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I have an ESP32 that has 3 MAX31855 sensors measuring 3 temperatures. All temperatures are reading excellent when connecting only the computer USB to the ESP32 module, but when I plug my external AC-DC 12V 8A 100W Switching Power Supply (https://www.aliexpress.com/item/4000846764734.html?spm=a2g0o.9042311.0.0.27424c4db1lms9) the temperatures get lower values.

From the 12 DC volts that I get from the power supply I then have a 12vDC to 5vDC converter:

https://www.aliexpress.com/item/1005003515877710.html?spm=a2g0o.9042311.0.0.27424c4db1lms9

And I use this 5v to power my ESP32. The MAX31855 sensors are powered from 3.3v from the same ESP32.

These are the values from the 3 sensors when only USB power is connected.

<<<<< Receive Data:

Bytes received: Request Type: Request Temp Data

Internal temperature of Thermalcouple 0 in C = 20.81

Temperature of Thermalcouple 0 in C = 18.25

Internal temperature of Thermalcouple 1 in C = 20.50

Temperature of Thermalcouple 1 in C = 17.25

Internal temperature of Thermalcouple 2 in C = 20.69

Temperature of Thermalcouple 2 in C = 19.00

When I plug the power supply then I get these values:

<<<<< Receive Data:

Bytes received: Request Type: Request Temp Data

Internal temperature of Thermalcouple 0 in C = 20.81

Temperature of Thermalcouple 0 in C = 16.75

Internal temperature of Thermalcouple 1 in C = 20.50

Temperature of Thermalcouple 1 in C = 15.25

Internal temperature of Thermalcouple 2 in C = 20.69

Temperature of Thermalcouple 2 in C = 14.50

And when I turn on the 12V motor connected to the same circuit then I get these values:

<<<<< Receive Data:

Bytes received: Request Type: Request Temp Data

Internal temperature of Thermalcouple 0 in C = 20.81

Temperature of Thermalcouple 0 in C = 9.25

Internal temperature of Thermalcouple 1 in C = 20.50

Temperature of Thermalcouple 1 in C = 7.00

Internal temperature of Thermalcouple 2 in C = 20.75

Temperature of Thermalcouple 2 in C = 11.50

What is very interesting is that the Internal MAX31855 temperature is never affected, the only one affected is by the K Type thermocouple temperature probe.

Any clue and any recommendations on what things to do in order to avoid this annoying situation?

Appreciate it.

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  • \$\begingroup\$ I don’t have oscilloscope but just bought this one 1,208.50 MXN$ 49% Off | FNIRSI-1C15 Professional Digital Oscilloscope 500MS/s Sampling Rate 110MHz Analog Bandwidth Support Waveform Storage a.aliexpress.com/_mPJbK9q will take 3 weeks to arrive and hope this let me see more clearly if there is noise or something wrong \$\endgroup\$
    – VAAA
    Jan 11, 2022 at 14:59
  • 1
    \$\begingroup\$ Why would you choose to buy your equipment from mali-express (or pee-bay or shamazon or bang-bad) when there is no datasheet, no guarantee of performance, a likelihood of poor design and reliability, probably no recognized adherence to safety and no adherence to any quality system. Why? \$\endgroup\$
    – Andy aka
    Jan 11, 2022 at 15:12
  • \$\begingroup\$ Ups, maybe because ignorance and is cheap but you are right. Maybe that won’t give me the right data I need. Anyway is on its way 😭 \$\endgroup\$
    – VAAA
    Jan 11, 2022 at 15:37
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    \$\begingroup\$ @Andyaka pee-bay made my day. Regarding OP's question... hope this let me see more clearly if there is noise or something wrong it's obvious that there's noise. However, your circuit might need some improvements as well since the design and layout affect the performance. \$\endgroup\$ Jan 11, 2022 at 16:03
  • \$\begingroup\$ I will try to do a schematic and update this ticket to see if someone can help me with this head pain \$\endgroup\$
    – VAAA
    Jan 11, 2022 at 16:25

1 Answer 1

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I found a way to solve it amazingly and easly and cheap. I bought an oscilloscope that haven't received yet but by experimental I added a 0.01uf between the leads of the thermocouple and now it works without any issue. Also now the 3 thermocouples readings are exactly the same. Before I had like 0.50 C difference between each one and now is perfect.

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