The Perils of cheap Ebay stuff, or, why doesn’t my MAX31855 thermocouple board work?

I’ve been playing around with Type K thermocouples for a project. These are cheap, commonly available, and can read a temperature range from –270C to 1260C (though it depends on the exact thermocouple).

It looks similar to this (you stick the metal pointy end in the hot thing):

The above is a Type K thermocouple, fresh from Ebay. It works fine: I actually received two of them. Its the cheap’n’cheerful option amongst thermocouples in that temperature range.

Its basically just a couple of funny pieces of wire though: you need a thing to turn that into a temperature reading. Enter the MAX6675:

This board came with my thermocouples and sends you a reading in Celcius over the SPI bus when you ask it. I knocked some code together from the datasheet, and all looked good.

However, for reasons, I needed to use its big brother, the MAX31855. Its pretty much the same idea, only it has a larger temperature range. It arrived from Ebay separately:

You need slightly different software to speak to it. Its also not pin compatible with the MAX6675 (foreshadowing).

I hooked it up to the same thermocouple as before, updated the code. However, it kept complaining the “thermocouple was shorted to ground”. Cue an annoying hour of fiddling, debugging, testing and swapping cables.

Finally, I checked online and found tons of people complaining about exactly this issue.

Closely examining the board above, you’ll see a trace linking two pins above (highlighted in red). This is linking the thermocouple -ve directly to ground, exactly what it is complaining about. Hmmmm..

Looking at the datasheet for the MAX6675, it says the following for pin 2: “Alumel Lead of Type-K Thermocouple. Should be connected to ground externally”. It does not say that on the MAX31855 datasheet.

If you look at the MAX31855 board above, you’ll see it says both MAX6675 and MAX31855 on the white silk screening at the bottom.

My guess is the manufacturer has assumed the MAX6675 and the MAX31855 are pin compatible – which they clearly are not – and just slapped the updated part on the old PCBs.

Result: annoyed customers, and this blog post.

I could just cut the trace, but I don’t trust this board at all now: its ripe for defenestration. I’ve ordered a somewhat more expensive MAX31855 board from a more reputable supplier.

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