Current limitation for stepper driver



  • Hi there!
    I was "upgrading" my A3 with some drv8825 to replace the A4988, so I measured the vref on those last and they were at 0.68v ==> (on most a4988 I=vref * 2.5) ==> so I was being limited at 1.7amps which is the max rated current of most nema17 steppers but when I looked for the specific nema17 model on the A5 it came up a max rated current of 1.3amps

    So for the time being I put the X stepper to 1.3amps and the 2 Y steppers sharing 1.5amps (I could have put 2.6 but 1st I don't think I need all the torque and 2nd 1.5amps is the max rated voltage for non forced cooling on the drv8825)
    To be honest I haven't yet tested it extensively, but so far so good.

    What do you think? Have you changed your current limitation? Your stepper driver?
    Share your impressions!


  • Developer of T2Laser Software

    @jorgemarmo Check the value of the sense resistor, that will determine the Vref and completely change your calculations.

    The drivers are rated at 1A without a heat sink, then forced cooling (fan) increases it further.


  • Active Member

    This post is deleted!


  • @mrmister sorry, I missed something... I didn't get your answer
    [EDIT]
    Ok, I got you now... It's actually a nema 17
    Here is the reference



  • Hi again,
    I look for the Current Sense resistor on the a4988 included with the eleks, its a 0.1ohm. so making the maths again
    Imax= Vref/(8*Rs) ; Vref =0.68; Imax=0.85amps

    For everyone else reading this, here are some useful links from reprap wiki from pololu

    But then again, should I re-set the drv8825 to 0.85amps or should I leave them as in my first post? Meaning 1.3amp for X (max of the motor) and 1.5amp shared for both Y motors (max I for one drv8825 driver without forced cooling) ???


  • Developer of T2Laser Software

    @jorgemarmo That's better, now you have the correct calculation. The drivers are 1A without a heat sink or forced cooling, 2A with both.

    http://forum.eleksmaker.com/topic/404/burnning-motors/2



  • @Zax so in that post you suggest 0.875amps and 1.75amps?? My both controllers were at 0.85amps


  • Developer of T2Laser Software

    @jorgemarmo No, I recommended a Vref of 0.7V which would be 0.875A for the Y-axis (2 motors) and half that for the X-axis.



  • @Zax thank you very much again. And sorry for the last question, I assumed a different value for your Rcs. It's all clear now.



  • So yesterday I played with my A3.... Between several other modifications I tryed the current settings proposed by Zax, but I found the X axis getting stuck on a "hard point" of the rail. So I set both controllers to 0.85 amps.
    I don't know why I have this "hard point" and is just slightly harder, but enough to make the stepper slip with 0.42amps.. seems to be working fine with 0.85A
    And I just wonder is the fact of using 1/32micro stepping (DRV8825) instead of 1/16 (A4988) might turn the stepper more sensitive to slip...


  • Developer of T2Laser Software

    It could be, but to be honest 1/32 is way too high resolution for these machines.

    You are trading torque for precision and top speed, and since the mechanical resolution is lower than the motors you aren't gaining anything.



  • Yes, I thought so as well, but I couldn't find a way to set the drv on 1/16 without having to cut the MS pins (or even worse, removing the tracks on the Mana) is there a "soft" way to ut the drv on 1/16?


  • Developer of T2Laser Software

    @jorgemarmo If the driver doesn't have dip switches then you would need to cut traces.


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