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Heated bed z-motor current

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Hello,

I upgraded my UMO with the UM heated bed recently. It works fine and printing ABS and PET works pretty well. With the new z-stage a new motor is shipped that obviously is different from the original one as the z-steps/mm is much different than for the original one.

But the new stepper is connected to the old driver and the z-motor is getting very hot (around 80°C during longer print). I wonder if not only the steps/mm need to be adjusted but if also the motor has a different specification regarding current? Especially as the z-motor doesn't move a lot I'd expect it's staying much cooler than the x/y-motors (they are around 70°C on longer lasting prints).

Is there a comprehensive guide showing how to measure the stepper current with a voltage meter? I've seen how to change the current but not how to measure it (which pins and what voltage is correct, and under which conditions - motor moving, keeping in place). I got the UMO with board rev 1.5.7 and the drivers shipped orginally.

 

Thanks in advance for any help!

- Christian

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You can google "pololu current" and there are articles out there in the general reprap community that are better than discussion of this on the forum. There is a test point on or near the trim pot (depending on which pololu you have) and there is a formula to convert that voltage to a current. That is a reference voltage and the driver is careful to never go over that mount of current. I would disconnect the servo, measure the voltage and turn the trim pot the tiniest possible amount such that the value goes lower (lower voltage is lower current) and then continue in that direction and lower the voltage maybe 10% or so then connect it to the stepper. Don't do this while connected to the stepper as you can blow your stepper in well under a second if you turn the current up too much. Many people who mess with this have to order a new pololu (not expensive). That's probably why they are socketed.

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Thanks for your help and for the links.

I've just ordered a couple ot the A4988 drivers, as one was dead after the last print this morning. Think I understood how to measure the voltage and will do it now on the remaining ones to see to what they are set.

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I measured the voltage and it was at around 0.64V on the remaining ones.

Then I got the idea to put the driver from the X stepper to the not working stepper and noticed that it is not working anyway. I switched back the working driver to the X stepper and it works still. Then I've connected the non-working stepper to the X stepper connector and it works as well. Then I took the driver I thought it's dead to the X stepper and it doesn't work.

So it seems that not only the driver for the extruder is dead but also the extruder part on the main board or the Arduino is broken. Do you have any idea how I figure out what isn't working?

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I plugged in a new Arduino board and installed the firmware. It's still the same problem. The extruder stepper works on the x or y-stepper connector but when I put the x stepper driver to the extruder and connect the working stepper on the extruder port it does not move at all. As the Arduino has been exchanged I assume the only thing left could be the main board. But I don't see anything that could break here. No melted path on the circuit, the capacitor looks fine (cap isn't beaten), resistors around the socket look good. I don't assume the voltage regulators could be connect to this.

Any idea please?

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I got the Pololu Black stepper drivers now and want to set the current limit. Just to make sure I'm not frying the next driver or stepper:

Pololu writes the current limit is calculated by Vref * 2,5. The stepper for the extruder is rated to 1.68A max and I read it should be set to something around 1.0 to 1.2A. They write on their webseite that in full step mode as used for the extruder the current is limited to 70% and that it needs to be included in the calculation.

So considering this I should set Vref to (1.2A / 2.5)*100/70 = 0,68V (or 0,57V for 1A)?

Edited by Guest

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Ok, I got it working. I just used the unscaled value not considering that 70% setting for the beginning and set it to 0,48V and did some tests. I also installed one of the Pololu black drivers on the z axis as this one got very hot from the beginning. I've set it to the same voltage and it doesn't get hot anymore. The steppers also got less loud now.

It would be nice if somebody could confirm whether the 70% calculation is valid for full step mode or not.

Thanks again for you help!

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I don't know anything about the 70% but UMO and UM2 use microstepping - 16 micro steps for all servos except the Z axis. On the UMO Z axis is 8 micro steps (you can set this with jumpers under each stepper - 16 steps should be quieter but I never messed with that).

So since we are using microsteps does that mean you don't need to do the 70% part?

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You're right. And even if it is for full step none of the jumpers is set to full step so it's not relevant. I did some further tests with fast retraction and it still works properly without running the extruder stepper even hot. I would say it is getting to 40-45°C after an hour printing.

Many thanks!

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Being honest I'm not sure why the z-motor got so hot. I was thinking that this motor had to do the smallest part of the work and does not need much power at all. But it got hotter than every other motor.

I tried to find out whether the old motor for the non-heated bed requires different current settings but I couldn't find anything. But now when I've set the current to 1.2A it does not even get very warm (maybe 30-35°C after an hour printing). I've never check whether the original motor got hot as well. However, with the 1.2A setting it works fine for me so far.

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By the way I'm having trouble with the heated bed as it doesn't hold the high temperature for ABS very well. When I prepare for ABS and heat up to 100°C it reaches this temperature after some time. It holds it during printing for a while but after 20 minutes or so it goes down to 95 and doesn't get up anymore. I just cancelled a print this morning because the ABS warped a lot. I've set the temp to 110°C now and it's the same. It heated up to 110°C first but now after 20 mins it went down to 102°C. I tuned it to 120°C now but it stays at 102-103°C and doesn't get higher.

I've even closed all sides of the printer with baking paper and some parts on top to keep the heat inside better and prevent draft. I'm using both power sources as I thought it's because the new one doesn't deliver enough. But it seems not related to it. The UltiController shows the current and the desired temperature properly.

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I'm not so sure about if this is wanted. I notice that many people do cooliing the steppers. I've changed my extruder to the Rollerstruder from MoonCactus on thingiverse and set the extruder current to 1.2A the same as for the z stage and now the extruder stepper doesn't get over 40-45°C anymore.

 

Even the second feeder motor on my umo+ gets that hot or more, and the motor isn't even moving. I suppose that's normal.

 

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Airflow should make it even worse as the bottom of the printer (under the bed) tends to stay at room temperature but if you heat that also you will need even more watts to keep the bed warm.

I don't know why your bed isn't up to temp but it's probably fans related - when you start printing the fans come on and cool the glass. For ABS you don't need much fan - the older "abs" slicers kept the fan off 99% of the time and only turned them on for a few seconds during printing bridging or overhangs.

Anyway I found enclosing the UMO very well - especially the sides - and letting the whole thing heat up to air temp at least 40C - makes it so the bed doesn't have as much trouble staying at 100 to 110C. Also put a box over the top (with large hole for the bowden).

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