Thursday, December 27, 2012

PrintrBot Plus Stepper Motor Current

If you have a multimeter, you can set your current limiting potentiometers to the theoretically correct values for the stepper motors by connecting your multimeter ground (black) probe to one of the unused connectors with black wires going from the power supply to the PrintrBoard, and measuring with the multimeter's positive probe at the potentiometer terminals shown in the PrintrBoard image below (red, X, should be 440 mV, yellow (Y and E) should be 400 mV, blue (Z) should be 825 mV:
Works for me, and the motors run without getting too hot.

The background for those interested

The Printrbot X, Y and E stepper motors are according to the bill of materials here of type 42BYGH4807. The Z steppers are of type 42BYGH011.

The X, Y, and E stepper datasheet specifies their current as 500 mA per phase. The Z stepper datasheet lists the current as 330 mA.

The PrintrBoard uses Allegro A4982 stepper motor drivers. The datasheet says that:
The maximum value of current limiting is set by the selection of RSx and the voltage at the VREF pin. The transconductance function is approximated by the maximum value of current limiting, ITripMAX (A), which is set by
ITripMAX = VREF / ( 8 * RS)
where RS is the resistance of the sense resistor (Ω) and VREF is
the input voltage on the REF pin (V). 
The sense resistors (RS) of the PrintrBoard (except for the X stepps for some reason) are 0.1Ω, and VREF is set by the pontentiometer. For the X, Y, and E steppers, we want an ITripMAX of 500 mA. Solving the equation gives a VREF of 0.4V for Y and E, and 0.44 V for X

For the Z steppers, we have two steppers each wanting 330 mA, which gives us an  ITripMAX of 660 mA. This gives us a  VREF of 0.66 / (0.8) = 825 mV (0.825 V).

Good luck, have fun!