Thermocouple usage

Hi
In my 3D printer I use type "K" thermocouples with AD595 board. (for Extruder and Bed).
I modified Marlin code:

# define TEMP_SENSOR_0 -1 //Extruder#1 with type "K" TC, and AD595 signal conditioner
# define TEMP_SENSOR_1 0 //disable 2nd. extruder
# define TEMP_SENSOR_2 0
# define TEMP_SENSOR_BED -1 //BED Heater with type "K" TC, and AD595 signal conditioner

I have 2 problems:
1. Every time the BED heater cycle is "ON" , it generates noise disturbing only on the Extruder temp reading .
When BED heater cycle is "OFF" , - Extruder reading back to normal.
- I used coaxial cables for the sensors, and for both TC's. All well grounded.
- Add 220uF capacitor on the DC supply to AD595 boards.
- twisted power lines cable for BED and Extruder to reduce EMI
with no help.

2. Extruder actual temp is +25^C over setpoint (at 200^C). (BED temp is calibrated up to 110^C - no problem).

3. I'll appreciate if anybody who experienced such problem can help
Thanks!

Meirbns Wrote:
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> - I used coaxial cables for the sensors, and
> for both TC's. All well grounded.

The voltage signal on thermocouple systems is very small so one has to take extra care and I see you have taken measures already. The signal is differential so as long as the pair travel together and are not greatly influenced by nearby electrical or magnetic fields they can be run for long distances.

The thermocouple wires must not be grounded. The only connections to the thermocouple wires must be the isothermal connections in the amplifier circuit.

Do not connect capacitors or other stuff to the thermocouple wires, they will generate false temperature offsets.

Electrostatic screens/shields on wires must only be connected at one end, usually the end with the more robust grounding.
Using a screen as a power common needs careful thought and cannot be shared between sensors and loads.
A screen used as a protective earth should not be common with signal grounds.

If you get it working let us know how you did it.

Edited 1 time(s). Last edit at 08/26/2013 10:25AM by KalleP.

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Kalle
--
Lahti, Finland
The only stable form of government is Open Source Government. - Kalle Pihlajasaari 2013

Im using a thermocouple, and I am not having these problems, also the temp readings are extremely quick, and after calibrating the pid settings i get only +- 0.1c deviations.

Hello!
I know it's some-kind off this topic, but does any of you know how to wire the thermocouple amplifier to RAMPS1.4?
You can take a look here:
[forums.reprap.org]

Thanks!!
iLyngklip