Tuning motor current
A4988 and similar
Per the A4988 datasheet, the calculation for the maximum trip current is:
I_TripMax= Vref/(8*Rs)
With Pololus, the sensing resistors are Rs=0.05 ohm, so a Vref of 0.4 should produce a maximum current of 0.4/(8*0.05)=1A. Clockwise increases the current which will make the motor run hotter and counterclockwise reduces it which will cool it down (see forum thread 159081).
As another example, aiming for 50% temperature rise on 1A rated steppers by using max 0.7A, so rearrange it as:
Vref = I_TripMax * 8 * Rs
or
Vref = 0.7A * 8 * 0.05 = 0.280V
With a measured Vref of 0.273V, I should expect 0.6825A, and I measured the current through one coil as 0.486A in full step mode, which should be 0.7071 of the full trip current, or I_TripMax= 0.486/0.7071= 0.687A, which seems close enough.
The Vref signal is accessible as the "VREF" pin on the carriers with voltage regulators, as the through-hole via on the carriers without, and also as the wiper on the trim pot itself on both carriers.
Note 1: for StepStick, the Rs=0.2 ohm, so you'd get 4 times the Vref at the same current.
Note 2: most "Made in China" A4988 Pololu knock-offs have Rs=0.1 ohm.
