Trimpo turning freely on Stepper Driver - what now?

Hi, I am new to 3D printing. I just finished assembling a Prusa i3 Steel.

I've had all axis working - but the stepper drivers (DRV8825) tend to get very hot and become unresponsive.

When trying to adjust the trimpo it was turning freely and I couldn't find the starting position. How do I manage to adjust the drivers like that? When I tried to adjust the x-axis "blindly" I experienced some rather random behaviour - X always went to home position, but refused to move to the right. I didn`t turn everything off before adjusting - hope I didn't ruin anything.

I would really appreciate some ideas about how to do this without causing any destruction...

Best regards,
Anna

Edited 1 time(s). Last edit at 02/02/2016 03:10PM by Anna & Karl.

Hi Anna,

Do you have a volt meter / multi meter?

The trim pot is for adjusting the current that is fed through the stepper motors. The voltage coming out of the trim pot is proportional to the stepper current. The desired stepper current depends on your exact motors but for nema17 it is usually between 1-2 amps.

I don't have specific experience with drv8825s as I use 498x drivers but the principle is the same, according to this guide you should set the trim pot voltage to 0.5v to give 1 amp of stepper current, which is a good place to start.

So, you need to connect the common / black probe of the meter to the -Ve power connection on your board - ideally attaching it so it stays on without holding.

Make sure your meter is set to DC voltage in a suitable range e.g. 0-20v. Test it on the power input to make sure it reads close to 12v.

Then, for each driver, carefully hold the red probe on the middle pin of the trim pot (you need the board to be powered). Then slowly adjust the trim pot to get close to 0.5v - I would aim to be between 0.45-0.55v if not closer. Note that turning the trim pot clockwise will probably decrease the voltage, which is kind of back-to-front!

If the stepper current is too low, you will get missed steps, which can end up with misaligned layers or lost extruder feed.

If the stepper current is too high, your steppers can overheat and the drivers might shut down.

When the current is right, it is normal for the steppers to get quite warm but not too hot to touch.

It is ok to adjust the trim pots with power on - in fact you have to - and you probably haven't killed anything.

Be very careful, when poking around with the meter probe, not to short any connections together by touching 2 at once!

Good luck!

Wow, thank you for the detailed explanation! This will be my next step. I really hope to get this thing printing soon!

Best,
Anna

That helped - everything moves smoothly now.

Wow that was easy! Now for the next 200 problems

Yes, we're up to our neck in problems - some of the selfmade, RTFM type, and other, truly bewildering ones - plus the problems caused by a impatient 12yo, who made me resolder the Mini USB today after falling over the cable - I didn't even know this could be done by mere mortals, but it looks I saved the board. Will try tomorrow if it still works...