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Original post by: Donna ,


While here doing some diagnostics on my X-axis, I just measured the Vref setting on my boards for all three (X, Y, and Z) axis and the Vref pin is measuring ~0.700 volts on all three.  The best I can tell, the Rs (sense resistor) value on these boards is 0.05.  It's difficult to measure them with my meter due to the low resistance value, but that's what seems to be written on it for a value, assuming my eyes are reading it right, and that matches the following web sources.

According to both the Pololu and RepRap websites for measuring/setting the current limit:

This comes out to be:

Itrip = Vref/(8*Rs)

Itrip = 0.7/(8*.05) = 1.75 amps

Pololu has a nice video showing how to measure and set it:

I cross-checked the resistance divider bridge with my meter, and the potentiometer appears to be a 10k pot, set to 4.3k (with respect to the ground side), in series with a 20k resistor.  This matches the schematic on the Pololu website at the above links.

It also verifies my voltage reading, as the divider bridge would be 4.3k/(10k+20k) = 0.143.  Multiply that by the 5 volt logic supply and you get 0.716 volts which is close enough to the 0.700 volts that I measured.

So -- from the best I can tell, they are set to 1.75 amps...

NOW...  for my big question... WHY did they clip the step-set pins on the drivers and not just set the jumpers on the Ultimaker 1.5.7 board correctly as it was intended to be done???  Can anyone explain that one to me???

I'm looking at replacing my X-axis stepper drive board to see if it solves a mystery I've experienced a couple of times during printing where my X-axis will suddenly shift.  The first time it happened was a really long (17+ hour) print and I was only monitoring via remote camera when it happened.  Even though it was PLA, I had some warping issues in the part that did cause one corner to pop up about the right amount to cause the 10mm X-axis shift that occurred and so I dismissed it as simply being a head to part collision that somehow shifted it.

But then last night, I was doing a quick 30-minute print with some Nylon 230 and everything was adhering perfectly and definitely was no head collision or binding problems.  I was sitting here watching the printer and all of a sudden halfway through the part, I realize my X-axis had mysteriously shifted by 5 or 6mm and it was printing in mid-air.

What was really frustrating was that the first layer and raft and all were absolutely perfect and the adhesion to the print bed was as perfect as one could get for nylon -- one of my best ever for getting it to stick, all after a couple of hours of failed attempts at getting it to stick....  and then I was robbed of my print because of the X-axis problem...

So... I'm going to be swapping out some stepper driver boards, but if anyone can explain why they cut the pins on the step-setting inputs and didn't just use the jumpers, please let me know!  I've looked at the Ultimaker 1.5.7 schematic and cross-checked with details on the driver board itself, and I can't see why they did that.