T0 Temp Stuck

Ramps 1.4
Marlin firmware

I decided to add connectors to my extruder stepper, hot end resistor, and thermistor so I can swap out extruders easily.

When I wired up the thermistor and thermistor I got them backwards. I briefly exposed the T0 pins on the RAMPs to the heater resistor. No power was going to it obviously because the resistor was hooked up to the T0 pins.

The "temp" reported in Pronterface shot up to like 700 degrees. I knew something was not right. Noticed the reversed connections and fixed it.

Now, T0 reports 128 degrees. Even if there is nothing attached. The thermistor did not get damaged because I tested in on the T1 pins.

Could exposure of the heater resistor to the T0 pins have damaged the RAMPS?

I have reset the Arduino. Reflashed. Completely unplugged the RAMPS and let it rest.

Thoughts? I can move the extruder temp to pin T2 that is not in use. I am very curious what is wrong with T0 however.

I would verify the thermister is good, with an ohm meter. Did you apply 12v to it ( turn the heater on while attached).
With it disconnected what does your ramps report?

You've damaged the thermistor input on the Arduino Mega with a voltage greater than 5V. If you want to use T0, you'll need to replace the Mega. If you're unlucky, it may have damaged other inputs as well.

This is sort of a known problem with RAMPS and most other electronics, that the thermistor inputs are usually not protected against voltages >5V. Some of the newer boards out there have this protection, but RAMPS is now a pretty old design.

Note: The way the hot end output works, there is ALWAYS 12V on the hot end, so even if you didn't turn on the hot end, it will definitely have exposed the thermistor to >5V.

Edited 1 time(s). Last edit at 02/08/2014 05:46PM by Cefiar.

See? I learned something today. I keep forgeting the mosfets go to ground.

Well there we go. The other inputs appear fine. I will just move the hot end temp to Temp2. Thermistor is fine and measures correct. I show the temp on Temp0 if the thermistor is on it or not. I am sure the pins are fried.

I have a plan B .. but lesson learned.

Thanks.

Yeah it's an annoying thing. One of the things I wanted with the RAMPS-FD project was to get protection into the board, so that we didn't have the same issue over and over again. Fortunately, the guy who did most of the work on it (bobc) agreed that it was a great idea and now the latest version of the board has such protection. Hopefully when the board is stable, and people start using it instead of RAMPS, we'll see this issue go away.

I'll buy that for a dollar!!!!