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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:

Power Supplies +

See also:

Protecting against short circuits +


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