>You would have used something like a dump resistor between the ULN2003
diode common and ground
> (and not connect the diode common to Vdd).

Good point to note - when driving a multiphase stepper from a ULN2003 the
common diode connection must *NOT* be connected directly to Vdd. This is
because the stepper acts like a transformer - when you connect one winding
between Vdd and ground the other winding which is between Vdd and an "off"
ULN2003 driver is transformer coupled to the on winding and assumes an equal
and opposite voltage to the "on" winding. As a result the voltage on the
"off" terminal rises to about twice Vdd !!!!!!!!!!!!! If you connect the
catch diode terminal to Vdd you are shorting out the off winding via its
catch diode. At the least this will drastically affect stepper operation
(and probably make it not work properly or at all) and at worst will destroy
the catch diode. Thereafter the winding will not be clamped in any way and
you will have 'trouble". In such cases the diode common needs to be
connected to something which can deal with the 2 x Vdd output or left to
float. (If left floating you have to deal with any leakage inductance spikes
some other way). Several possible energy dissipating terminations are a
resistor to Vdd, a capacitor (reduces spikes)plus discharge resistor or a
suitable zener.


        RM

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