RAMPS + USB (PC) + Power (12v 30A) = meltdown

I have had this problem several times now and I just can't seem to get to the bottom of it.

I have a RAMPS 1.4 connected to a typical cartesian printer, e3D lite 6 hotend, MK1 6" x 11" heated bed. If I run this setup via RPi + OctoPrint, no problems. If I connect the RAMPS to a PC USB, Immediate meltdown at the USB port. This doesn't always happen. At first I thought it was bad USB cable, but that same cable works fine connected to RPi. On top of that, the same thing happened to a Smoothieboard recently (yes, ouch).

This is the MEGA for one of the RAMPS that melted down. You can see the passive resistor or whatever it is just melted away.



Same thing happened with this smoothie clone:

This happens because of the ground loop formed when you connect a PC and a 3D printer via USB. This ground loop causes two problems:

1. If the ground wire between your electronics and PSU is too thin or it has a bad connection, then the ground current will flow through the USB cable and mains ground to the PSU. When the heated bed is on, this is a lot of current, so the cable gets hot and USB components may get burned.

2. If other electrical equipment generates ground noise, this can flow through the USB cable and upset the electronics, typically causing it to hang or reset. Avoid it by powering the PC and the printer and nothing else from a power distribution block, and by using a USB cable with a ferrite bead on it.

You can avoid the ground loop by not using a USB connection. I control one of my 3D printers via Ethernet (which unlike USB is galvanically isolated) and one via WiFi, so no need for USB.

Edited 1 time(s). Last edit at 11/30/2016 02:51AM by dc42.

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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 David
Bottom line it sounds like there's a ground loop somewhere. This happens immediately once the PSU is powered up so it's not impacted by the heated bed.

The only thing I'm not clear on is that I can plug into the USB on the RPi and there's no problem. In fact, all of my printers are driven by RPi zero with a USB/enet hub. It's a slick setup. But there are times when I want to take printers offsite and just connect to a laptop. This is when I fried those two boards.

You are probably running the RPi from a 5V power brick that doesn't have a connection to mains ground. So no ground loop is formed. Anyway, check the wires and connections between the negative output terminals of your PSU and your RAMPS. Chances are that there is a bad connection there.

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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].

Figured it out:

The 30A power supply I'm using has mounting points on the bottom. The M4 screws I was using are steel. They were bottoming out and grounding the board to the case. Replaced them with resin screws. Problem solved.