I'm looking at a 6V unipolar stepper motor, 1.2A/phase, 5 ohm/phase
If I work it all out, and leave the center tap disconnected, I should be able to power it with a 12V supply right?
The motor is http://www.robotshop.com/ca/en/rbsoy09-soyo-unipolar-stepper-motor.html, and I'll be controlling it with a ST micro L6470PD (found in a few stepper motor controller boards) over SPI
If I work it all out, and leave the center tap disconnected, I should be able to power it with a 12V supply right?
No.
Without the center tap you no longer have the four coils you need for it to be a unipolar motor, just using the two coils means you have to drive it as a bipolar motor.
and I'll be controlling it with a ST micro L6470PD
But that is a bipolar driver. And it has current chopping so you can drive it with any voltage from 8 to 45V. The higher voltage you use the faster you can make it move.
I know it wouldn't be a unipolar motor if I omitted the center tap, but I could drive it at 12V in bipolar mode right?
Long and the short of it is from a 10-14.5V supply and that chip, I can drive that motor in bipolar mode, correct?
Yes it looks like you can.
great, thanks 
um... why buy a uni-polar when you want a bi-polar ?
also, why use such a high voltage motor with such a low voltage power supply ?
Because it is not rearly a unipolar motor that is just the marketing, a lot of these motors have a center tap so you can configure them as both.
You can run a motor at any voltage you like, it is just that the speed is limited by how fast you can get the current flowing. A high voltage helps for that no matter what the coil resistance is.
You may find something useful in stepper motor basics
...R
Grumpy_Mike:
Because it is not rearly a unipolar motor that is just the marketing, a lot of these motors have a center tap so you can configure them as both.You can run a motor at any voltage you like, it is just that the speed is limited by how fast you can get the current flowing. A high voltage helps for that no matter what the coil resistance is.
bi-polar are becoming easier to find, lower in cost and higher efficiency.
an 84oz-in on E-BAY can be found in a NEMA-17, bi-polar,
also, is it not the voltage, but the ratio of nameplate to power supply.
a $10 e-bay special, 84 oz-in hybrid motor, 2.8 volts
with a 12v power supply will deliver more than twice the power of the 10 volt, unipolar listed.
or, worded differently, one would need to use 40 volt power supply to get similar performance.