K Type Thermocouple Daughter Board Schematic?

AJ Quick

I was searching, but couldn't find it. Can anyone direct me in the location of the schematic and files for the new V1.1 Thermocouple daughter board?

I need to make a few custom PCBs for my printers and wanted to include the thermocouple connections on a custom designed board. I know the design specs for the MAX31856 are pretty straight forward, but I just like to double check so that I don't make a fundamental mistake somewhere.

Here is the board I am talking about: https://duet3d.dozuki.com/Wiki/Thermocouple_daughter_board

Thanks!

dc42

https://github.com/T3P3/TempDaughterboards/tree/master/MAXTempTC

AJ Quick

Thank you!

TLAS

@dc42

Quick question - I've created a similar board, but copied the Adafruit reference design. There are really two differences:

Pull-ups on the MOSI, SCK, and CS lines
100 ohm resister between the negative thermocouple to the bias input.

The circuit performs fine when connected to an Arduino, but I can't get it to function with the Duet. I'm thinking it has to do with the pull-up lines. I'm able to connect it, and the M305 command shows comes back without any errors - but then the temperature reading stays at 2000C. On the Arduino, using the same cables, it reads correctly.

Any thoughts?

dc42

Are you using the correct X parameter in the M305 command, depending on which CS line you are using?

The pullup resistors shouldn't matter unless their values are too low.

TLAS

So after debugging this all day, I think I've got a lead.

Based on the Max31856 spec, it could draw about 0.2A each at 3.3V. The ethernet cable I was using happened to be a 32 AWG cable - the actual voltage at the card was closer to ~2.3V by the time it ran through the cable. Spec min is 3.0V.

I happened to notice that the Arduino would read fine, but when I connected my smart effector as well, the reported temperatures dropped by 2/3rds. Then it started having trouble during boot - thinking it was type J instead of K and all sorts of weirdness.

I'm going to increase the gauge of the ethernet wire (to 24+ AWG) to see if that resolves the issue.

On a side note - what's the maximum current that the 3.3V line can handle? I currently have 4 TC boards (~0.8A max) + a microcontroller (~0.4A).

dc42

@tlas said in K Type Thermocouple Daughter Board Schematic?:

Based on the Max31856 spec, it could draw about 0.2A each at 3.3V.

The MAX31856 datasheet that I have says on page 2 that the maximum current draw is 10mA. I suspect you have a bad or incorrect connection.

The maximum load on 3.3V that can be tolerated will probably be limited by heating of the 3.3V regulator. The regulator has a rated output current of 800mA or 1A depending on the revision of the Duet.

TLAS

I was looking at the maximum power on page two for current estimates.

In any case, I just made a duplicate board and connected it to my test printer with a 24 AWG, 3ft ethernet cable and it worked just fine. Literally just plugged it in and it worked. I have a 7ft cable for my main printer coming in on Friday - I expect it to solve the problem.

All ethernet cables are NOT created equally...