Setting the right current limit - A4899 Stepper Motor Driver Board

Hi! I bought these cheap stepper motor driver boards of Aliexpress: A4988 Compatible Stepper Stepstick Motor Diver Module With Heat Sink For 3D Printer Controller Ramps 1.4(Pack Of 5Pcs)|Motor Driver| - AliExpress

The last thing i need to do, before running the stepper motor, is setting the current limit by adjusting the potentiometer on the board in order to get the right VREF-value. I saw this article about adjusting the current limit: VREF adjustment A4988 - E3D-Online. It says the VREF is equal to: Rated motor current x 8 x Rsense

The rated motor current is 1.5 Amps as far as i can see from the motor specifications i got with the motor (see attachment: MotorSpecifications), but the Rsense-value is what i can’t find. On the driver’s chip, there is written “HR4899sq B3642A” i managed to find a datasheet for it: HR4988SQ datasheet(3/31 Pages) YONGFUKANG | Micro-stepping motor driver chip with built-in converter and overcurrent protection, but it doesn’t state the value as far as i can see. Is it related to the boards resistors and not the chip? If so, does anyone know how? - I can’t find anything about it. Theres a picture of the board attached (see attachment: DriverPicture)

Hope someone can help me, thanks in advance!

Pololu has instructions for setting the current on their A4988 page.

This rep rap page has more info on the Chinese A4988.

The sense resistors are the ones labeled R100 (0.1 Ohm).

Thanks for the reply!

groundFungus:
The sense resistors are the ones labeled R100 (0.1 Ohm).

According to the links this resistor is what defines the Rsense-value right? That means my value is 0,1 and thus my VREF-value should be 1,5 x 8 x 0,1 = 1.2V as far as i understand - Thankyou!


Correct me if I'm wrong, but as far as i understand, this VREF-value should be the maximum value for this motor, but it could always be lower if I need to run it cooler and not much torque is needed right?

Correct me if I’m wrong, but as far as i understand, this VREF-value should be the maximum value for this motor, but it could always be lower if I need to run it cooler and not much torque is needed right?

Yes, that is right. Be aware that a heat sink and active cooling (fan) is recommended for coil currents above 1A. The DRV8825 driver can supply up to 2.2A (heat sink and cooling required over 1.5A).

Have you measured the voltage drop across one of the motor windings ?

Have you measured the voltage drop across one of the motor windings ?

Hi, sorry I didn't see your comment before now... Why would I need to measure the voltage drop across the motor windings? As far as I know, the voltage drop differs when the motor is running due to back-emf, induction and so on. That's why I need a driver, to regulate the AMPs -right?

Thanks for the feedback & help btw!

current sense values

current sense values

Oh I see. You are referring to the first method the Pololu-page mentions. Though I find it a little confusing because of the "70% of actual current limit"-thing. I will just use the second option and calculate the VREF and adjust potentiometer from that.

But thanks for the help!

Didn't you say this: ?

but it doesn't state the value as far as i can see. Is it related to the boards resistors and not the chip? If so, does anyone know how? - I can't find anything about it

The values are the link I posted.

VREF=8⋅IMAX⋅RCS

Yes exactly - I wanted to find the Rsense value (or RCS in the equation above). GroundFungus told that it was found on the boards resistors, which the Pololu-page also explained - so everything should be fine!

Sorry, I just got a little confused when you asked:

Have you measured the voltage drop across one of the motor windings ?

As if I was missing something in order to run the motor correctly - but the potentiometer should be tuned right when the VREF value on the board is measured & set to 1,2V (or less) with my motor and board. :smiley: