Current Requirements-Current Regulated Controllers

This application note describes how to calculate the power requirements for your system when using our current-controlled products, such as our BC2D20, BC4E20, BC6D20 and BC6D25 controllers. 

1.  Determine the individual motor requirements.

The first issue is to determine the current requirements for your stepper motor.  This number is provided by the motor manufacturer (such as 0.6 amps/winding).  This figure is used to select which of our controllers can handle your motor --  in general, you would normally want to obtain the least expensive controller which fits your needs.

Once you have determined the motor current (and controller to use), then you will need to determine how you intend to run it via our product offerings.  We have four modes of operation, which provide for three levels of power per motor.  These modes are controlled by the "o" command, which specifies the technique used to drive the windings.

Update Order Current Multiplier
0 (single winding full step) 1.4
1 (half step: alternate 1, 2 windings) 2.5
2 (full step: 2 windings at a time) 2.5
3 (microstep) 2.3

Obviously, if you are going to run multiple motors off of one supply, you will need to add together all of the currents needed in order to determine how large of a supply to use.

2. Determine the voltage for your motor power supply

No, this is not "who is buried in Grant's Tomb".   The motor manufacturer will document the minimum voltage at which the motor can operate, and will often specify a much higher voltage which was used to document the top speed and torque of the motor. 

Our BC2D20, BC4E20 and BC6D20 controllers can handle up to 34 volt power supplies, while the BC6D25 can handle up to 40 volts.  If the motor voltage used to document the performance of a given motor is above the limit for the given controller, our controller will not be able to operate the motor to its documented level.  As long as the power supply voltage is at or above the minimum voltage for the given motor, our controller will normally operate the motor correctly, to within the limits of the motor's behavior at that voltage.

For all of the above controllers, you get best microstepping response from motors when the voltage of the power supply is from 1.3 to 5 times the 'nominal' voltage for the motor.  Your highest speeds are attained in the range of 3 to 8 times the 'nominal' voltage for the motor.  In general, you probably do not want to go above 10 times the nominal motor voltage: the controller may not adequately regulate the current under this circumstance. 

From the point of view of determining the nominal  voltage requirements for your motor, our system is best modeled using the standard resistor-only based formula (ignoring inductance) of:


That is to say, the voltage which would be used to exactly generate the target current in the motor would be the result of multiplying the requested current (I) by the resistance of the motor windings (R).  This value can be much smaller  than that claimed by a given motor manufacturer, since most of them assume that you are using a current-controlled system to run their motors.

For example, if you have a 3 ohm resistance in your windings which is specified as requiring 2 amps of current, then the motor will "draw" 2 amps if 6 volts (2*3) is connected across its coils.

In general, the higher the voltage you use on your power supply, the higher the top speed of your motor.  However, the higher voltage power supplies cost more, and extra cooling of our board may be required at the higher voltages.

3.  Determine the logic supply requirements

The current needed by the logic portion of our BC2D20, BC4E20, BC6D20 and BC6D25 series of product offerings  is at most 0.5 amps, on a 6.5 to 15 volt supply if you are not using a fan assembly.  Add 0.5 amps if a fan assembly is used that is powered off of the board logic.

4. Determine the power supplies you will be using

Your choices are dependant on the desired voltage to the motors, and on the board which you have purchased from us.  In all cases, we strongly recommend that regulated linear supplies be used: switching supplies are not very good when used with inductance based loads, and non-regulated supplies may cause the board to fail!

Single Supply.

If your motor power supply voltage is from 6.5 to 15 volts, then you may choose to use a single supply to operate the system.  Obviously, the current capabilities of the supply must exceed the sum of the current requirements of the motor(s) and the logic circuits.

Dual Supply

You may separate the motor supply from the logic supply. 

Depending on board options ordered, you may use any one of the following options to power the board logic:

  • Use a regulated supply in the voltage range of 6.5 to 15 volts on the logic supply which you have available, to reduce generation of waste heat on the board.
  • Use a regulated supply that operates at exactly 5 volts
  • If you are not using a fan assembly powered off of the board, then you may also be able to use the power provided by the USB connection to power the board logic.

The motor supply should be above 7 volts for the BC2D20, BC4E20 and BCC6D20.  For the BC6D25, the motor supply voltage must be at least 10 volts.   If the supply is to drive multiple motors, please remember to multiply the current needs by the number of motors being operated.

Triple Supply

The BC4E20, BC6D20 and BC6D25 series have the additional capability of allowing use of three different power supplies.  It is quite possible to have one set of motors operate at a different voltage and current level than the other set, if this turns out to be a requirement.