I was trying to drive a Nema 17 stepper motor using an arduino uno and an A4988 driver. I had it wired up according to this instructable (http://www.instructables.com/id/Drive-a-Stepper-Motor-with-an-Arduino-and-a-A4988-/) at step 2 without the motor power supply wired to the bread board yet. ALSO, I hadn't wired the sleep to reset yet (I just didn't read directions carefully). The board was connected to my computer via USB and I had uploaded the sketch on the instructable to it.

Then, because I misunderstood the directions, I plugged my 12V 8A power supply into the power input on the arduino. All of the sudden, my computer turned off. My computer is fine after restarting, but now the arduino isn't recognized at all by the computer when I plug it in.

Given this information, I am pretty sure that my arduino is fried. However, I can't understand why plugging in the power supply to the power input, which has a voltage regulator, would cause the board to fry.

  • That's an odd problem. If you power the arduino from a 9V/12V power supply to the jack on the board, with no USB connected, test the arduino chip and the voltage regulator with your finger; do they get hot to the touch? That's the best indication that one has fried.
    – cortices
    Commented Jan 2, 2016 at 3:03
  • When I look at picture 2 of step 2, there is a connection Vin of the Arduino to the motor power. On the picture 3 this is missing. How was your cabling?
    – ott--
    Commented Jan 2, 2016 at 4:06
  • 1
    Are you absolutely sure you used the RAW pin and not the VCC pin?
    – Aloha
    Commented Jan 2, 2016 at 5:49
  • Do any other usb devices work?
    – MadHatter
    Commented Jan 2, 2016 at 6:35
  • I had tested other devices and know that neither my computer nor the cord are the issue.
    – Jere82
    Commented Jan 3, 2016 at 5:31

2 Answers 2


The Uno is not one "thing" that fails. It has various components, some of which may have failed. I wrote a lengthy page about how to test various parts:

Have I bricked my Arduino Uno? Problems with uploading to board

I plugged my 12V 8A power supply into the power input on the arduino.

Perhaps you reversed the leads, and shorted the USB power to ground, thus causing the PC to turn off? This would probably also blow most of the components on the Uno because they would have had -12V applied to them, assuming it got past the voltage regulator.

However there is a reverse polarity protection diode on the power input jack. Did you use that or the Vin pin? (or the Vcc pin?)

Note that the reverse polarity protection does not protect you if you use the Vin pin.

  • Thanks for the info page. What do you mean by reversing the leads? I used the 2.1 mm center-positive barrel plug connector on the arduino so wouldn't that be impossible? Also, I didn't have anything wired to the vcc pin, and the Vcc pin was wired to the input voltage on the stepper motor driver.
    – Jere82
    Commented Jan 3, 2016 at 5:43
  • You said I plugged my 12V 8A power supply into the power input on the arduino. I assumed you had to make up a lead for a heavy-duty power supply, and might have got them the wrong way around.
    – Nick Gammon
    Commented Jan 3, 2016 at 20:36

I wonder if you destroyed your Arduino's voltage regulator.

If the Arduino was connected to the stepper motor via the A4988 when you powered the Arduino with a 12V supply, the stepper motor might have drawn current from the Arduino's regulator. This current is likely ~2As, which is far more than the tens of milliamps the Arduino is designed to output. This kind of current draw could quite possibly destroy the regulator on your Uno.

This could also explain why your computer restarted. USB ports on computers are designed to provide an output voltage to their USB devices, so if you provide a voltage on the USB power lines your computer is also driving, you can short your ports if the two voltages are not exactly the same. This is actually why USB cables have the A vs B end design - they mark which side should supply power (A), and which should be powered (B). The Arduino might have provided a voltage on the USB cable's 5V line and thus shorted something in your computer that provides the voltage for your USB ports (I believe Unos are not designed to be powered by both USB and the 12V connectors simultaneously).

If the regulator fried on your Uno, you might have had a voltage that was way off of the expected 5v on the USB port. A well designed USB port will usually try protect itself in a situation like this, often causing the computer to shutdown or the USB bus to deactivate until restarting the computer.

You could check your Uno's voltage regulator with a multimeter or better yet and oscilloscope to see if that is in fact the problem. If so, you could solder a new one on pretty cheaply and quickly. However, there is no guarantee nothing else got fried, in which case buying a new $10-20 Uno looks like a pretty appealing option.

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