# What is the correct method to set the current limit?

My motor (17HS4401) have 1.7A as rated current

and i want to know the correct method to set Vref :
1-Method
1.7A / 2 = 0.85v (vref)

2-Method
The motor use the full step mode so the current will be 0.7 times (70%) the current limit :
current limit = 1.7A / 0.7 = 2.43A
Vref = current limite / 2 ==> 2.43/2 2.43 = 1.21V

3-Method
vref = Inom * 5 * Rsense *0.7 so Vref = 1.7 * 5 * 0.1 * 0.7 = 0.6V

what do you think?

You did not specify what stepper motor driver you are using; however, from your previous posts and the equations you used here, it sounds like you are using a DRV8825 carrier.

If that is the case, your first two equations look correct (depending on if you are accounting for the 70% in full-step mode); however, as Derrill mentioned in your previous thread, the DRV8825 carrier should be limited to 1.5A per phase without additional cooling. A VREF setting of 0.75V would set the current limit to 1.5A per phase. If you are only using full-step mode, you can increase the VREF to about 1.05V. This results in a current limit of 2.1A per phase, but since both coils are always on and limited to approximately 70% of the current limit setting in full-step mode, it only ever reaches about 1.5A per phase.

By the way, to correct your 3rd equation, you would need to move the 0.7 to the VREF side of the equation (VREF × 0.7 = I × 5 × Rsense). Then it simplifies down to your second equation.

Yes, I use DRV8825 with additional cooling.
Please can you tell how to calculate vref in my case: bipolar stepper motor (17HS4401) 1.7A as rated current like mentioned in the datasheet

The equation stays the same; if you are adding cooling, you will need to figure out what current limit to set based on your setup and what it can handle.

Brandon

But wich method i will use , my motor is bipolar and have 1.7A as rated current

The difference between what you labeled as method 1 and method 2 is just whether or not you account for the driver only using 70% of the current limit when in full step mode. I recommend starting with the lower current limit (i.e. the value calculated in your 1st method) and testing to see if that will work for you. If you need more torque for your system and are only using full step mode, raising it to the value you calculate using your 2nd method might help.

Brandon

For the same motor but with a A4988 driver would the formulas be similar?

Thanks

Hello, Sanyamo.

You can find the VREF formula for current limiting for our A4988 carriers in the “Current limiting” section of the driver’s product page.

Please note, other manufacturer’s drivers might have different formulas based on the components they use on their carriers.

-Derrill

Hi there I’m new to this forum and Arduino/stepper motors somewhat, have a few questions if anyone can help please.

1. I have a DRV8825 stepper driver and the same 17HS4401 stepper motor as above with a 12v,
2 Amp power supply connected to the stepper driver, and have calculated the vref voltage at 0.75 however I’m using 1/8th steps and I’m having a few issues with missing steps randomly and also a lack of torque when driving the motor (its driving a turntable and if there is any (even the smallest) type of resistance it hasn’t got enough oomph to keep driving it and slips past then speed sup again. is there a safe, sensible way to increase the torque and reduce the missed steps? someone mentioned to me increasing the vref or increasing the driving voltage of the power pack to 36v but I’m not sure f the stepper drive for my motor for that fact will support that. hope some one can help me please as this is a project for my local club and I’m struggling at present. cheers Kevin

Hello, Kevin.

As you mentioned, increasing your supply voltage could also help get you more torque while the stepping since the current will be able to ramp up faster. It is fine to use a higher voltage than the stepper motor is rated for as long as you are using a current limiting driver like the ones we carry.

Please note that microstepping can significantly reduce how much torque a stepper motor can apply per microstep. Here is an application note with more details about that. So, if you still have problems with a higher supply voltage, you might just be nearing a limit of what your stepper motor can do while using a 1/8 microstep. You could try reducing that to 1/4 or 1/2 to see if it helps.

Brandon

HI Brandon, many thanks for your reply that’s very helpful , I will try a 36v supply I think i have one here.
Can I ask if you were putting a solution together than needed the 18th microstepping, and a Nema 17 style motor, what motor/brand/specification would you recommend/choose?

Many thanks Kevin

There are a lot of factors that go into selecting an appropriate motor, many of which depend on your particular application, so I do not have a specific recommendations. In general, the performance metrics and specifications in the manufacturer datasheet for stepper motors (such as a pull-out torque curve) are probably the best approximation of what the motor is capable of.

Brandon