Reading a thermocouple ?

Mesa BSPI to a MAX3155 ought to work, too, if the bit-banged component offends you.

Hello together,

it is a very late reply to this topic, but actually we have a problem exactly on this topic.

We are using a MESA 7i76 with one of the analog inputs to measure a NTC100k@25deg thermocouple. We have wired as following:

+12V --- NTC --- Analog0-Input
Parallel to the Analog0-Input, we have wired a 100k resistor to GND.

We expect to measure around 6V at the Analog input @25deg.
As long as the potential between NTC and 100k-resistor is NOT connected to the Analog0-Input, this is the case.
But as soon as we connect the potential to the Analog0-Input, the voltage drops to around 2V.

I cannot explain myself this behavior since i assume that the input impedance should be (theoretically) infinite.

Can somebody please explain, why this behavior appears or what i am doing wrong?

Thank you very much in advance!

That is a 100k thermocouple, so it's resistance is 100Kohm or 100000 Ohm, so the input of the mesa board will drop down the voltage as it also needs a bit for it to function, and the input resistance/impedance is never infinite.
That can easily be addressed with a potentiometer instead of the resistor, so you can adjust it's value to get 6V at the input.
Do you plan on reading negative temperatures with it?
If not, better leave it as is as you will have more headroom for positive values.

The 7I76 input impedance is 20K ohm. This is why you see the large voltage
drop.

Thank you @tommylight and @PCW

Meanwhile i have recognized that the input impedance is 20k --> www.mesanet.com/pdf/parallel/7i76man.pdf --> Page 14.
Sorry for that, i should have read it more carefully.

Now, i have wired as following:
+12V --- 100k NTC@25deg --- (20k) MESA-Analog0

Knowing that, i have developed my own formula to calculate Temperature from Input-Voltage, based on Measuring the Temperature with NTCs
The question is: Where can i implement my calculation?
My aim is to measure the temperature of the extruder of my 3d-printer (max 220deg) and to display the temperature via PyVCP.
I am using this thermistor.

I have read that there exists a Py-Interface to LCNC. Maybe it is possible to do this kind of calculations with it. However, there is no easy-to-understand tutorial on how to realize this kind of stuff. Is it possible to post a short example/howTo or maybe show me some links, where i can find more informations about that.
So the aim is to read in the analog input with "ain" in python, do the calculations in python and finally return the result to PyVCP.

Thank you very much in advance.

I would always prefer a thermocouple to a thermistor for that sort of job.

But the answer to converting ADC counts to Degrees is likely to be the "lincurve" HAL component.
That can approximate any function between known limits.