Polulu A4988 behaving differently from clone

I was having extruder problems, and one of my diagnostic steps was to replace the clone A4988 stepper driver that came with the kit with a genuine Polulu.

The problem turned out to be something else, but the extruder behaved weirdly with the Polulu, both when printing and manually extruding.

I didn't change anything else -- the vref was tuned the same (0.35) and all jumpers were installed with both. I could hear the motor stuttering, and the extrusion was wavy. (Didn't take a picture.) Also, I couldn't turn the motor shaft by hand until I powered off. I remember reading what this last thing indicated, but forgot what.

I put the clone back on, and the motor ran smoothly and extruded fine. Do I have to do something else to get the Polulu to work? Does it require a different vref? I did play with that value, with no effect.

Thanks.

Prusa I3 from Folgertech
RAMPS 1.4
NEMA 17 2.6kg/cm Stepper Motor

Compare the shunt resistors of the clone and the pololu. If they are different, also Vref has to be different .
-Olaf

Is there a reference guide, or formula, for determining the correct vref for a given shunt resistor?

For the A4982 and A4988, Vref = I * Rs * 8 as far as I recall.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

"I" is what? (I'm not an electronics guy.)

I tried values between .35 and .5 with no difference in motor behavior. I know the vref gives the motor enough power to do the job right, not too much and not too little -- but what would cause the stuttering/staccato motion? What does being unable to turn the shaft by hand indicate?

Thanks.

V=volts (voltage)
I=amps (current)
R=ohms (resistance)

V=IR

- Tim

If the genuine Pololu is misbehaving at 0.4v. then I suspect the chip is faulty.

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Waitaki 3D Printer