
> >your DC voltage. or your ac less the drop from the diodes or
> >rectifier, then times 1.414.
> >
> >Dave
> >
> >
> >
> >
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