a4983 setting current limit

I have read the product pages, spec sheets etc, but I just am not picking up the math correctly. the stepper drivers I am using are the a4983 pololu 1201 carrier boards.

My understanding is that actualCurrent * .7 = meteredCurrent

also, the stepper’s rated current is what can be applied to both coils? not a combined output?

the stepper I am using is 1.68a stepper, using that math, setting the driver to a metered current of 1.2, actually means I am actually supplying 1.7a of current?

Hello.

This is not correct; the current you are measuring is the “actual current”. The relationship is:

Current Limit Maximum * 0.7 = Measured Full-Step Coil Current (one coil)

where Current Limit Maximum is the maximum current the stepper motor driver will allow through either coil (this value is called “ITripMax” in the datasheet). When in full-step mode, neither coil will ever see this full current because both coils are being limited to 0.7 times this value. When using microstepping, however, there are steps where one coil will be on at this maximum current limit and the other coil will be off. Basically, what the stepper motor driver is doing is keeping the magnitude of the current vector constant at this maximum value. At each step, the coil currents are selected such that they satisfy the following constraint (if possible–if your current limit is set too high, the coil currents will never reach the limit):

(coil1 current)^2 + (coil2 current)^2 = (current limit maximum)^2

Does this make sense?

The way you phrased this isn’t so clear to me. I would phrase it as: the stepper’s current rating applies to each coil independently. If your stepper has a 1.68 A rating, each coil can safely carry up to 1.68 A, independent of what the other coil is doing.

- Ben

Hello, ofcsilencer,

It seems that Ben edited and overwrote your post by accident, instead of quoting it in a reply as he probably meant to do. I have restored what you wrote originally (from Google’s cached copy of the page) and split Ben’s reply into its own post. Sorry if this caused any confusion.

- Kevin

Thank you! Even though I couldn’t phrase it very well you still got me the answers I was looking for. Thanks again!