Author Topic: Large excursions in thermocouple readings with MAX6675  (Read 1311 times)

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Offline amateur_25

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Large excursions in thermocouple readings with MAX6675
« on: December 23, 2016, 04:44:52 pm »
I was told the max6675 is supposed to be used with a ungrounded thermocouple.
If I wanted to used a grounded thermocouple then the max6675 should be made floating.

However despite using a adum5401 (with isolated power), am still getting major excursions when reading the temperature.

See the attached for a graph of the readings.

The test firmware (i got off the net)  uses the max6675 library to read the temperature in Celsius then another separate reading in Fahrenheit .

Notice how the line in red sometimes reads a much higher temperature than the ambient in my bed room but this is then not reflected on the blue line.
Hence it's a separate reading. This tell me it only occasionally goes berserk.

Any help will be much appreciated.


 

Offline GBowes

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Re: Large excursions in thermocouple readings with MAX6675
« Reply #1 on: December 23, 2016, 11:30:29 pm »

Not sure where you got this idea of isolated thermocouple.
The Max6675 data sheet shows a grounded thermocouple in their "typical application"
For the Open Thermocouple sense to work the data sheet says that T- MUST be grounded.
Note that they do want the ground separate from the CND (pin 1)of the chip.

https://datasheets.maximintegrated.com/en/ds/MAX6675.pdf

The data sheet also has some comments about noise that may be contributing to your "large excursions"

Graham


I was told the max6675 is supposed to be used with a ungrounded thermocouple.
If I wanted to used a grounded thermocouple then the max6675 should be made floating.


 

Offline Marinated

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Re: Large excursions in thermocouple readings with MAX6675
« Reply #2 on: December 24, 2016, 12:43:12 am »
This is a case where posting a picture of your setup is no less important than posting a schematic. These are sensitive devices and there could be noise coupling into the system from all over the place.
 

Offline shteii01

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Re: Large excursions in thermocouple readings with MAX6675
« Reply #3 on: December 24, 2016, 05:27:00 am »
https://www.adafruit.com/products/269

"Friends, don't let friends use MAX6675!"
 

Offline amateur_25

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Re: Large excursions in thermocouple readings with MAX6675
« Reply #4 on: December 24, 2016, 01:08:33 pm »
Maybe some information on why you think the max6675 isn't a good chip?

Do you simply mean it's now obsolete?

 

Offline shteii01

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Re: Large excursions in thermocouple readings with MAX6675
« Reply #5 on: December 24, 2016, 05:46:01 pm »
Maybe some information on why you think the max6675 isn't a good chip?

Do you simply mean it's now obsolete?
I have used those boards that I linked, with the MAX31855.  Hooked up like they show, two thermocouple wires attached to the ic.


 


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