How to set the current limits correctly

Talk about assembling your printer

How to set the current limits correctly

Postby erl » 2012-Dec-Thu-16-Dec

If you have a multimeter, you can set your current limiting potentiometers to the theoretically correct values for the stepper motors by connecting your multimeter ground (black) probe to one of the unused connectors with black wires going from the power supply to the PrintrBoard, and measuring with the multimeter's positive probe at the potentiometer terminals shown in the PrintrBoard image below (red, X, should be 440 mV, yellow (Y and E) should be 400 mV, blue (Z) should be 825 mV:

Image

Works for me, and the motors run without getting too hot.

The background for those interested

The Printrbot X, Y and E stepper motors are according to the bill of materials here of type 42BYGH4807. The Z steppers are of type 42BYGH011.

The X, Y, and E stepper datasheet specifies their current as 500 mA per phase. The Z stepper datasheet lists the current as 330 mA.

The PrintrBoard uses Allegro A4982 stepper motor drivers. The datasheet says that:
The maximum value of current limiting is set by the selection of RSx and the voltage at the VREF pin. The transconductance function is approximated by the maximum value of current limiting, ITripMAX (A), which is set by
ITripMAX = VREF / ( 8 * RS)
where RS is the resistance of the sense resistor (Ω) and VREF is the input voltage on the REF pin (V).

The sense resistors (RS) of the PrintrBoard (except for the X stepps for some reason) are 0.1Ω, and VREF is set by the pontentiometer. For the X, Y, and E steppers, we want an ITripMAX of 500 mA. Solving the equation gives a VREF of 0.4V for Y and E, and 0.44 V for X

For the Z steppers, we have two steppers each wanting 330 mA, which gives us an ITripMAX of 660 mA. This gives us a VREF of 0.66 / (0.8) = 825 mV (0.825 V).

Good luck, have fun!
  • 3

erl
Loading filament...
 
Posts: 13
Joined: 2012-Oct-Thu-01-Oct
Reputation: 3

How to set the current limits correctly

Sponsor

Sponsor
 

Re: How to set the current limits correctly

Postby lwalkera » 2012-Dec-Thu-16-Dec

All of the stepper drivers should have the same sense resistors. If you don't then your PB is defective or was assembled incorrectly.
  • 0

Laine Walker-Avina
Printrbot Beta-tester and Printrboard Designer
lwalkera
Honeycomb infill 25%...
 
Posts: 291
Joined: 2012-Feb-Sat-04-Feb
Location: Folsom, CA
Reputation: 33

Re: How to set the current limits correctly

Postby erl » 2012-Dec-Fri-05-Dec

lwalkera wrote:All of the stepper drivers should have the same sense resistors. If you don't then your PB is defective or was assembled incorrectly.


I have not measured my resistors; I went by the schematic here (linked from the RepRap PrintrBoard Wiki page) which shows R17 and R18 (X current sense resistors) as 0.11Ω, and R16, R19, R24, R25, R30 and R31 (Y, Z, and E) as 0.1Ω. That is the Rev B version, I guess I should check the Rev D.

In any case, I don't think a 10% deviation will be very important.

Do you know what the actual resistor values are?

/Erland
  • 0

erl
Loading filament...
 
Posts: 13
Joined: 2012-Oct-Thu-01-Oct
Reputation: 3

Re: How to set the current limits correctly

Postby lwalkera » 2012-Dec-Mon-13-Dec

They should all be the following or similar: http://www.digikey.com/product-search/e ... S.11FCT-ND The values on the schematic are mainly for indication, the actual values used are on the BoM here: https://docs.google.com/spreadsheet/ccc ... 3FLb3dmTUE
  • 0

Laine Walker-Avina
Printrbot Beta-tester and Printrboard Designer
lwalkera
Honeycomb infill 25%...
 
Posts: 291
Joined: 2012-Feb-Sat-04-Feb
Location: Folsom, CA
Reputation: 33

Re: How to set the current limits correctly

Postby Alexcheve » 2013-Jan-Wed-16-Jan

My Printrbot plus kit came with five motors all with same numbers, I received it a couple of weeks ago. I have seen other posts suggesting to turn all potentiometers counterclockwise to "four oclock" but since I have not been able to print a thing mine are quite cold for the moment. And motor cable connectors can connect in both directions to board so it is difficult to find the right side.
  • 0

Have Printrbot+ kit struggling assembly from January 12 to 18, use ABS for now. Cracked glass with three bolts with strings at HotBed. Am not able to print a thing yet but getting closer thanks to you.
User avatar
Alexcheve
Loading filament...
 
