USB Electrical Noise


I am currently working on building a cnc machine powered by an Arduino running GRBL. For the electronics, I am using a 12v 25A power supply from eBay, 3 Pololu A4988 stepper drivers, an Arduino Uno, and a CNC Sheild (Arduino CNC Shield – 100% GRBL Compatable | I have everything connected properly, but I am getting some strange behaviours when I attempt to run the machine.

The first issue I'm having is that my power switch is not working properly when the machine is connected to a desktop computer with USB. I am currently switching the negative line coming out of the power supply and going to the CNC shield. This works fine unit you plug in the machine to usb. When this happens, the power bottom no longer has any effect on the power of the machine.

The second issue I am having is that the machine sometimes seems to randomly move while it is connected via usb.

Finally, when the machine is connected to my computer, it makes things behave oddly, such as the touchpad being un responsive or the screen showing distortion and noise.

What could be causing these issues?


Is this some shockingly poor PSU off eBay by any chance? It certainly sounds like it. Never skimp
on mains power supplies, your safety matters and a poor supply can destroy everything it is
connected too (not such good value then).

You must switch the positive supply rail, or both rails, not switch just ground (unless its only
switched on the mains side). It probably should be fully isolated (which it clearly isn't by the
heavy interference shown).

You may also have problems due to ground loops or a lack of understanding in routing
high current paths away from sensitive logic and sensors. A star-ground confuguration is
generally to be desired

You would normally have PE (protective earth) connected to your circuit ground at the machine
chassis (star ground point) for something like this, because large metal electrical machines are
not safe otherwise (you want any random bit of main wire that shorts to the chassis to blow the
fuses instantly before your heart goes into fibrillation).

Its also possible that your computer has a poorly behaving supply itself and this is the first time
its been put to the test.

If this is something like an ATX or other computer supply they have both minimum and maximum
current ratings on all the outputs which you have to observe for correct operation. They are not
designed for general purpose use and have to be cajouled into behaving with dummy loads as