# A4988 Question

Hello , as i checked how to set current limit for A4988 from here pololu.com/product/2128 i see this formula Current Limit = VREF × 2.5 ,
My question is where 2.5 coming from ? , is it resistance of motor coils ? or what ?
im using 2 different stepper motor in my 3dprinter one with 6.2 ohm resistance coil and 1.2A 4V and another one 1.9 ohm resistance coil and 1.68A 2.8V , so i think that formula with 2.5 not working for me .

[quote]so i think that formula with 2.5 not working for me[/quote]What evidence do you have that the formula does not work?

My Main question is where 2.5 coming from ?
"Another way to set the current limit is to measure the voltage on the “ref” pin and to calculate the resulting current limit (the current sense resistors are 0.05Ω). The ref pin voltage is accessible on a via that is circled on the bottom silkscreen of the circuit board. The current limit relates to the reference voltage as follows:

Current Limit = VREF × 2.5"

i guess for different resistors 2.5 in formula will change

Hello.

The 2.5 multiplier is determined by the value of the resistors on the board used to measure the current. It does not depend on any of the properties of the stepper motor attached.

-Nathan

[quote=“nathanb”]Hello.

The 2.5 multiplier is determined by the value of the resistors on the board used to measure the current. It does not depend on any of the properties of the stepper motor attached.

-Nathan[/quote]

Thank you , is it any formula to calculate that

[quote]any formula to calculate that[/quote]Yes. You can read about the current sense operation in the A4988 data sheet, while consulting the schematic for the A4988 driver board. See the section “Internal PWM Control” on page 9 of the data sheet.