[Resolved]Dangerous problem with USB/Marlin [Fire Hazard]

Greetings all,

I have a Lepfrog Creatr Dual which I've recently changed the original electronics to a Megatronics v3.

One of the reasons that finally pushed me into the conversion was that the previous electronics board (from Leapfrog, which was already a replacement, and had several problems with losing connection) shorted out when I connected it to an old PC at my University that is running Windows XP.

When this happened something else that's very weird happened as well, which is that the heater for one of the extruders switched on as soon as I connected the board to the USB port (I'm not sure if it was immediately or after I installed the drivers), and without any sort of temperature control. In fact, that's how I noticed the problem, because smoke started coming out of the extruder, and then I realized the MOSFET LED light was on without the computer even running any printer control software.

Because the board had been having problems I attributed this to it and the faulty electronics, however I have since switched to the Megatronics and connected it to the same old PC, and low and behold the exact same thing happened (although luckily it didn't short out the board). So now I'm thinking that the old PC is the root cause of the problem, and that either XP drivers are faulty or the USB hardware on that computer is faulty.

Then today happened. I connected the machine to a Raspberry Pi 2 B running OctoPi, and as I was trying to configure the machine profile in the settings page I hear the sound of plastic crackling, and see the residue plastic oozing from the extruder and the MOSFET LED light turned on. I switched it off and disconnected the USB, then turned the machine on again for the fans to dissipate the heat. I tried reconnecting it to the Pi and as soon as I did the MOSFET LED came on again. Once more, the software had not even established a connection to the printer yet.

The weird thing is, this doesn't happen when I connect it to my MacBook Pro (2009 17 inch model, last of the Intel Dual Core), using the exact same cables (I've used two different ones throughout all this).

Are there any documented cases of this happening before, especially on other firmwares (the Leapfrog also uses Marlin)? I was sure the problem was on my end. Is this a driver problem or hardware?

Any help would be appreciated.

Edited 2 time(s). Last edit at 04/21/2016 10:13AM by Outbound.

The problem is probably caused by the ground loop you get when you connect a PC to a 3D printer via USB. This ground loop is one of the reasons why I control my printers over Ethernet from a browser, and only connect the USB cable when debugging firmware. Ethernet is galvanically isolated unlike USB, so no ground loop.

Here are some things to check:

1. That the wire between the negative output terminal of your 12V or 24V printer power supply and the electronics is thick enough and secure at both ends. I suspect that is where the main problem is.

2. Power the PC and the printer, and preferably nothing else, from the same mains distribution block.

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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].

Here are some things to check:

1. That the wire between the negative output terminal of your 12V or 24V printer power supply and the electronics is thick enough and secure at both ends. I suspect that is where the main problem is.

2. Power the PC and the printer, and preferably nothing else, from the same mains distribution block.

I think number 1 could have been the problem as I did use a wire that I wasn't sure was appropriate in order to use a quick connector when I was constantly plugging the board in and out and wasn't screwed to the printer yet, but this happened previously when the original wiring was in place.

As for 2, the only other thing being powered from the same block was a monitor.

I wish I could use ethernet but Megatronics doesn't have that. The new Ultratronics seems to have it, but I'm committed to making this thing work. I just have to update the firmware though because I mistakenly used an old Marlin distribution instead of the stable Marlin for Megatronics v3 and the LCD pins don't seem to be properly configured, so after that I won't have to connect it to a computer at all. It just sucks because I've used the Pi before and it worked beautifully. I wonder if the fault isn't with the electrical installation here at the Mechanical Eng. Departments lab. There's a lot of heavy duty equipment around here and quite a few heavy duty 5 pin A/C outlets. I wonder if that's having any effect.

OK so I just realized that you're right and it's totally a grounding issue.

What I did was connect everything to the Pi (monitor, which was plugged into the mains) and the Megatronics via USB, but I didn't plug in the Pi power supply.

As soon as I switched on the machine the hot-end LED came on. I disconnected the monitor's VGA cable and connected the Pi power supply. I turned the machine back on and the LED didn't fire up this time.

I really have to look into properly grounding this thing.