Hi
## Background
I bought an old printer based on the Reprap Darwin, since buying it I've upgraded the controller to an Arduino Mega & RAMPS 1.4 running Marlin 1.1.9 and I've upgraded the extruder to an E3D Titan Aero (V6 & Extruder combination). This has been working great and I've done a number of successful prints lasting upto 12 or more hours.
## Problem
Recently I've started getting "Thermal Runaway" errors on the hotend. These are seemingly random but it is generally after several hours (3-6).
The first few times I noticed that the final temp measured before the shut down was roughly 195C (5 below target), following this I changed the Thermal Protection Settings to increase the Hysteresis to 10C but the failure continued only at 189C on a 200C target.
I don’t have a time frame for the temperature drop as it never happens when I’m watching and the graph on Octoprint doesn’t log long enough.
## Steps to Resolve
* Check Thermistor selection
```
5 : 100K thermistor - ATC Semitec 104GT-2/104NT-4-R025H42G (Used in ParCan & J-Head) (4.7k pullup)
```
* PID Autotune Completed & updated in Firmware.
```
// Rapman
#define DEFAULT_Kp 27.6
#define DEFAULT_Ki 2.2
#define DEFAULT_Kd 86.5
```
* Adjusted Thermal Protection Settings
```
#if ENABLED(THERMAL_PROTECTION_HOTENDS)
#define THERMAL_PROTECTION_PERIOD 80 // Seconds
#define THERMAL_PROTECTION_HYSTERESIS 10 // Degrees Celsius
```
* Checked wiring connections for Heater and Thermistor
* Measured resistance of Thermistor - 70kOhm @ 30C
The printer is currently reprinting the last file but with the parts fan disconnected to see if it avoids the problems. Beyond this I am out of ideas how to solve so am looking for any assistance / suggestions from the community.
Thanks
James
Twenty2a
Checkout the story behind the Rapman at www.twenty2a.co.uk