Posts: 21
Joined: 2012-Dec-Tue-00-Dec
Location: Queretaro, Mexico
Reputation: 0

Re: How to set the current limits correctly

Postby jeromedayton » 2014-Jan-Sun-06-Jan

I've read elsewhere to not use conductive tools to adjust the pots as you might short some pins. So I ordered nylon tools, "anti static alignment tool kit", from GC Electronics and discovered that they are made of "conductive nylon" to deal with possible static on the board. So have I ordered the wrong kind of tool?
  • 0

Jerome
Retired Software Developer
Printrbot Simple operational since Xmas 2013
Printrbot Jr v 2 with alu extruder March 2014
User avatar
jeromedayton
Waiting for extruder temp...
 
Posts: 82
Joined: 2014-Jan-Fri-15-Jan
Reputation: 2

Re: How to set the current limits correctly

Postby RetireeJay » 2014-Jan-Sun-07-Jan

The "conductive" feature for anti-static actually means resistance in the megohms, not ohms, so you should be fine. Or, you can make adjustments with the power off if you really want to be careful.
  • 0

Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3
User avatar
RetireeJay
My next printer is...
 
Posts: 4936
Joined: 2013-Jan-Wed-13-Jan
Location: Greenville, SC
Reputation: 495

Re: How to set the current limits correctly

Postby REPRAP SQUAD » 2014-Jan-Mon-11-Jan

Alexcheve wrote:My Printrbot plus kit came with five motors all with same numbers, I received it a couple of weeks ago. I have seen other posts suggesting to turn all potentiometers counterclockwise to "four oclock" but since I have not been able to print a thing mine are quite cold for the moment. And motor cable connectors can connect in both directions to board so it is difficult to find the right side.


There are lots of diagrams etc that show motor plug direction etc. The 12o clock setting that comes stock is a good place to start but some fine tuning will help your motors run to the best of their abilities. I've turned some down and some up but all of my motors run nice and cool.
  • 0

Twitter: @REPRAPSQUAD
Check it out: Build logs, 3D printer news and all things Printrbot--http://reprapsquad.wordpress.com
PC Case Mods 3D printed:
http://www.enviousmods.com

RepRap Squad's The 1st Simple (XL)
* Custom built that towers over the LC

Printrbot Plus V2 W/
*Dual extruder PB Version (2-Ubis 1.75mm hotends)
*Heavily Modded and all axis extended
User avatar
REPRAP SQUAD
Metal hotbed butt
 
Posts: 1629
Joined: 2013-Feb-Sun-11-Feb
Location: Pacific NW Oregon-US
Reputation: 71

Re: How to set the current limits correctly

Postby Hadesblue » 2014-Feb-Sun-23-Feb

my pots are way off, reading from the left.
786mv, 825mv, 1137mv, 1111mv
  • 0

Hadesblue
Levelling print bed...
 
Posts: 30
Joined: 2013-Mar-Thu-20-Mar
Reputation: 0

Re: How to set the current limits correctly

Postby Hadesblue » 2014-Feb-Wed-21-Feb

just for reference purposes this is what happens when the current limit is too low, and steps get skipped. I had to increase Y slightly higher than 400mv, probably because I'm using the aluminum build plate and it needed more power.
  • 0

You do not have the required permissions to view the files attached to this post.
Hadesblue
Levelling print bed...
 
Posts: 30
Joined: 2013-Mar-Thu-20-Mar
Reputation: 0

Re: How to set the current limits correctly

Postby H315M4N » 2014-Apr-Tue-07-Apr

I'm glad I found this post. Instead of measuring 440, 400, 825, 400 mV across the four pots, I was measuring 850, 650, 1450, & 1425 mV. No wonder my motors got so warm! It runs quite a bit quieter now, too.
  • 0

H315M4N
Waiting to connect to printer...
 
Posts: 2
Joined: 2013-Apr-Mon-16-Apr
Reputation: 0


Return to Assembly talk

Who is online

Users browsing this forum: No registered users and 1 guest