Hello, I need some help. How do I set the current limit in my A4988 driver for my Nema17 stepper motor? I saw a formula (Vref = Imot * 8 * Rs) that requires the current sensing resistor to get the Vref, but my driver's datasheet doesn't have the current sensing resistor value. My stepper motor has a rated current of 1.68A. Is there another way to determine the Vref for my stepper motor?
Post a close up, focused picture of the motor driver, and forum members should be able to identify and tell you the value of the current sense resistors.
The A4988 can't deliver more than 1 A/winding without overheating, so set the current limit to that or lower.
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Here sir. I searched about how to determine the current sensing resistor of the A4988 and one of them says that I should read the value on top of the resistor, which in my A4988, it is R100. Does it mean that the current sensing resistor of my driver is 100mΩ or 0.1Ω ?? Correct me if I'm wrong.
Instructions for setting current limits on stepper drivers.
You are correct. R100 means 0.1 Ohms which is the usual value for cloned Chinese A4988 stepper drivers.
My stepper motor has a rated current of 1.68A. So using the formula Vref = Imot * 8 * Rs
Imot = Rated current of stepper motor
Rs = current sensing resistor
Vref = 1.68A * 8 * 0.1 ohm
Vref = 1.34V
So I will set my current limit on my driver to 1.34V? Correct me if I'm wrong.
It's a little more complicated that that.
In full step mode, that formula actually sets the current per coil to be 0.707 x Imot.
If you want to set the coil currents to 1.68, then Imot should = 1.68 x 1.414.
However, if 1.68 for Imot provides enough torque for your project then it will make the motor run cooler (not get so hot)
Where did the 0.707 and 1.414 come from sir?
Is my formula wrong? Based on the reference and youtube that I watched, they used the formula (Vref = Imot * 8 * Rs) to determine the Vref value to set up on the A4988. And they get the value of Imot based on the rated current of the stepper motor that they used. Is it correct or there's an accurate way how to know the Vref value for my stepper motor with a 1.68A rated current?
This webpage explains:
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No, because the A4988 cannot deliver 1.68 Amperes. Set Vref for 0.8V.
How did you get the 0.8V ? Please enlighten me.
Enlightenment follows:
Please take the time to read and absorb the responses to your posts. This is a technical forum.
Did you understand the explanation on the pololu webpage?
It can be confusing.
Yes. Thank you for your response and the webpage that gave. However, I'm a little confused about the specification of my stepper motor. It only says that it has a rated current of 1.68A, does it mean that 1.68A current per phase? Here's the specs of my stepper motor
QUICK SPECS:
Step moment angle: 1.8 ° Current: 0.84A
Resistance: 5.75 Ohm Inductance: 9.3mH
Static torque: 28N.cm
Shaft diameter: 5mm
Motor weight: 300g Body length: 34 / 40 / 48mm
NEMA 17
Model 42HB34F08AB
Rated power: 200 (W)
Product Type: 2-phase hybrid stepper motor
Rated voltage: 7-24 (V)
Rated current: 1.68 (A)
Rated speed: 0-900 (rpm)
Rated torque: 3.5 (NM)
Sorry, I'm just a beginner and just starting to learn about this thing.
Yes. That is the maximum steady state current, per winding.
Which the A4988 cannot deliver without additional cooling. To get the maximum available torque out of that motor, you need a stepper driver capable of supplying more than 1.68A/phase.
I have an additional heatsink for my driver, is it enough? Some references that I visited that the A4988 maximum current per phase is 2A with additional cooling.
Not without a fast, noisy fan blowing air directly on the cooling fins, and probably not even then. Don't believe the hype.
For professionally designed boards and honest advice from a responsible seller, check out Pololu's offerings.
It is simple enough to do the experiment. Set Vref to above 1 A/phase and see if the A4988 shuts down after a bit. Or, read through the complaints on this forum titled something like "my stepper doesn't work".
Okay. Thank' a lot
I have read from a reputable source that the heat sink alone is not sufficient. It needs active cooling (fan) as well. Running components at the ragged edge of their specifications is a recipe for trouble.
Like others have said, get a driver that can comfortably handle the current that you want. And I second Pololu. They have instructional pages for each of their products and good tech support. You cannot say that of Amazon or Ebay.
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