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DRV8825 current limiting math issue

Discussion in 'RUMBA' started by AlexCphoto, Aug 12, 2014.

  1. AlexCphoto

    AlexCphoto New Member

    Joined:
    Aug 12, 2014
    Location:
    Seattle, Washington
    Something about my step drivers seems off,.

    I was not able to find the current limiting math anywhere on the reprap wiki "rumba", nor on the reprap wiki" DRV8825" pages, so i searched around, and found the Pololu page for their DRV8825 step stamps (http://www.pololu.com/product/2133). (i also searched here and found nothing)

    on the Pololu page they say that the math is a fairly simple (Current Limit = VREF × 2) as long as the current sense resistors are 0.100Ω which i checked and yes they are on the rumba DRV8825 so even though the brand is different the main chip and the sense resistors are the same,. which as far as i am aware means the math should be the same?,. however,.

    I am using fairly standard (1124090) 1.5A kysan step motors on X,Y,and both Z's and a (ALTN1728S56) 1A OSM step motor on the extruder.

    This should result in a reference voltage (measured at the pot) of 0.75v for the 1.5A Kysans, and 0.5v for the OSM extruder motor... so the DRV8825 with its 2.2-2.5A max should be capable of handling either of these motors. however,.

    The pot on the stepper chip is maxing out at 0.725v which results in a max current limiting control of 1.45A, now technically this should still be enough to move the X,Y, and Z's, and it is moving them,. but they are giving out this horrid whining noise i usually associate with steppers that aren't properly current limited,. and everything i read about the 8825 tells me that its max should be closer to 2.2-2.5Amps,. so you should be able to get the pot to give a max reference voltage of ~1.1v,... (0.725v should not be the max)

    Now even stranger, the E motor that i have set to 0.5v for its 1A current limiting, is barely producing any force at all, so its acting like its extremely under powered with that math,...

    Essentially iv got to have the math wrong,. or,. something is not working properly,.

    Do i have the math correct? any other thoughts?

    I am not seeing any version numbers on my Rumba,. knowing my luck its on the back and ill have to completely de-wire to get to it,.. the DRV8825's state "Cool DRV r2" on top, nothing on bottom.

    FYI it would be really really helpful to have the math for the current limiting pot on the rumba or DRV8825 Wiki page.

    any help appreciated, thanks
    Alex
     
  2. AlexCphoto

    AlexCphoto New Member

    Joined:
    Aug 12, 2014
    Location:
    Seattle, Washington
    Found a short on the board from the factory soldering job, all bets are off once you find a short so,.. n/m,.. going back to the other boards i like better anyway.
     

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