Two ADC Failures

Equipment failures happen, and there isn’t always an immediate explanation or a clear path by which you can obtain one. What I have discovered today, an apparent failure of the Analog to Digital Converter (ADC) chip in my 3d printer, happens to be similar to a failure I discovered a month and change before which also involved a failure of an ADC, except that that one was for my Roomba.

Other than the similarity in failed part, there seems to be little in common between the two. The failed parts are both on unshielded low voltage DC computers, both ADCs were connected to relatively distant sensors (a few inches from the computer boards) by wires that were properly run side by side (which limits susceptibility to induced currents). The sensors on the 3d printer were 100k thermistors and the ADC circuits measured the impedance of these. In the Roomba, the ADC circuits are connected to Infrared Diodes which were cleverly used to detect dropoffs to prevent the device from dropping down stairs.

The failure of the Roomba was initially detected when the robot was set to clean mode and it immediately announced a please relocate error which occurs when it cannot find a way to move itself to safe ground. I then eliminated failure of the IR diodes or wiring as possibilities by taking it apart, and testing the sensors and wire leads. They were in good working order.  This left the ADC chip itself as the most likely failure point.

The failure of the 3D printer ADC was detected by the LED screen announcing a max temp error in combination with the detected temperature being displayed as zero despite the room and thus the thermistors being at about 25C. I disconnected the thermistors from the board (there are two, one for the heated print bed, and the other for the extruder hot end), and tested them. They both had resistances which were appropriate for room temperature, and the wiring checked out. Then, I made sure the printer’s settings weren’t to blame, by re-inputting the correct values for maximum permissable temperature and thermistor values. Now again, I have a device failure which is mostly a failure of an ADC chip.

I can’t imagine a way in which these two failures could be related. The only shared vulnerability seems to be to really powerful magnetic field changes causing an induced voltage on the ADC terminals high enough to damage the sensitive chips. But the only way that could feasibly happen for these is if someone took one of those really big rare earth magnets and waved them around the printer and the roomba. Now, I didn’t do that myself, and to my (quite accurate) knowledge, neither has anyone else. So it remains a mystery for now, and I’m stuck doing my own vacuuming and using InstaMorph for my odd plastic jobs, at least until I can make some replacements.

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