power supply for stepper motors

I'm in the process of looking for a power supply to power 5 stepper motors for a robotics type application I am developing. The five
motors are size 17 bipolar hybrid stepping motors and I am powering them with 5 microstepping drives. My specific questions involve looking at the power requirements for my drives and motors and using those figures to determine what I'm going to need in a power supply.
The drives are rated at 12-24 VDC, and have an adjustable max current of .25 to 1.4 Amp RMS/phase or .35 to 2.0 Amp/phase peak. The motors were tested at 1.4 Amp RMS at 12 volts and have plenty of torque at this range to satisfy my calculated requirements. I intend to start at 12 volts and hopefully this will be enough, but it would be nice to have a power supply that went up as high as 24 volts if I need to get more torque out of the motors than I expect.
The dives also require 5 volts at 7 mA for the logic circuits. According to the manufacturer if a separate 5 volt supply isn't provided it's acceptable to use a resister to drop the voltage. A separate computer power supply might also be a possibility to power the logic circuits of the drives.
My ideal solution would be to have one power supply to power all five drives. Something with an adjustable voltage or separate channels between 12 and 24 volts, or possibly even a separate 5 volt source for the logic circuits. I think my amperage requirement would then be 2 amp/phase x 2 phases/motor x 5 motors = 20 Amps. This is assuming I need to use the peak amp figure and not the RMS figure. I think this would then give me a power requirement of 12 volts x 20 Amps = 240 Watts (using 12 volts) up 480 Watts (using 24 volts). But maybe it is not the most economical, practical, or even possible to use the same power source for all five motors.
So, my questions are:
Did I calculate the correct specifications for power requirements for a power source?
Does anyone know good places to look for the power source described? The more economical the better.
Would I be better off with 5 smaller power supplies rather than one big one, and if so where can I find those?
Also, am I correct in this assumption: A 20 Amp power supply, supplies UP TO 20 Amps, but only when 20 Amps are being drawn. I need a 20 Amp supply to hook up 5 2 Amp/phase motors with 2 phases each, but in situations where I just want to move one motor at a time then only a maximum of 4 amps will be used for that time and the power supply will only put out 4 amps. Is that correct?
This isn't exactly my area of expertise and any help would be greatly appreciated.
Thanks, Jason
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Is this for a stationary robot, or a mobile robot? Five steppers sounds like a 5-axis arm, but before spending time answering, are you asking about an AC-operated power supply or a DC power supply driven off a battery?
-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza
jpoge66 wrote:

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This is actually for a 5 axis mechanical hand. I'm actually looking for a power supply that runs off of regular AC input. This is going to be a bench top type application for now.
- Jason
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This is actually for a 5 axis mechanical hand. I'm actually looking for a power supply that runs off of regular AC input. This is going to be a bench top type application for now.
- Jason
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This is actually for a 5 axis mechanical hand. I'm actually looking for a power supply that runs off of regular AC input. This is going to be a bench top type application for now.
- Jason
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jpoge66 wrote:

Steve noted a PC power supply but I'm not aware of too many that deliver 20 amps at 12 volts, which is the minimum you'll want for the motors. The +5 output is more than enough for your drives.
But, since older PC power supplies are almost free these days, if space isn't a problem you could get five, one for each motor. Remember that most PC supplies require a load on at least the +5 output, and some older ones need a minimal load on all outputs. A properly-selected power resistor across the output(s) will suffice.
At 20 amps you definitely need some fold back protection against shorts, some decent fuse protection, good-size caps for filtering, yada yada. I'd also spec the supply at no less than 50% worse-case, or 30 amps.
I'd look for a good, used HP (or other quality made) bench power supply. You can find them on eBay for $100 to $175, and sometimes less if you're patient, and you find a seller who doesn't know what he/she has. A 20 amp at 12 vdc DC power supply is actually fairly brutish. You could make one, but IMO a store-bought surplus supply would be better, and safer.
-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza
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It sounds like you can use a PC desktop power supply for this application. Sized smaller than half a shoe box. You can buy a PC supplies from your local computer store for less than 50 bucks. Or A used one is perfectly fine and you could get one from an older PC for almost free.
You are correct in your understanding that a Motor only draws up to that specified current amount.
2 amps DC x 2 coils (phases )= 4 amps per motor 4 amps x 5 motors would consume a max of 20 amps
A power supply rated at 20 amps will provide any where from 0 to 20 amps depending on the load. The power supply will get warmer as you approach its maximum rated current.
Most PC supplies will provide atleast 10 amps on the 12 volt connector. they also provide 5 volts.
You can probably connect two separate supplies in series for more voltage , but you should not ground them together.
Stepper motors draw current when they are moving AND they draw current when the are commanded to be stationary.
Another potential solution is to use some medium sized 12 volt lead acid batteries or gel cells.
A 12V 7 AH battery will provide 20 amps for 12-15 minutes, a larger AH battery will provide proportionately more run time.
Two batteries can be connected in series for more current.
Have fun
jpoge66 wrote:

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This is true the vast majority of the time, but smart controllers can reduce the current used to hold or even turn it all the way off in applications where friction will provide enough stability.
The linistepper controller (an open source system using a PIC Microcontroller) has a low power hold mode and can be modified to shut off completely when the motor has not been active.
http://www.piclist.com/linistepper
The advantage of this is not only that the power supply needs supply less current, but also that the motor will heat less and will heat evenly.
Another (minor) point has to do with the linearity of microstepping: Believe it or not, if a stepper motor holds for a long time on one set of coils, the difference in temperature in the coils will cause e.g. half steps to be not exactly half way from one step to the next.
James Newton.
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