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Electronics => Beginners => Topic started by: amateur_25 on December 23, 2016, 04:44:52 pm

Title: Large excursions in thermocouple readings with MAX6675
Post by: amateur_25 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.

Title: Re: Large excursions in thermocouple readings with MAX6675
Post by: GBowes 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

Quote from: amateur_25 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.

Title: Re: Large excursions in thermocouple readings with MAX6675
Post by: Marinated 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.

Title: Re: Large excursions in thermocouple readings with MAX6675
Post by: shteii01 on December 24, 2016, 05:27:00 am

https://www.adafruit.com/products/269 (https://www.adafruit.com/products/269)

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

Title: Re: Large excursions in thermocouple readings with MAX6675
Post by: amateur_25 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?

Title: Re: Large excursions in thermocouple readings with MAX6675
Post by: shteii01 on December 24, 2016, 05:46:01 pm

Quote from: amateur_25 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?

I have used those boards that I linked, with the MAX31855.  Hooked up like they show, two thermocouple wires attached to the ic.

(https://cdn-shop.adafruit.com/970x728/269-02.jpg)