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Stepper Drivers + Potentiometers

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While attempting to debug my machine which is repeatedly pausing printing and then resuming when "disable steppers" is selected through the Ulticontroller, I noticed that the potentiometer on one of my stepper drivers had been damaged, and as I tried to put it back together it broke apart completely. I now have a few questions...

- Is there a US source for UM stepper drivers? The shipping from Europe is just too much for such a small piece.

- Is it going to be possible to replace the broken pot with a 2.5K resistor (the measured resistance of my other pots) and bring the board back to life until a replacement comes?

- Is there a P/N for the pot so I could replace it completely?

- Is there an available schematic or layout for the driver PCB so I can see what I'm doing?

- What do I need to know about using a Pololu driver instead of an UM driver? Anything? I ordered 2 as a short term solution in case I can't repair this one.

Any help would be appreciated!


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Sooo..... I appear to have just fried two stepper drivers and the Extruder 1 connector of my PCB.

Apparently they kill themselves if you put the driver PCB in backwards? (...for the Extruder driver, backwards = the same orientation as the other steppers. Brilliant! Should be a keyed connector there, or an orientation indicator screened onto the PCB...).

Annnnnyway... It seems like I should be able to climb out of this hole I've got myself in by changing the firmware to use Extruder 2 as the primary extruder for the machine when I receive my two pololu replacement drivers.

Can anyone comment on this? Have I possibly fried my board entirely?


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The Pololu drivers did not come with heatsinks.

Supposedly, they (the black versions) are rated up to 1.2A without a heatsink, which is also supposedly around where they should be tuned for an Ultimaker... so I might be ok. The potentiometer seems to be of a different resistance value though, I think 20K instead of 10K. My plan was to just turn the pots until they matched resistance values, but since neither the pots nor the wiring is the same, I need to meter the current.

Can someone explain where I need to meter current when tuning the potentiometer?


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I looked this up a while back. There are several different versions of pololu I believe. Anyway there is a test point (voltage) on or near the pot (it's one of the pins of the pot). You can measure that voltage and there is a formula to convert to current. The formula should be in the specification of the pololu that you bought. If you get the wrong test point it will probably measure at a rail voltage e.g. 0V or 5V or 19V. I think the formula might have been i=v meaning if you set it to 1.2V you get 1.2Amps.

Anyway, google "pololu pot current test point" or something similar and there are tons of great documentation out there.


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AAaahhhhhgghh balls...

Got one of the Pololu drivers installed properly, but fried another in the process. I followed the Pololu instructions, putting the stepper into full step mode (all jumpers removed) and tuning the pot until the test pad voltage was 0.65Vish, but it still wasn't working properly; the movement was very jagged even back in 1/16th step mode. I then started making (...admittedly arbitrary) adjustments to the pot and something fried. I'm not exactly sure what happened - I may have set the current limit way too high, or I may have just accidentally shorted something while ham-fistedly metering the test pad.

Anyway, if other people ever need to do this, here's the pragmatic, low risk set of instructions for installing your pololu driver that I used successfully on the second driver I purchased:

1) Turn the pot fully counterclockwise

2) Install it in the machine... in the correct orientation, you idiot!

3) Turn on the machine and connect it to Cura

4) Jog the machine through the print interface. Listen to the sound the motor makes. It will sound wimpy and quiet.

5) Make a small (1-2°) clockwise turn of the pot and jog that axis some more. Listen to the motor. It will sound less wimpy.

6) Repeat step 5 a few times.

7) Eventually, it will sound less wimpy, and more angry. Dial it back until both the axis you are replacing and your stock driver axis sound equal amounts angry/wimpy.

8) Print yourself some victory.

Now I just need to replace the second stock driver that I broke when I was trying to debug the first broken driver... Still just one stepper driver away from a fixed machine...


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BTW, if you want to heatsink the pololu drivers, I believe this is the correct tiny heatsink:


I have also been successful at twisting off the heatsinks from the dead drivers and cleaning them off my carefully scrapping off the old compound with an xacto knife and then cleaning with 99% alcohol. HOWEVER, before cleaning, note how little adhesive was used.

Then I used artic silver adhesive to attach. Note that you only need a very, very small amount of the adhesive component probably like 1/2 cubic mm or less. You don't want so much that it squeezes out onto the pins. However, I have had success with cleaning the tiny pins of uncured epoxy and a fine bristle brush. At least it hasn't gone up in smoke yet... Start with a tiny amount and see how much it spreads. Adding more is easy, removing excess especially if it gets on the pins, is more difficult.

I used this: http://www.arcticsilver.com/arctic_silver_thermal_adhesive.htm

However, I wanted to use the cheaper alumina product but the store I went to didn't have it: http://www.arcticsilver.com/arctic_alumina_thermal_adhesive.htm

Probably overkill for the "black" drivers but shouldn't hurt anything.


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