Hardware Peripheral Input/Output

Stepper Motor Driver Power Supplies

The current required to drive a stepper motor depends on the motor, motor wiring, the driver, and the drive mode. To be safe, supply twice the motors rated current (and always include a fuse or circuit breaker).

Motor Rating: Manufacturers can rate their motors under different systems. Most of the old round unipolar motors you pull out of old equipment have label ratings for 2 phases on. So 1.5A means both phases always running at 1.5A. But newer motors (like most square ones) are rated per phase, so 1.5A means 1.5 amps maximum with one coil running.

Driver Type and Wiring: The type of driver and how it is connected to the motor can also affect the required drive current. A bipolar driver in series mode needs 0.707 * rated peak current but in parallel mode needs 1.414 * rated peak current. Unipolar drivers typically drive the rated current. See: Stepper Motor Connection Options for more on this. However, that is just the peak current. The average current, which is what you care about for power supply, can be quite a bit less in a bipolar driver, because it can route the remaining power from the magnetic field stored in one coil, into the other coil, when it moves from one position to the next. This reuse of back EMF means that a bipolar system typically needs 1/3 (for series mode) to 2/3 (for paralllel) the rated current of the motor.

On top of that, the drive electronics can require slightly more or less depending on the type of current regulation. A chopper drive can require slightly less than full current on average while a linear driver will need up to 1.5 times the full standard rated current.

Drive Mode: Even within one driver, the mode can change drive current requirements. In full step mode, if both coils are on all the time, the current draw is doubled. In microstepping modes, the current draw is typically about 1.4 times the rated current. But there are exceptions and each driver may be a bit different. For example, the Linisteppers full torque half stepping mode will use between 1 and 1.5 times the rated per phase current depending on which step in the sequence the motor is on, but it will provide more torque from the same motor!

Taking the Linistepper as an example of a non-chopper type, microstepping, linear driver (but one with /active/ current regulation): The average current when running in microstep mode is 126% of the single phase current. For any particular microstep the minimum is 100% and the maximum is 155%. In FULL step mode the current will be 200% at all times. The half-stepping mode the LiniStepper uses is 140% average current. Double the current using full step mode, but 140% is the best generalisation for half stepping and microstepping modes.

Battery / Charger Supplies

One of the best power supplies for stepper motors (or any application where peak current is much more than average) is a stack of batteries. Stack up 12v car or SLA batteries to make whatever voltage is needed, and then buy or build a charger for that voltage. The charger won't need to supply more than an amp or two to keep the batteries charged, and the batteries can provide nearly unlimited current on demand. Even old batteries seem to work nicely. Some people have had excellent luck with free used car batteries from automotive repair businesses, or SLA's from old UPS units. They don't have to be full strength, since they are always charging.

12 volt chargers are availalbe everywhere for next to nothing. 12, 36 and 48 volt chargers are commonly available for those "mobility carts" for under $50.

Of course, fuses are /very/ important when working with batteries. Fuse each battery and the entire pack. Put the car batteries in battery boxes to keep any leaks or fumes out of the work area.

The down side of this method is that individual batteries may not charge equally when charged in parallel. As a result, one battery can get more and more "out of whack" and fail. It might be best to include a multi-gang knife switch to re-connect the batteries in series to a second small charger when not in use, in order to balance the batteries with each other.

Recommended supplies:

• http://www.antekinc.com/ps-8n28-800w-28v-power-supply/

Power Supplies +

See also:

• http://ebldc.com/?p=86 Decay modes.
• http://motion.schneider-electric.com/support/general/choosing_power_supply.html
• joeimai at spamhotmail.com refers to

  http://www.cnc-arena.com/en/forum/ps-steppers--140174.html Hello, I have a Gecko G320X servo driver controlling a RS-550 DC motor. My setup runs fine when powered by a DC Power supply at 24V 3Amps. But since the DC power supply could only generate 3Amps max and the RS-550 was needed about 10Amps to turn it's load I switched my power supply to using two 12Volt car batteries connected in series to make 24Volts. The rated CCA(cold cranking amps) for each battery is 370A with 45Ah.
  I thought this would be more than sufficient to supply my needed 24V 10 Amps for the motor.
  
  But when I switch to this car battery setup, for some reason the motor is not able to turn the load and is drawing less current than the previous setup's DC power supply setup at 24V 3Amps max.
  
  I read your car battery set up at cnc-arena:
  
  If you may have any insight on this I really appreciate help.
  
  Thank you,
  Keita
  

  James Newton of MassMind replies: That doesn't make sense... it should work much better. The only thing I can suggest is checking the connection and make sure your connecting wires are nice and thick. Check the voltage at the battry while the system is running and see if it is dropping. Then check again at the driver. If you see a drop of more than a volt, you have a bad connection or a thin wire. It's probably getting hot as well. Let us know what you find?+

  +

• http://www.procooling.com/index.php?func=articles&disp=52 Running multiple (ATX) Power Supplies in Series at 18 or 24 Volts! Note that is requires removing the ground connection (either by using a 2 wire power cord or by cutting the ground on the AC side inside the power supply case) and doing that is potentially dangerous. There is a reason why the AC side of a power supplies are grounded.+
• http://www.rcgroups.com/forums/showthread.php?t=1170784 Converting hot swap PC power supplies into high current 12volt bench supplies.+
• http://web2.murraystate.edu/andy.batts/ps/powersupply.htm Converting a standard ATX PC power supply into a 12volt bench supply
  +

Protecting against short circuits +

Questions:

• xheight at spamlive.com asks:

  Linistepper with regulated powersupply - gets pulse for movement.
  
  Hello! With my Plotter project i lately wanted to save some time and bought AC 240V to DC 5V Voltage Converter that is meant for LED lights. It is a switching power supply. Only flaw is that when i hooked all things up, motor started to erratically turn without any inpulse on "step" pin if i connect with regular "wall-wart" weaker(2A) PSU all works normally.
  What can i do to fix this problem? Seems like that particular PSU creates spikes that linistepper counts as step impulses? can it be this way?

  James Newton of MassMind replies: Yes, a power supply that isn't smooth can certainly cause this sort of problem.+

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