T-962 reflow oven 31 – Cold junction compensation
After you flashed the firmware with the “UnifiedEngineering” version, you can place a cold junction temperature sensor for better temperatur management.
The existing controller makes the assumption that the cold-junction is at 20 degrees Celsius at all times which made keeping a constant temperature “a bit” challenging as the terminal block sits on_top_of_an_oven with two TRIACs nearby. We can fix this by adding a temperature sensor to the connector block where the thermocouples are connected to the controller board. It turns out that both an analog input and at least one generic GPIO pin is available on unpopulated pads on the board. GPIO0.7 in particular was very convenient for 1-wire operation as there was an adjacent pad with 3.3V so a 4k7 pull-up resistor could be placed there, then a jumper wire is run from GPIO0.7 pad to the
Dq pin of a cheap DS18B20 1-wire temperature sensor that gets epoxied to the terminal block, soldering both
Vcc and ground pins to the ground plane conveniently located right next to it. Some hot-glue may have to be removed to actually get to the side of the connector and the ground plane, someone seems to have been really trigger-happy with the glue gun!
More info here: https://github.com/UnifiedEngineering/T-962-improvements/wiki
Menu screen before the mod:
Menu screen after the mod: