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Topic: Reducing current limiter on an A4988 stepper board (Read 383 times) previous topic - next topic

Miniflyer

Hi guys. I'm using A4988 stepper drivers on an arduino nano to drive 10mm micro stepper motors at 5V.
They are not specified for amp draw, but i have set the limiter to the lowest setting and am still making the motors glow. I suspect they need only 50-60mA. The controller wont let me go below about 120mA.  I need to either add resistors to burn up the surplus energy, or change the onboard poti to allow me to lower the current.
Any suggestions? Which ones do i need to change?

Robin2

The Sparkfun Easydriver uses a stepper driver chip with a max current of 750mA (IIRC) and it may be adjustable to a lower current - however check with Sparkfun before spending money.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

gilshultz

There is no guarantee that this will work, you are doing this at your own risk, increase R1 and R2 in value from 0.75 to 1 ohm and measure what happens, the current should go down.
This response is to help you get started in finding the problem, not solve it for you.
Good Luck & Have Fun!
Gil

MarkT

Hi guys. I'm using A4988 stepper drivers on an arduino nano to drive 10mm micro stepper motors at 5V.
They are not specified for amp draw, but i have set the limiter to the lowest setting and am still making the motors glow. I suspect they need only 50-60mA. The controller wont let me go below about 120mA.  I need to either add resistors to burn up the surplus energy, or change the onboard poti to allow me to lower the current.
Any suggestions? Which ones do i need to change?
Its a constant current driver, a resistor isn't going to do anything till the driver runs out of voltage headroom.
But that's probably already happened, try two 68 ohm resistors, one in series with each winding.  Maybe lower values will be OK, but those ought to prevent overheating.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Miniflyer

#4
Nov 23, 2019, 06:10 pm Last Edit: Nov 23, 2019, 06:11 pm by Miniflyer
Hi Gil.
Which resistors do you mean? Is there a diagram how they are hardwired? Then i could calculate the values instead of trial and error....i need to order the resistors specifically, so im trying not to guess too much.

Mark: how do you get the 68 ohm?

Miniflyer

Looking at the physics,  wouldnt it make most sense toadd a resistor in paralell wirth the same value as the motor has? Voltage remains constant and current is reduced by half.

Of course setting up the hardware to limit the current is still the #1 option, but noone seems to have experience on that....

MarkT

That assume motor windings are resistors, and that constant-current motor drivers like driving paralllel RL, I think that could cause all manner of problems with hysteresis chopper circuits.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

gilshultz

Miniflyer, I looked at the schematic for the A4988 stepper driver, not sure where I found it but I think maybe Adafruit, they are good about posting the information. If I remember to change the pot you will have to go to a higher value, I believe the CW pin is connected to the ref and the CCW  goes to ground via a resistor. I my memory is correct lowering the value of that resistor will also help.This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil

Miniflyer



I've read a lot about the chopper circuits used, and am giving up on using external resistors.......i need to find an onboard solution to cut the settable values down to 1/10th of the values (now 0,15-2A, needed 20-200mA).

I have yet to figure it out thoough....since i cannot find any board documentation despite intense google searches.

104 resistors are 100kOhm, the 103 is 10kOhm and the 203 is 20kOhm. The 2 R22 are 0.22Ohm.....

Miniflyer

#9
Nov 26, 2019, 05:03 pm Last Edit: Nov 26, 2019, 05:14 pm by Miniflyer
This is my actual board. It shows different resistors used....




instead of the 20kOhm is a 30kOhm, and instead of the dual 0.22kOhm is dual 1 Ohm resistors.

My understanding is that the 203 (303 on my board) sets the Vref together with the Poti (100kOhm according Pololu, but since the other values dont correspond to my board either i am not sure it really is). The other 2 resistors measure the current as sensor inputs....so the solution to reducing the Current limit is

a) reduce the VRef
b) increase the 2 sensor resistors

is that correct?

Robin2

@Miniflyer, have you studied the Allegro A4988 datasheet?

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

Miniflyer

Yes i have.....just having a hard time figuring out what is what on the board......

My status now:
Doing the Math i get a Vref of 0.06V to achieve 100mA stepper current. Reducing Vref makes it less accurate.....
So i will need to change the 2 resistors. I have found this calculator:
Stepper Calculator

Vref in basic setup for a 2A motor should give a Vref of 1.28V

Changing both resistors to 1.3 Ohm should reduce the range to max 150mA whith a Vref of 1.25V

Now i just need to figure out how to unsolder the tiny SMD resistors and replace them....

gilshultz

Thanks for the link to the calculator, I did not know there was one for stepper motors.  The best I can suggest is measure R1 and R1 or whatever they are on your board and then adjust the value down by about 25% and see what happens.  I believe they are the center top two resistors in you picture labeled R100. These resistors setup the current measurement from which the current regulation is derived. I hope this response helps you progress in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil

Miniflyer

Yes the R100 seem to be the dual sensing resistors that can be quoted in the calculator.
I used these values to get the 1.3Ohm required to reduce my range.
Just in case i will also order some resistors with inbetween values....

Any hints as to unsoldering SMD resistors? They are 0805 types....so pretty tiny....

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