> > A ULN2003 is a open collector bipolar driver.  There are 7 darlingtons
> > within the package and each can switch 50 volts at 500mA.
>
> A darlington is a particularly bad choice in this instance.  The high on
> voltage is a significant portion of the 12V supply voltage.  At the
voltages
> and currents mentioned here (12V, 3A), FETs are the best choice.


If using FETs and driving directly from PIC you will need "logic FETs" which
(typically) accept a 5 volt gate level.
A standard FET will need the better part of 10 volts.
You can use extra driver transistors and standard FETs but the extra
complexity is hardly worth while.


A Darlington is OK as long as you use a transistor able to handle the
dissipation and current.
A decent darlington in a TO220 pkg should drop no more than 1 volt (probably
less) at 3 amps. This is a 3 watt dissipation (and probably less as duty
cycle per phase will not be 100% usually) so a fairly minimal heatsink will
suffice.


regards

                Russell McMahon

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