Power supply calculations questions

[cnczoner]

Hi all,

I'm designing a power supply for my to-be-CNC'ed mini lathe, and trying to get a handle on how much power I need to drive the 2 stepper motors. For just one motor, I have the following specs (for a Powermax M21NxxA in bipolar parallel mode):
I=5.6A
R=0.23ohms
Therefore, V=I x R = 1.29 Volts.

[ Currently working on my own controller/driver design to see if I can better the Gecko in bang-for-the-buck, but I may still use a Gecko. ]

I've seen recommendations of using 20 to as high as 30 times this required voltage for the chopper circuit. I can't find the links right now, but I'd like to know how this is derived. Also, Xylotex states that 12V or 24V can be used, but this is not dependent on the specific motor. Anyone have the low-down on how the ideal voltage is determined?

Also, the motor has a label on it that says "Vs(dc): 35V". Is this an indication of the voltage I should run across it? If so, should it be derated?

For current, in bipolar mode, only one coil would be energized at one time, so I should only need 5.6A from the power supply (for a single motor). Gecko's G201 manual indicates that this can be derated by 67%, which seems reasonable and I've seen this elsewhere. Does this (derating factor) change if the motor is run in full-step, half-step, or microstep mode? I'm guessing it would be higher for half-step, but that's not a scientific statement.

Thanks,
-Neil.