Hello,

I want to pilote a stepper motor with microtsep, and,that’s why, I want to use a kind of A4983/A4988 like this one:
pololu.com/product/1201

I have read several topics about these boards, but I don’t have the answers at my technical questions.
In fact, I don’t understand how we configure the current limit.
Indeed, my stepper motor have a phase current of 0,12 A and a phase resistance of 80 ohm.
Moreover, I would like to use VDD=3.3V…
So, if I understand well, I have to put the driver into full-step mode and to measure the current running through a single motor coil without clocking the STEP input. I have just to tie my ms1 at the low voltage since MS2 and MS3 are pull-down resistors and I have to measure the current flowing in one coil.

So I wonder what is the difference between the two ways to set the current limit because in the schematic the trimmer is used for the ref pin.
Als there is any importance in the choice of Rsx resistor values.

Regards.

Hello.

Your understanding of how to set the current limit of one of our stepper motor drivers by measuring the coil current seems correct. The one thing you forgot to mention is that to get the current limit you should divide the current you read from your meter by 0.7, since it will only be 70% of the actual current limit.

Both methods of limiting the current should work, but we recommend the method using Vref since it can be done without connecting your stepper motor. That method is described in more detail on the A4988 stepper motor driver carrier’s product page.

The Rsx resistor is the current sense resistor, you can find more information about Rsx in the A4983 datasheet.

By the way, the A4983 stepper motor driver carrier is discontinued. It has been replaced by the A4988 stepper motor driver carrier.

- Grant

Hello Grant,

Thank you very much for your help
You are right, I forgot to mention the fact that you need to divide the measured current by 0.7.
In my case, I read 0,11, so, the current limit is 0,11/0,7.
Moreover, if I use Vref=3,3/2, according to the formula in the DS, I have Rs=Vref/(8*3,3/2) hence 2 Ohm.

Just another question, how can I manage well the speed of my motor?
By playing on the Rosc (and toff since toff=Rosc/825) which set up the chopper frequency?

regards.

I don’t follow how you got the numbers for your calculations involving Vref. You should not need to calculate the resistor value. It is given on the product page, and you can set the current limit without it by measuring Vref and following the instructions in the “Current limiting” section on the A4988 stepper motor driver carrier product page.

To control the speed of the stepper motor, you should change the speed at which you pulse the step pin on the stepper motor driver. You should not need to change the Rosc resistor.

By the way, your current limit seems very low, and I am not sure how well the stepper motor driver will work at current that low.

- Grant

Sorry, you are right, I made a mistake in my formula above. Indeed, it’s rather Rs=Vref/(8*(0,11/2)) p7 on the DS, and it works with your example (Vref=0,56V , 1,4A and Rs=5e-2ohm).

Yet, as you said, I have well follow the instructions in the “Current limiting”.
So sorry for my difficulty to understand well, but, for the first method it’s in fact just to measure current and divide by 0,7?
And for the second method, I have to measure Vref without my stepper motor, but in this board, Vref=R9/(R5+R9)*VDD.

Regards.

Yes, as the instructions indicate, you can divide the current measured on a single coil by 0.7 to find what the current limit is set to. For the second method, you can enter the voltage measured on the “ref” pin into the following equation:

Current Limit = VREF × 2.5

- Grant