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Is it possible you need the ferrite beads like in the eval board schematic?
Quote from: CrabxCore on May 21, 2014, 03:46:28 amIs it possible you need the ferrite beads like in the eval board schematic?I had not seen an eval board for it, it should be easy to cut the Vcc track and insert a bead as a test.
Your cold junction compensation is off? What does it read when short circuited with no TC in the socket, and when the TC is at room temerature along with rom temperature from another thermometer.
Quote from: SeanB on May 21, 2014, 03:56:41 amYour cold junction compensation is off? What does it read when short circuited with no TC in the socket, and when the TC is at room temerature along with rom temperature from another thermometer.My take on it is that the MAX chip does the cold junction compensation and applies it to the thermocouple portion of the digital reading. You can read the comp reading separately of course, but as far as I know you don't have to do anything with it.Hmmm... if I disconnect the TC and short the input, the reading is 26°C which would be about right for my office at the moment.EDIT: Adding the bead to the Vcc (before the cap on the MAX) made no difference.
Sample from Maxim?
It really does look like the chip is the wrong curve.I bought the chips from Digikey; 2 pieces originally then another 2 a month later. The two prototypes have one from each purchase, but they are probably the same batch.Unfortunately neither RS or Farnell have stock, so can't easily get another chip to see if that's the issue. I'll see if I can get one somewhere else and go from there.
Quote from: ve7xen on May 21, 2014, 07:01:57 amSample from Maxim?That would take weeks. I'll get a breakout board from Wiltronics in the mean time as I need to get this prototype running ASAP.
boiling water ? water is conductive.... make sure you have a thermcouple that is fully electrically isolated ...
I have been using the MAX31855 in a project and have produced a number of prototype boards using it and have had no problems up until this point. I have just however produced a new batch of these boards and am seeing the exact same problem you have described on 75% of them (i.e. reading ~78°C at 100°C input from an Omega thermocouple calibration unit). The new board has some very minor alterations from previous versions, none of which should be relevant to this behaviour from the MAX31855.
Replacement parts should not be from lots ending in either "253" or "107"
not entirely sure what to make of Maxim's email saying the 107xx batch are also bad.
they're not sure exactly which ones,
So it looks like the trim issue is only on MAX31855KASA+ units with lot codes 253AL and 253AK with date code 1352A2. I also notice that data code 1345 is also with the same issue. other data code should be ok.