Makerbot Industries documentation:
- Stepper driver board V3.3
- Stepper driver board V3.4 has no obvious differences; the schematic and pictures seem to be identical.
- Thing-O-Matic driver setup values
- MK6 Stepstruder
Copied directly from those pages for quick reference:
| Stepper Motor | REF Voltage (V) | Rotation between Max and Min |
|---|---|---|
| X & Y | 1.5 | Between 1/4 – 1/2 |
| Z | 0.617 | Between 1/8 – 1/4 |
| Stepper Motor | PFD (V) | RC1 (V) | REF (V) | RC2 (V) |
|---|---|---|---|---|
| X & Y-axis | 1.952 | 0.953 | 1.5 | 0.955 |
| Stepper Motor | PFD (V) | RC1 (V) | REF (V) | RC2 (V) |
|---|---|---|---|---|
| Z-axis | 2.311 | 0.952 | 0.617 | 0.963 |
| Stepper Motor | PFD (V) | RC1 (V) | REF (V) | RC2 (V) |
|---|---|---|---|---|
| Stepstruder MK6 | 2.31 | 0.94 | 2.26 | 0.94 |
The Allegro A3977 Microstepping Driver datasheet documents how the chip at the heart of the MBI stepper driver boards actually works.
The A3977 uses a pair of current-sense resistors (RS) to control the winding current. The REF trimpot sets a voltage level that the A3977 compares with the voltage (VREF) on the sense resistor, so the maximum current is directly proportional to the REF setting:
Max current = VREF / (8 * RS)
The MBI boards use 0.25 Ω resistors, so the equation reduces to
Max current = VREF / 2
So you can determine the maximum winding current by simply dividing the REF voltage in half.
However, keep in mind that the supply voltage and motor winding resistance also limit the current; additional voltage drops in the drivers and resistance in the wiring count against the maximum. The REF trimpot has no magic ability to force more current into the motor than Ohm’s Law will permit.
The Smell of Molten Projects in the Morning 
Huh. I just built a 3977 board to drive a stepper for the new feedscrew on the lathe, and it seems to work quite well. I’m glad you pointed me at this since it’s always good to see how other people do hardware.