# Basic Current Limiting Doubt - A4988 Microstep

Hi, is my first time dealing with motors and drivers, and I’ve already read some guides and questions to introduce myself and understand some theory, however, there’s some doubts that I have yet (maybe can be very basic for you):

Vref= Imax*(8*Rs)
I’m trying to understand how to set the Vref by this formula when I’m planning to use the 1/2nd or 1/4- 1/8- or 1/16-, -th microstep mode. I know that I’ve to calculate the final Vref multiplying the Vref of the formula by 0.7 as the Table 2: Step Sequencing Settings of the A4988 Datasheet says but what I’ve understood is that that factor (0.7) is for the full-step mode (maybe I’m wrong) and in my case I want to use the microsteps.

Values:
Imax=1.7A (from the Datasheet of my NEMA17)
Rs= 0.1 Ohm (R100)

Another doubt is that I currently have a Bipolar NEMA17 stepper motor with 1.7A/Phase, I know that the chip itself can handle up to 2A with sufficient additional cooling (that I will implement), considering that, do you recommend using the DRV8825? In that case, could you specify the calculation for the microstep mode too, please?

My application is the extrusion of a liquid with low viscosity, I think that it wouldn’t need to much torque so I can change the motor later but is what I’ve at the moment, but the microstep mode is very relevant (and learn how to set the Vref values in that mode).

Thanks

Hello.

When microstepping, stepper drivers typically use up to 100% of the full-scale current limit set with VREF, so you do not need to reduce the VREF setting below the rated current of your motor. (When full stepping, many drivers use only 70% of the full-scale current, so you can increase the VREF setting in that case.)

Since you are microstepping, I recommend setting the current limit so that the full scale current (Imax) is equal to the rated current of the motor, unless of course that is more current than your driver board can handle.

For a stepper motor with a 1.7A current rating, I would suggest using our one of our TB67S249FTG driver boards (either the full breakout or the compact carrier should be fine). You can find their current limiting equation on the product page.

- Patrick

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