component selection; inductor for K Type Thermocouple

Sparkfun's MAX31855 BOB found here, Shows these ferrite beads in the schematic. schematic found here

I'm trying to build a custom board with 3 of these MAX31855's and am using this schematic. Would this Ferrite Bead work for L1 & L2?

I don't know what they do really, so I'm not sure what specs to look for. But both Adafruit and Sparkfun's BOBs have them.

They are to reduce radio frequency interference. Most likely you don't need them.

My custom board has a wifi WEMOS PRO D1 sitting right on top of it…

I can’t figure out why my max31855 aren’t showing a reading, and I thought maybe it was these inductors?

I would look for a simpler explanation, but you can always power down the wifi module to see if that fixes the problem.

I ALWAYS make sure that all the individual parts work, each with its own little test program, before putting them together in a more complex project.

Think about it.
You must know that any wire acts as an antenna. The pair of thermocouple wiresvpick up any RF in the vicinity so the ferrite beads ,
being inductors , impede any rapidly changing current, thus filtering out RFI.
That,s RF-101

That,s RF-101

No, that's RF102. RF-101 is where they tell you to NOT sit on a radiating antenna

It's not the wifi module. I haven't got that far yet.
Yes, I'm bench testing just the max31855, with only it's components on the board.

Back to my original question, does anyone know if those inductors are the right thing there? However, I removed them from the equation, and it still doesn't work. I'll keep testing.

PS. My K type thermocouple is grounded, and somewhere I read that this can cause problems. I'm using this one.

https://www.ebay.com/itm/K-Type-Thermocouple-6-150mm-Probe-with-90-Bend-Pointed-Tip-for-BBQ-Smoker/253192345495?hash=item3af3709797:g:ZdIAAOSwna1Z1iZW

This is the Thermocouple I'm using

My K type thermocouple is grounded, and somewhere I read that this can cause problems

Absolutely! Follow the MAX31855 manufacturer's wiring recommendations exactly, or it will not work.

You do know that the red wire is NEGATIVE on most TCs above ice point

Thermocouple is producing some voltage obouth 0.05 V DC, try connect it directly to analogue input using my voltmeter

SouthernAtHeart:
Back to my original question, does anyone know if those inductors are the right thing there?

https://www.ebay.com/itm/K-Type-Thermocouple-6-150mm-Probe-with-90-Bend-Pointed-Tip-for-BBQ-Smoker/253192345495?hash=item3af3709797:g:ZdIAAOSwna1Z1iZW

The inductors and capacitor are for preventing AC hum, your MAX is not working.
Try this; 1.5V battery + voltage divider 1:50 ratio, so you get 0.03V , use it instead thermocouple, when you get it working connect thermocouple.

Hi,
Can you post your code and a schematic of your project please?

Have you just tried with ONLY one 31855 on the PCB and communicating with it.

Thanks.. Tom.. :slight_smile:

Ok, don’t go away. I’ll post my code and schematic. It seems to me that it should work with a grounded TC, but just not very accurate.
I’ll post my setup...

Hi.
OPs PCB.
8e851c7e7d66e6544c50abbde58f8c6303211d6b.png
Check the pinouts of the IC, Pin 1 is gnd, but I see no connection to pin1.
3588.jpg
Tom… :slight_smile:

Here’s the schematic, and the board layout. All parts from Digikey. (I have enough troubleshooting without China knockoffs).

GND on pin one is seen in photo with the copper pour.

schematic.pdf (42.3 KB)

board.pdf (28.4 KB)

…continuation from post 15

These 2 screenshots show the basic test I did. The one shows the schematic of the only parts I populated, and the other show them on the board.
Then I hooked them up to my UNO. Powered by the 3.3V pin, and ran the example serialThermocouple sketch found in Adafruits MAX31855 library.

How ever, this test would have failed, because I forgot, the data sheet says the IC’s SPI I/O pins are only 3.3V tolerant. So that explains why this simple bench test on a bare board didn’t work. I will have to try again with an MCU running on 3.3V. Or level shift on a breadboard.

It looks like you have a lot of unconnected ground islands on the board. You are going to need a bunch of vias or reroute traces to let them connect

tinman13kup:
It looks like you have a lot of unconnected ground islands on the board. You are going to need a bunch of vias or reroute traces to let them connect

post 15 covers this... there’s a ground pour on top and bottom

I would add another blue polygon and Name it GND so you have GND plane on both layers.
Sprinkle some vias and Name them GND also connect the two layers.