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If I have a bipolar stepper motor driver (currently based on a L293, but I may be switching to a A4983) and I would like to control two 4 wire bipolar steppers from it (but not at the same time and the speed of the switching is irrelevant), can I just use a DPDT switch or relay to switch 1 wire of each pair between two motors on the output of the driver ? Apart from just using another driver per motor, are there better options here?

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Having just addressed this myself I'd have to say separate drivers for each motor is the most sensible approach.

If you remove the power from one motor, is has no holding torque and will 'flop' under any force.

You may also have issues with the state of the outputs not being correct for the current rotor position, which will make the motor jump a bit when you switch it.

And as Dave mentioned, if you have power on a coil when you switch it, it will arc inside the switch which will eventually destroy it.

It's also way more impressive to have both axes moving simultaneously if you can :)

Your application may be able to cope with some of these issues, but if you need reliable accuracy then I'd recommend separate drivers. The A4988 (update of the A4983) is only $3-4 from Digikey if you're building your own board.

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You make excellent points, especially the twitch from the outputs not being correct for the motor - that would cause me greater grief than the lack of holding torque (which I'd already considered). My biggest concern with the doing the A498X on my own board is that I've not done surface mount soldering before. Pololu makes a board for the A4983, but its uneconomical to purchase one for every motor I need. Maybe its just time to bite the bullet on SMD... – rfusca May 11 '11 at 5:32
@rfusca - while SMD isn't always difficult to do, leadless packages like QFN may prove frustrating as a first experience. When the occasion arises you could start with something like SOIC. – stevenvh May 11 '11 at 7:14
@rfusca sounds like the perfect opportunity to learn SMD soldering! Watch this helpful video to get a head start: store.curiousinventor.com/guides/Surface_Mount_Soldering/101 – Dave May 13 '11 at 6:01
@Dave - Thanks, I've watched that and about a million other tutorials on it lol. I've decided to do it, I've got some SMD parts on the way and so then I'll etch some simpler circuits to practice on. – rfusca May 13 '11 at 12:52
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I don't see why not, as long as the switch you're using is rated for the amount of current that your motor is going to draw. I also wouldn't switch over unless the driven motor is first stopped, so that you don't have any arcing in the switch.

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