Issue with A4988 coil tension

Hi all,

I use Pololu A4988 unit with ATX PSU as 12V/5V power source. To control A4988 I use USB245 unit. I set coil current to 0.7A value.

Wiring of my layout is shown here:

I have soldered for the moment only one A4988 unit, upper on layout.

I have measured coil’s tension on both outputs, manually sending pulses to STEP contact.
Obtained dependence for full microstep mode is shown in attached image.

Issue is related to absence of output tension for the coil 1, while tension of coil 2 seems to be correct.

I tested tensions on all contacts of A4988, they are seems to correct, at least they corresponds to layout.

I would be very grateful for constructive ideas


Additional information for my question. I have measured coils’ tension for the case, when stepmotor is disconnected. Corresponding dependencies are drawn in below image.

Like to the case of connected stepmotor, tension on the coil 1 is very close to 0, it’s on the level of a few mV. Tension on coil 2 is practically constant and equal 13 V ± 20 mV.

Is it known, how to check working state of A4988?
Are here some resistance, tensions’ values that can be used to verify it?

Thanks for help and information


Can you post some pictures of your drivers and your setup?

- Patrick

Thank you for your reply and help.

Sure, you can see both sides of my setup. I added some labels of power and ground lines.

What about A4988 verification tips?
Best regards

It looks like the driver you have is a knockoff and not manufactured by us; we have only ever made our A4988 drivers with green or black PCBs. That makes it difficult to say for sure what might be causing the problem, and we cannot help you troubleshoot it since it is likely different than ours.

If you want to try a genuine Pololu A4988, then you can find them on our website, or you can get them from our authorized distributors. If you have the same problem with one of our boards, I would be happy to help you continue troubleshooting.

- Patrick


Thank you for your reply. I understand your point, but provider assured me that that was Pololu unit. Nevertheless, could you give me some information, how to verify working state of A4988? Do you have some tension, resistivity, etc. values, that enable such kind of examination?

Thank you for your help

Even if we knew your board was using a genuine Allegro A4988 (we have come across many knockoffs that do not) we do not have a definitive way to check if the driver is working other than to test it with a stepper motor, as you have already attempted. You could try measuring the resistance from each output to VMOT and GND with the board unpowered and disconnected. If they are inconsistent, it could suggest a problem, but it is difficult to draw any definite conclusions.

- Patrick

Thanks a lot for your information.

You could try measuring the resistance from each output to VMOT and
GND with the board unpowered and disconnected. If they are
inconsistent, it could suggest a problem, but it is difficult to draw
any definite conclusions.

This is what I need and what I was asked about. Could you precise what is the resistance range for genuine Pololu boards and A4988?

Will and how this resistance depends on applied multimeter polarity?

I appreciate your valuable help.

Best regards

I was only suggesting you could check the consistency of the values. That is not characterized in the A4988 datasheet, so I do not have any precise expectation for what it should be.

- Patrick

Thanks for your reply. But I don’t understand, what do you mean for “consistency”? Is it equality of resistivity values? Or do you mean something else?

Sure, I know, but I suppose that you have some experience of touching, testing such Pololu units, and my question deals with this experience, not datasheet data. And I would be satisfied not in precise values, but, at least in order of value.

And, if you have direct experience of handling of such units, are they very sensitive to static charges? Do I need to use anti-static bracelet to take them or it is not strictly necessary?

Yes, see if any of the resistances are much lower than the rest. I would expect them to be in the kΩ to MΩ range, so anything much lower could mean a problem. If you are still unsure whether your driver is bad, you should check with the manufacturer or seller of your drivers. However, I suspect your knockoff driver is not worth all this troubleshooting effort and it would be better to just order a replacement (preferably from us or one of our distributors so you know it is genuine).

Electronics like this are susceptible to damage from electrostatic discharge (ESD). I do not want to go so far as to say using an anti-static bracelet is “strictly necessary”, though we do use them throughout our manufacturing facility, but I would recommend taking as many ESD precautions as you reasonably can.

- Patrick