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
    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.

  • @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.

Log in to reply

Looks like your connection to Offical Forum was lost, please wait while we try to reconnect.