Russell McMahon wrote:
>
> 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).

  Autotransformer effect here, OK, makes sense.  Long weekend, no spare
brainpower this AM <G>  Saw it in the reading I've been doing, I keep
thinking in terms of 4 separate windings, need to remember that those
6-wire jobs are NOT so, at all, but have 2 center taps.  OK, good.

> 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

  Good to know those gotchas that I forgot, I remembered all the easy
stuff like how to step things <G>

  Mark