On a circuit taken from this page http://www.opend.co.za/tutorials/steptut1.htm
The last circuit shows two diodes positioned in reverse polarity to the current.
It is supposed to protect against the inductive kickback of each coil when its driving transistor is turned off.
But when one transistor is closed and the diode is not connected in parallel to the coil (as most flyback diodes I've seen) how can a closed circuit be formed between the two sides of the coil to discharge it?
Best Answer
The answer to your question really depends on what type of stepping motor you are using.
In this case from the web link you provided they are discussing a unipolar stepping motor where the phase coils are bifilar wound and act very much as a transformer.
Here is a schematic with the simulation so you can see the waveforms.
simulate this circuit – Schematic created using CircuitLab
Notice that the back EMF is clipped by the diode on the OPPOSITE transistor to the one just turned off. So D2 clips the back EMF caused by Q1 turning off and D1 clips the back EMF when Q2 turns off.
The website is a very poor tutorial since no attempt is made to clarify the differences between stepper motor configurations. The technique for example could not be applied to a 2 phase single winding stepper motor. It uniquely applies to a dual wound (or center tapped) winding configuration driven as a unipolar stepper.