USB from arduino sparked on laptop

I had my 3d printer plugged into the wall and went to plug the usb from the arduino into my macbook and when it touched the metal case it sparked and the arduino stopped working. This is the third board I've killed in a couple months but this is the first time I've seen it spark. Ive looked for shorts but cannot find any. If anyone has any idea what could be causing this id love to hear it because I'm stumped.
Thanks

... the GND's of your printer and of the macbook have different potentials/voltages, what kills the "weakest" device in the connection
You can measure the difference with a voltmeter.

Serach a better PSU for your printer ...

---

Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Also:

1. Check that the wire connecting the negative 12V output terminal of the PSU to the power input of the electronics board is securely connected at both ends.

2. Make sure that the AC mains cable connecting the PSU to the mains includes a ground wire.

3. Connect the negative 12V output terminal of the PSU to the ground terminal (in ATX PSUs this is normally done already inside the PSU). This means that any leakage (e.g. due to the capacitance of the transformer) passes to local mains ground instead of through the USB cable.

4. Power the PSU and the macbook, and nothing else, from the same AC mains distribution block.

Ground voltage differences between 3D printers and PCs are a frequent source of problems when a USB connection is used. An Ethernet connection avoids the problem. So does running the laptop off its battery while it is connected to the printer..

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for the replies. Ill get a new PSU (the one I am using is a few years old) and try everything else you guys have said. I had no idea the difference of ground voltages would cause a problem like this.

Notebook's PSUs usually are galvanically isolated. May be MAC's PSU damaged too (or it is Apple's feature).

The metal frame of 3D Printers should be attached to GND (ground, negative or earth connection) and the main power cable fused, so in the case of a short, the fuse is the only thing that lets out the "magic smoke"

Any voltage (5, 12, 24, 110, 220-240, 440 volts) just loves to be grounded. IE go to earth aka it says "take the path of least resistance" upon it travels around the circuit

I know of a project which "took out and killed 3 Arduino Megas" because the live wire of the 15v power supply to the stepper motors touched the metal frame of the project.

---

Supporting 3D Printers with Parts and Build services.
Printer: Ormerod 2 (528.4) Duel extruder set-up with Aluminium X-Rib, RRPro Firmware v1.11-ch (2016-04-08)