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MAX31855 breakout board powered by 3.3V
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

MAX31855 breakout board powered by 3.3V

by zbelding on Fri Aug 21, 2020 12:36 pm

In the product description, it states: "3.3 to 5v power supply and logic level compliant!"

In my opinion, this is incorrect. This is from the LP2981 datasheet:
"The device operates if the input voltage is equal to, or exceeds, VOUT(TARGET)+ 0.6 V. At input voltages below the minimum VIN requirement, the device does not operate correctly and output voltage may not reach target value."

Powering the device with 3.3V via Vin does not meet the min requirement of 3.3+.6= 3.9V.

I have been chasing my tail for weeks trying to understand why I have been getting such erratic readings. I think this is the issue. Vout appears to be around 3.2V when powered with 3.3V supply.

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Re: MAX31855 breakout board powered by 3.3V

by adafruit_support_bill on Fri Aug 21, 2020 12:54 pm

To power from 3.3v connect your 3.3v supply to the 3.3v pin on the MAX31855 board.

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Re: MAX31855 breakout board powered by 3.3V

by zbelding on Fri Aug 21, 2020 1:03 pm

Thanks for your help. I am confused. The breakout board clearly states: "Vin: 3-5VDC" and "3Vo: 3.3Vout".

Looking at the schematic, I understand we can power through the 3Vout, but I feel this should be documented and the "Vin: 3-5VDC" silkscreen on the board should be changed.

Image

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Re: MAX31855 breakout board powered by 3.3V

by zbelding on Fri Aug 21, 2020 1:52 pm

Other documentation that should be fixed:

From Arduino Code section of learn: "If you're planning to use the MAX6675/MAX31855, there's a little more work to be done. First off, Vin and GND must connect to a 3-5V supply."



Also from CircuitPython Microcontroller Wiring section of learn shows 3.3V input on Vin:

Image

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Re: MAX31855 breakout board powered by 3.3V

by ChliHug on Fri Aug 21, 2020 1:55 pm

Yeah, I'm no fan of rounding the voltages like that. Other boards have similar silkscreens.

The schematic might be really old. The photo for the product shows an LDO regulator with a QT33 marking which probably makes it a MIC5225. What marking does yours have? It most likely doesn't make much of a difference anyway in regard to this issue (it is just an LDO regulator after all), but it's obviously better to have the numbers for the right device.

Technically, the MAX31855 should still work with 3.2 V. Its datasheet specifies the minimum at 3.0 V. But, as you mentioned, there's no more regulation and now we don't enjoy the noise rejection of the linear regulator anymore.

These questions come to mind:
Do you have a noisy power supply? The MAX31855 datasheet says something about being sensitive to power-supply noise. There seem to be 20 µf on the 3.3 V side, but maybe these capacitors don't catch all the noise?
Do you have the erratic readings when you power it above 4 V?

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Re: MAX31855 breakout board powered by 3.3V

by zbelding on Mon Aug 24, 2020 9:53 am

ChliHug wrote:Yeah, I'm no fan of rounding the voltages like that. Other boards have similar silkscreens.

The schematic might be really old. The photo for the product shows an LDO regulator with a QT33 marking which probably makes it a MIC5225. What marking does yours have? It most likely doesn't make much of a difference anyway in regard to this issue (it is just an LDO regulator after all), but it's obviously better to have the numbers for the right device.

Technically, the MAX31855 should still work with 3.2 V. Its datasheet specifies the minimum at 3.0 V. But, as you mentioned, there's no more regulation and now we don't enjoy the noise rejection of the linear regulator anymore.

These questions come to mind:
Do you have a noisy power supply? The MAX31855 datasheet says something about being sensitive to power-supply noise. There seem to be 20 µf on the 3.3 V side, but maybe these capacitors don't catch all the noise?
Do you have the erratic readings when you power it above 4 V?


The regulator is labeled with a QT33, but as you said, it doesn't change the fact that feeding 3.3v into this type of regulator will cause it to not operate correctly.

I have tried bypassing the regulator and feeding the breakout board with 3.3v from my Arduino MKR NB 1500. I am still having issues... The cell modem on the MKR NB can use high current in short bursts. The antenna for the MKR NB is located 25 feet away from the arduino/max31855. I am occasionally seeing thermocouple readings 20 deg F lower than they should be... The internal/junction readings are exactly as they should be.

I guess I need to put a scope on my 3.3v rail and see what is going on.

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Re: MAX31855 breakout board powered by 3.3V

by zbelding on Tue Aug 25, 2020 9:52 am

3.3V from the Arduino was clean. The problem ended up being EMF from the modem(even thought the modem antenna is mounted far away). This graph shows before on left and after on right. I extend the SPI lines about a foot away from the Arduino and this seems to have removed the issues. I don't know if the cell modem was inducing voltage in the Max31855 chip itself, or into the thermocouple wires...

Image

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Please be positive and constructive with your questions and comments.