I’ve blown up a few printer control boards throughout this project. It turns out they were all caused by one tiny little short on the corner of my heated bed. This took quite some time to find.

I first discovered the issue when I had the printer plugged into my PC. At the time, I was trying to print direct from Repetier-Host rather than the SD card. I started printing and a short time later the printer froze. As I moved over to check it, I brushed past the USB cable and discovered that it was piping hot! I quickly unplugged it and discovered that I had well and truly let the smoke out of the Arduno MEGA.

I immediately suspected that my cheap imported power supply was to blame. It was supposed to have a floating ground so I thought that its negative would sit at the same potential as earth. I’d read somewhere that some cheap power supplies don’t have a floating ground and this can cause dangerous ground loops.

I replaced the Arduino and checked all of my wiring for shorts. I couldn’t find anything that would have caused it. Measuring the voltage across the heated bed and hot end MOSFETs was very confusing because I didn’t realise they were sinking current. I mistook it that the power supply was providing 12V at “ground” and 24V at “12V”, giving a 0 to 12V range.

I decided not to use the USB cable anymore and the problem disappeared. If I ever needed to flash the firmware I used my laptop that was not connected to anything else. Everything went fine.

At a much later stage, after a trip to Shenzhen, I installed a new power supply in the hope that it would not suffer from the same ailment. I got it all wired up and plugged into the computer. To much celebration, it didn’t fry the Arduino MEGA. Success!

Not long after that I upgraded the RAMPS/Arduino control board to a Smoothieware-based MKS SBASE. I was confident that I would not have any issues and the transplant went smoothly. One day, before printing, I plugged in the USB cable to update the configuration file… and let the smoke out! I couldn’t understand why the problem had re-emerged after so long AND I had replaced the power supply. I decided to do some tests since the board was fried anyway. It seemed that the problem only occurred while printing. This was the first clue. I did some probing of the heated bed with my multimeter and found that there was a dead short between the two wires! I then discovered this:

The culprit!

The small washer that I had used for the spring mounts had cut through the trace and was shorting out. I suspect that while the printer USB cable was disconnected, the whole ground potential of the printer was much higher than earth but everything would be fine. When the USB cable was connected, it would short the 30 amp power supply to ground through the braided shielding on the cable. This in turn would blow up the USB chip and/or the main processor. I’m lucky that my PC USB ports were not fried as well!

After isolating the cause, and purchasing a replacement MKS SBASE, the problem never emerged again and I don’t have to worry about the USB cable anymore.