Wouter wrote:

> But would that have any negative consequences when the motor's
> Vcc is 12V and the PICs uses 5V derived from the 12V via a 7805?
> At least I used this setup for some time without noticing any
> problems. The impedance of my 12V is very low (nicads, big C).

  You are interpreting "Vcc" as "the supply for the motors" and your
observations are almost correct as such.  Dave's objection was to the
implication that the flyback diodes were returned incorrectly to the +5V
logic (PIC) supply.  This would of course feed current via *all* the
motor windings through the flyback diodes to the supposedly regulated
supply.

  The nasty trick to this mistake is that the steppers would still
operate (poorly) as the "sneaker" currents would tend to balance out.
OTOH, the logic would be running at excess voltage.

  As previously explained, when unipolar drive is applied to a stepper,
switching one winding on causes, by transformer effect, the opposite
winding to kick up to twice the supply voltage.  Limiting this by diode
to the supply voltage serves to delay current build-up considerably and
cause excess heating overall.

  Further, the "flyback" of each coil is already limited to some extent
by the inverse diode across the driver transistor.  (I am unsure from
the Allegro datasheets why such diode is shown dotted however.)

  The common for the flyback diodes should return to a Zener of twice
the supply voltage.
--
  Cheers,
        Paul B.