Thermocouple Temps decrease to 0 when heat turns on

So I've got my E3D hotend setup with a K-Type ptfe insulated thermocouple, I installed the thermocouple and modified the firmware according to the instructions here
http://reprap.org/wiki/ExtThermoCouple_1.0

Now when I boot up Repetier, the initial temperature reading seems correct, the extruder is at about 18-20 degrees celsius. However, as soon as I set a temperature to heat up the extruder, the temperature reading begins dropping down to 0 until MINTEMP is triggered and everything stops. I restarted the printer after letting the extruder cool off and tried again, same result. Initial reading is correct, but then temperature drops once the extruder begins to heat up.

Anyone have any idea what could be causing this? Firmware problem maybe?

 

This is an example of what happens. Here I had started up the printer, set the temperature to a simple 25 C, and then right after it begins to drop.


Blowing hot air on it causes the temps to go down instead of up, seems like maybe its reversed.

 

So in case anyone has this problem in the future, it turns out the Diagram on that Wiki page was wrong, you have to swap the green and white wires around, otherwise the polarity is reversed.

Thermocouple is working great now!

 

Curious as to why you want to use a thermocouple?

 

Bad experience with a thermistor, and the thermocouple seems more durable and reliable, even if it's a bit less accurate. And of course it's possible to read higher temps for some exotic materials.

Installation was a breeze, compared to all the soldering I would have had to do with the thermistor. I'm actually thinking about installing a second thermocouple to my hotend as a redundant temp reading.

 

Time to print PET!

 

hi, i have the same problem, and i did't understand from your post what wire was wrong,

 

The green and white wires of the thermocouple. When you plug them into the thermocouple board if your temperature goes down when you start heating the extruder you need to swap them.

 

I just received my E3D V6 hotend with thermocouple and pcb.

How did you insert the thermocouple junction into the heater block? just naked or insulated? into the hole?

 

Yes just put it into the hole where the original thermistor was supposed to go. I didn't use any insulation.

 

Hi Thanks!, and did you put the green/white sleeve under the screw+washer?

 

I switched out my board to a Megatronics so my situation is different

 

I know this is an old post, I had exactly same problem, swapped the wires and was working fine. I found this was the problem commented on a web site about the thermocouple boards. It says to put the green wire on the positive side, but as it says below, they put the + on the wrong pin.
Connect Thermocouple


Wire up your thermocouple to the board. Thermocouples have polarity, so you need to insert it the right way to get a proper reading. If you insert it wrong, it will simply report the reverse of the real temperature.

Please note: the v1.0 board has a silkscreen error, and the +/- signs on the board are reversed. Sorry!!!