I’m trying to make a standard test setup with a Pololu A4988 an Arduino Mega and a NEMA 17 stepper motor. But there is something I don’t understand about the VDD, reading the diagram I conclude that i should connect the VDD to the Arduino micro controller 5V output and the GND on the A4988 to the GND on the Arduino board. But i must have misunderstood something here, because when i apply 12V on VMOT / GND i can measure ~5.5V (~1.5V when using a 9V battery) across the VDD / GND. I discovered this when i had the Arduino board attached to the A4988 without power on the Arduino board but with 12V connected to the VMOT the Arduino light up. I have wired it the following way.
RESET and SLEEP is connected together.
DIR to pin 12 on Arduino
STEP to pin 13 on the Arduino.
The NEMA 17 is attached coil 1 on 1a 1b coil 2 on 2a 2b
12V to VMOT / GND
I haven’t had the circuit up and running yet, because I’m not sure this is the correct behavior.
Ok so I tried the following setup. I was not able to reproduce the 5V on the logic VDD GND but I could still measure around 1.89V when the drive was supplied with 12V.
12V on VMOT and GND (the GND for VMOT)
With SLEEP and RESET “jumped” and the stepper motor attached I could measure ~1.89V between VDD and GND
With the drive only supplied with 12V and no other connections or wires I still could measure ~1.89V between VDD and GND.
But this is not a correct behaviour either, right? There should be 0V as the VDD and GND should be supplied with power from the Arduino board.
That behavior does seem unusual, and your stepper motor driver might be damaged. I am not sure what might have happened, but it looks like you do not have the capacitor we recommend connected between VMOT and GND to prevent damage from LC spikes.
If you send us an email with your order information and a reference to this post, and I can see if I can get you a small discount on a replacement.
As I live in Denmark it is easier for me to order a new one here. I did a lot of other setups before I figured out about jumper between the reset and sleep. Also I initially only had access to 2 X 7.2V 1500mha batteries used for RC cars before I attached the power supply and yes I did not put in a capacitor… usually I get away with alot before I burn off electronics…
The important thing here was just to clarify if I should expect that behaviour…
