Ramps 1.4 on Wanhao I3 thermocouple reading

Hi everybody,

I've just throw out the melzi board on my Wanhao I3 for a ramps 1.4,
I also changed the thermistor for a thermocouple type K with the AD597, But I have strange reading.
It heat up great, i can go up to 280° without problem (i'll print around 270 for the taulman 645) but as soon as I turn on a stepper (the extruder is the worst) my reading drop.

If i'm at 200°C,

With the X-Y-Z axis motor : it drop to 160-180
with the extruder, it drop to 50° and to 150 when I ground the shield of the thermocouple wire.

So i need your help, i'm kind of new with the 3d printing world...

Thanks!

I found this : https://ultimaker.com/en/community/1...solved-totally

I'll try it soon

I don't think it would explain the large error you are seeing, but when I experimented with a thermocouple board with a MEGA2560/RAMPS setup, I realized there's a fundamental flaw that can lead to at least some error.

The AD595/AD597 sensors provide an output that is fairly stable as supply voltage to the chip varies. This would normally be good. The problem is that full-scale on the analog-to-digital converter in the ATMEGA2560 chip is hardwired to the 5V VCC for the board. At least with Marlin, VCC is assumed to be EXACTLY 5.00 volts. If your VCC is off by 5%, it will lead to your thermocouple readings being off by 5%.

I'm with Repetier, but I assume that's the same thing.
I've already changed my voltage regulator for the l7805cv (5v output, 1.5amp, same max/min volt than the ams1117-5.0) since I burned the original.

I could try to use capacitor to clean the 5v on the megta 2560 (0.33uf and 0.1uf if i follow the diagram on the datasheet), but I don't think that will stop my problem.
I bought the Bowden tube kit to put away the extruder stepper, hope that i'll reduce the noise, I'll machine a new extruder holder to fit with it.

http://www.st.com/content/ccc/resource/technical/document/datasheet/41/4f/b3/b0/12/d4/47/88/CD00000444.pdf/files/CD00000444.pdf/jcr:content/translations/en.CD00000444.pdf

MODERATOR NOTE: Post is late to appear due to unexpected delay in obtaining moderator approval.

Ok, so now it is working like it should.

I put a pull down resistor of 10k ohm 1/8w between the negative side of the thermocouple to the ground of th AD597 board.
My big mistake was to ground the thermocouple shield on the printer body, now that i ground it directly on the ground of the AD597 board, the temps stop to drop.

Now it is incredibly steady and it gave me a pretty good accuracy, better than my old themistor.

MODERATOR NOTE: Post is late to appear due to unexpected delay in obtaining moderator approval.