11.0 Commissioning & Calibration - Reprap Prusa Mendel Build Manual
©2012 NextDayReprap
Prusa Mendel Kit Build Manual V1.01 - 07/2012
The Pololu A4988 Stepper Drivers have already been assembled, calibrated and mounted to
your RAMPS Board. But it may be necessary to adjust them slightly if you are experiencing
certain issues, such as missing steps ( when the print staggers in an Axial direction) or if you
are experiencing extrusion consistency issues). Either way the following is for your reference to
help you understand how the stepper drivers affect the printer.
We use low voltage Nema 17 Stepper Motors which can take a maximum current of 1.68A at a
2.8V the Polulo A4988 Stepper Driver can drive up to 2A, this is far higher than the level
required, resulting in the stepper motors running a lot cooler.
There is a very fine line between too much current and too little current in relation to the
accurate operation of the stepper motor itself.
The best starting point is about 0.7x the rated current, this is the maximum current the stepper
driver can output before requiring a heat sink ( n.b Heatsinks are already fitted).
You can calculate the approximate required Vref Voltage value by applying the following
calculations :
Vref = Stepper Motor Max Current x Factor Current x 0.4
Vref = 1.85A X 0.7A x 0.4 = 0.47V
So now select DC Current on your Multimeter to two decimal places
11.4 Adjusting Stepper Motor Drivers