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Peter Norberg Consulting, Inc.

Calculating Power Requirements

This application note describes how to calculate the power requirements for your system.

This application note is for use with our "estimated current" products, which includes our SS1010 and BS1010 series of controllers.  Note that the BC2D20 and BC4E20 current regulating controlelrs are not restricted by these rules.  For those series, please review our calculating motor current for current controlled products faq for power information.

1.  Determine the individual motor current requirements.

The first issue is to determine the current requirements for your stepper motor.  Since our system does not monitor current at all (it only estimates current, using a PWM-like technique), the current ratings as seen by our board may not match those specified by a manufacturer who is assuming that current-monitoring based control is being performed.

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

    V=IR

or, rearranging terms in order to find I,

    I=V/R

That is to say, the current (I) as seen by our board equals the voltage (V) from your power supply divided by the resistance (R) of your motor windings.  This value can be much greater 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, then the motor will "draw" 6/3 or 2 amps if 6 volts is driven out of it, and it will draw 12/3 or 4 amps (per winding) if 12 volts is generated.

Once you have determined the motor current, 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

Note that the above table includes a "fudge factor"; we always recommend using a power supply which is somewhat larger than the absolute minimum required, in order to avoid overloading issues.

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

This is fundamentally the same calculation as above.  To calculate the voltage requirements for your motor, our system is best modeled using the standard resistor-only based formula (ignoring inductance) of:

    V=IR

That is to say, the voltage (V) from you your power supply which is required to generate the requested current (I) as seen by our board equals the current (I) times the resistance (R) of your motor windings.  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.

3.  Determine the logic supply requirements

The current needed by the logic portion of our product offerings depend upon the product.  All of the units except for the BiStep2A need at most 0.4 Amps for their logic (that provided via the Vc or Va connection); the BiStep2A requires 1 amp for this connection.

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 linear supplies be used: switching supplies are not very good when used with inductance based loads.

Single Supply.

If your motor power supply voltage is from 7.5 to 15 volts, then you may choose to use a single supply to operate the system.    Note that the current capabilities of the supply must exceed the sum of the current requirements of the motor(s) and the logic circuits.

Dual Supply

For all of the other units, you may separate the motor supply from the logic supply.  If you do so, we suggest using the lowest voltage in the range of 7.5 to 15 volts on the logic supply which you have available, to reduce generation of waste heat on the board.

The motor supply should be above 7 volts in all cases (due to some signal requirements on the board), and otherwise is as calculated under sections 1 and 2, above.  If the supply is to drive 2 motors, please remember to double the current needs.