Stepper Driver Vref adjustment help

I am building my first printer (Folgertech Kossel 2020) and just completed the building portion. I am attempting to set the Vref on the stepper drivers and am unable to attain the correct voltage necessary.

The build manual says to set to 350 mV for print head steppers and 550 mV for the extruder stepper. Just before the pot hits the dead zone and voltage drops to zero the maximum voltage I am getting is ~300 mV for all of the drivers on the RAMPS 1.4 board. The power supply is outputting 12.2 VDC. I rechecked all of my wiring and it all seems to be correct with all jumper caps installed

Any help would be much appreciated!

in my folgertech... 350mv was way too high and warped the corner pieces because they got too hot... i would keep it lower and then make them stronger if they start slipping

what do you have the driver for the extruder set at? Even that driver will only go to ~310 mV

I have been able to go from 0 to at least a volt. This is based on the 5v supply, so if the reg is bad, or an endstop is shorting it out, you may not get there. Heck, you can set Vref with nothing but a USB connection . . . 12v isn't involved.
I run .35 on the axis, and .9 on the extruder, same printer . . . .35 does run hot, but comes in at the rating of the motor. I added heatsinks/fans to the axis motors to deal with the PLA corners until I can get metal ones.

The other possibility is that you are reading from the wiper on the pot to the wrong pin . . . try it to the negative power input terminal on the RAMPS.

Can you connect to the Arduino, or get anything else to work, like motors to move? Drivers in backwards will screw this up as well . . .

- Tim

Edited 2 time(s). Last edit at 07/28/2015 11:16PM by tadawson.

I am reading to the correct pin, I wish it was that stupid of a mistake...

I did try to use the power input terminal as my ground as well. Everything is in correct, I even tried with the steppers/switches unplugged to ensure no shorts/bad wiring.

With the drivers on the board and not powered I am getting 7.77 k ohms where the voltage should be max. If you get a chance to check yours I would greatly appreciate it. I don't believe this to be the actual pot resistance but it does change as I turn the pot and at least gives a place to make a comparison.

Hopefully though this will all be inconsequential as the arduino is shot. Even with shield taken off the board it does not power up and sets off a high usb power alarm on my pc...I have a couple of megas at work I will try tomorrow and take it one step at a time to ensure that is the problem and not something with my wiring which I have checked about a dozen times.

I will update tomorrow after trying new board.

Thanks for your help

Reading to where? When at max on the pot, from the wiper, you should see 20K resistance to the 5v line (as per Pololu schematics - who knows what the Chinese cloners have changed) and 0 to ground when at minimum.

High USB power error indicates a short on the 5v rail, which will cause this - low 5v, low Vref, plain and simple. I'm not sure what you have plugged in, but if you have not tried it, unplug everything and see if you still have the problem (including stepper drivers) and if things then work OK, reintroduce things one at a time until you find the problem. No matter how you power the Arduino, 5v to the RAMPS comes from it. If still bad, separate the Arduino and RAMPS and see if you can talk to the Arduino . . . if you can, the problem is on the RAMPS, and you should look for shorts on the 5v line (it doesn't go too many places - the endstops and servos are the only off-board presences I recall, and the jumper to route it to the servo ports vs. external 5v). If the Arduino still fails, try the other one, but be careful - if a short on RAMPS killed the 5v reg, the new one may be at risk as well, so test quickly . .

An endstop connector turned 180 degrees will do this in a heartbeat on this kit . . . the normally closed lead goes from +5v to ground in that case, and not sense to ground as intended.

[www.pololu.com]

- Tim

Edited 5 time(s). Last edit at 07/29/2015 12:01AM by tadawson.

It's a shame that Folger, like most (but not all) kit manufacturers, ships ancient Arduino/RAMPS electronics, instead of modern electronics that let you set the current in software. All for the sake of saving $20. Lots of RepRappers have difficulty setting the the right motor currents, and some blow up the drivers in the process.

If it was just one pot going abruptly from 300mV to 0V then I would say it was a bad pot. But it it is all of them, it must be something else., or a whole batch of bad pots.

---

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].

Tim
thanks for the help, when I get home tonight with a new mega I am going to power the mega through pc and start adding components one at a time. The connectors they supply for end stops only connect one way (I will check to make sure they are all the same).

If you have the same Folger parts I got, the only keyed connectors on the endstops are on the endstop end - the three pin connnectors will fit the RAMPS either way, since bare three pin headers cannot be keyed . . .The 5v line is the row of pins farthest from the board edge, and if your cables match mine, that would have the red lead to the inside of the RAMPS, black in center, and signal (white?) to the edge . . . In any case, pulling these off and coming to life would confirm this is the problem in any case, and never assume that the endstop harnesses are correctly built, verify, verify, verify! You can also try putting black/white(? - not red) on the outer two pins, and leave the red in mid air. If the endstops then work, you have verified that at least the NC endstop contact works . . . the 5v is only used to run the LED on the endstop itself . . .

And check the old Arduino on your computer alone . . . it may well be fine.

- Tim

Edited 2 time(s). Last edit at 07/29/2015 01:51PM by tadawson.

The pins are oriented per the build manual online on the ramps board. Will confirm endstop wiring tonight. Even with everything disconnected however and trying to set vref it is the same issue.

When connected to pc with power supply off:

arduino with ramps with everything connected: doesn't recognize arduino in ide (usb device error but no power warning)

Arduino only: usb power warning

Was that the new or old Arduino that gave the warning? Or both? Certainly sounds like a 5v fault on the Arduino, unless both did it, in which case I'd question the computer being used as well. A blown 5v reg on the Arduino could well cause that . . . not sure how the regulator and USB 5v are isolated, if at all . . .

And never assume the manual is absolutely right . . . likely, based on other evidence, the endstop wiring is OK, but I hit that snag as well, and unplugging them is an easy test, hence my suggestion.

- Tim

Just a quick update...Thanks for everyone's help and input. The arduino voltage regulator that I received from Folger Tech was bad. I replaced the mega and all is well. I was able to set Vref to the correct voltages and am about to upload firmware and go through calibration process.

Wonderful! Have fun with your printer!

- Tim