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MCP9808 Temp Sensor has no sensitivity
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MCP9808 Temp Sensor has no sensitivity

by Jeffs_acr on Sun Mar 05, 2017 12:07 pm

I'm having a strange problem with the MCP9808 temp sensor, and I've having it with more than one, which would seem to suggest software, but I can find anything.

I'm using two identical sensors on two different boards. Both communicate. Both report the temperature. Both seem insensitive to change.

They tend to idle in the mid 70's at room temperature. Both seem to report around 76 degrees at room temp. I assume this is a bit of self heating from the chip itself.

Example:

If I place my finger on the sensor, there is no change. I've held it there like 30 seconds and it has changed perhaps a few tenths of a degree, sometimes 1 degree.

If I aim a heat gun at it from 6 inches away, the temp will rise, and quickly, but quickly return to the mid 70's where it seems to idle.

if I squirt some flux cleaner on the chip (which gets cold when it evaporates) the temp will fall into the 60s or 50s but quickly return the 76 degree range.

I've had one sensor running for several days in a room whose thermostat cuts the temperature during unoccupied times, and given this cold snap we are having this weekend, the room will fall into the 60s. Since I started the program Friday, the temperature always being reported is 76.55 degrees.

I'm using the example program for the 9808 breakout board/sensor.

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Re: MCP9808 Temp Sensor has no sensitivity

by franklin97355 on Sun Mar 05, 2017 5:46 pm

Post a picture of your setup and connections and we can look. Also, have you changed the code at all? What are you using to read the sensor?

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Re: MCP9808 Temp Sensor has no sensitivity

by Jeffs_acr on Sun Mar 05, 2017 8:41 pm

I'm getting the same results with the unchanged example code or that code placed into my program.

Ive tried with two different MPUs, an Arduino Mega and a Feather Cortex M0+.

in both cases the chips are on two custom PCBs, not a breakout board.

I can get some photos tomorrow, although I don't know why. Electrically the software is communicating.

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Re: MCP9808 Temp Sensor has no sensitivity

by franklin97355 on Sun Mar 05, 2017 10:35 pm

two custom PCBs,
What else is on the PCB and how is it connected?

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Re: MCP9808 Temp Sensor has no sensitivity

by Jeffs_acr on Mon Mar 06, 2017 10:54 am

The I2C lines drive two OLED displays, one BME280 pressure/humidity sensor, one BH1750 light sensor, one 24C64 EEPROM, and the MCP9808. All are working and communicate. All, except the MCP9808 deliver correct data.

Here, the temperature is shown just sitting at room temp:
IMG_20170306_082450.jpg
IMG_20170306_082450.jpg (38.76 KiB) Viewed 343 times


Now, with a finger on the chip, about 25 seconds before picture was taken:
IMG_20170306_082633.jpg
IMG_20170306_082633.jpg (40.42 KiB) Viewed 343 times

As you can see, no change. There is also no change as room temperature varies (as when the thermostat lowers the temp), the temp just stays put,

But, if you put some flux cleaner on the chip, and it gets cold when it evaporates, or point a heat gun at it from a foot away, you can clearly see the temperate change is registered. It's like the chip has no sensitivity to "minor" changes (although I wouldn't think my finger is a "minor" change).
Image2.jpg
Image2.jpg (39.82 KiB) Viewed 343 times


I also have another chip on a different board, on an old prototype of something different, and running the example program for MCP9808, the behavior is basically the same. I'm not including pictures of that at this time unless you feel it's necessary.

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Re: MCP9808 Temp Sensor has no sensitivity

by franklin97355 on Mon Mar 06, 2017 4:10 pm

Did you get the chip from us? didn't know we sold bare MCP9808 chips

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Re: MCP9808 Temp Sensor has no sensitivity

by Jeffs_acr on Mon Mar 06, 2017 6:16 pm

No, bought a dozen from Digikey.

By the way, mis-operation is not limited to your library. Tried several libraries. Results identical.

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Re: MCP9808 Temp Sensor has no sensitivity

by Jeffs_acr on Thu Mar 09, 2017 10:35 pm

Nobody has any further thoughts on this?

I have access to an environmental chamber, so I'm going to put the board with the sensor in it an try changing the temperature to some known values +/- 50 degrees F from ambient and see what results I get. I'll probably go from 30F to 125F. I don't want to damage the Feather board or the OLED displays, and I'm assuming everything can handle that range of 0 to 50C. I'll check the spec sheets for the SAMD21G18 and ATWINC1500 before going to 50C.

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Re: MCP9808 Temp Sensor has no sensitivity

by frodesto on Fri Aug 10, 2018 12:24 pm

Hi Jeffs_acr

Did you find a solution to your problem? I'm experiencing the same thing (custom PCB with chip from Digi-Key)

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Re: MCP9808 Temp Sensor has no sensitivity

by danhalbert on Fri Aug 10, 2018 12:43 pm

I found an answer to this problem in a previous thread: viewtopic.php?f=22&t=72294
The 250ms delay between readings is not quite enough. Increase that.

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Re: MCP9808 Temp Sensor has no sensitivity

by frodesto on Sun Aug 12, 2018 2:17 pm

Thanks for responding.

I think I found the reason for my problems. The MCP9808 is probably working just fine and is reporting the correct temperature. My sensor is mounted on a PCB together with an ESP8266 module, and I suspect the PCB is being heated by the ESP and other components (voltage regulator, USB-driver etc.). Putting the ESP in deep sleep and increasing the poll interval seems to help.

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