Hey, all. I've been having an issue with my Prusa where the hotend thermistor would read at max temp all the time. I ordered some new thermistors to rule out a faulty one, and thought I was making headway when the new one read 0 degrees. Come to find out, my thermal paste was shorting the wires together. I cleaned it all off, and the new thermistor was reading around 357 C when cold. I did some looking around, and it seems that this is a prime symptom of having shorted the thermistor circuit to 12V. Is there a way to test the RAMPS board/Arduino to see if this is the issue, or am I looking at buying a new board to rule that out?

Any help would be greatly appreciated.

With nothing connected it should read 0. With a short circuit it should read a very high temperature.

Connect a 100K resistor across the thermistor input and it should read about 25C.

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[www.hydraraptor.blogspot.com]

I didn't have a 100K resistor, but I tried with a 10K. When the resistor was connected to the thermistor pins, I got a steady 357C reading. When I disconnected it, as in the attached 'disconnect' image, the temp reading dropped and started fluctuating between 105C to 135C in a pattern shown in 'open'. I reconnected the resistor, and the temp reading jumped back up to 357C. This is with the ramps board plugged into the arduino and connected to the computer, no power, nothing connected to ramps other than the resistor.

Looks like the input is damaged then.

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[www.hydraraptor.blogspot.com]

Gotcha. Thanks for the help in narrowing this down!