Excellent tutorial on stepper driving from Tom but there's a
+ACI-gotcha+ACI- here
which has to be avoided -

+AD4-A diode should be connected across each coil winding. The Anodes
+AD4-all connect to the Common wire, and the Cathodes connect to A B C
+AD4-and D.
+AD4-
+AD4-The diode will supress high voltage spikes when the coil is
+AD4-switched from it's ON to it's OFF state.


As noted, effectively you have two coils which are centre tapped and the
two centre taps are connected to supply. Now, each individual centre tapped
winding acts as a transformer. When you put ct to supply and ground one end
the OTHER end goes up to 2 x supply voltage. IF you connect a diode from
this +ACI-high+ACI- end to supply it will prevent the end rising more than
a diode
drop above supply. The system will not be happy. If you want to use anti
flyback diodes like this there are several options. Each must produce the
same result - the +ACI-open circuit+ACI- ends must be allowed to rise to 2
x supply
at least. Some possible ways are is to connect all diode cathodes together
and then connect this point to a capacitor to make a 2 x supply point.
Connect a resistor from here to ground to dissipate flyback energy. Or just
use a resistor here. Or take each diode to Vcc or ground via a resistor
each. Or leave open and ensure that flyback voltages can be handled by your
driver (be careful). Also remember that flyback voltage loading will affect
the time constant of the coil and MAY affect your stepping. Time constant
+AD0-
L/R so smaller R's gives lower voltage peak but longer time constants
(opposite of case for capacitance).

In one design I drive a small unipolar stepper similar to this one with a
ULN2803 darlington driver with built in diodes and I leave the common diode
point open. In this case the driver is happy. YMMV :-).


regards

            Russell McMahon