Ok so my DRV8825’s finally arrived so I decided I’d see if I found a similar VRef to current ratios to what you came out with.
In full step mode, at 0.5v VRef I was finding around 0.5A in the coil. I adjusted until the coil current was 1A and the VRef at that point was 0.97v.
I then put it into 1/32 mode with the same VRef and measured 1.43A
That at least shows that full step current being 70% of the microstepping current is true. 1.43Ax0.7=1.001A
So maybe it is the case that the ratio on the ones that we have are slightly different to the Pololu ones.
The resistors on the ones that I have are the same as yours. They have .1 ohm sense resistors and the 152 and 103 resistors in the same locations as yours. The 2nd set of resistors aren’t really relevant to us anyway as they’re not in place of the ones on the Pololu board, they connect to a different pin on the IC if you follow the track on the PCB.
edit: Just had a quick read over the datasheet for the IC. From what I see the only 2 pins that should affect the current ratio are pins 6 and 9, “ISENA” and “ISENB”.
That’s where the sense resistors are connected on both boards and both are .1 ohms.
I wonder if the reading we’re ending up with is an RMS value of the waveform being fed to the motor coil.
For the 1A current that I read when set to .97v that would work out as (1 x √2=1.41A). That’s only 71% though due to being full step, so (1.41/0.71= 1.98A)
That value is much closer to what I would’ve expected from the VRef setting. The RMS value would also be for a pure sine wave which isn’t what we have here so that would effect that final value a little also but I don’t by how much.