Greetings, dear gurus. I have 6 thermocoupler type K interfaced with MAX6675 module. Everything is working fine, but i'd encountered some weird issue.
Whenever the thermocouplers (with metal sheathing) touches the power supply casing or anything earthed from the AC plug, the value drop down to 0.00 Celsius. *Even the metal sheathing contact to anything earthed it show the same problem!
I suspect it introduce "ground" to the sensor probe, and having grounding issue. Meanwhile, I'd searched the internet and realized that my thermocoupler might be the grounded type, which the probe tip is welded to the metal sheathing, which gives fast reading but the drawbacks of grounding issue.
I used continuity mode from multimeter to check whether the sensor probe was welded to the metal sheathing, each -ve and +ve line with the metal sheathing, but surprisingly, none of it were linked. So I'm confused, i'm not sure my thermocoupler is grounded or non-grounded type. But very weird it is conductive and it grounded my sensor whenever the thercoupler (tip or metal body) touches the earth!
I'm worried if the sensors mount to anything "earthed", the reading would be all gone...
If the housing is not connected to any of the wires, I'm not seeing how it could be an issue. Unless there is more to the story. Please post a picture of your device and link to the datasheets.
Confess I missed that, apologies - my excuse is I do this from my phone; and I jumped to that conclusion because .....Depends how the thermalcouple was tested with a “continuity” checker - there may be some resistance to ground - I had this with an oven I’m making , exactly the same symptoms, stopped working when the sheath was earthed down; yep I’d bought a cheap thermocouple.
There is another issue relating to these boards - there is often a short between two of the pins on the chip, which can pull one side of the thermocouple to ground . This is worth checking . The boards are very poor quality , with the tracks coming out of the chip pins at an angle and not getting any clearance. I think it is between chip pins 1&2 or between 3&4 -those pins nearest the thermocouple connector anyhow ( check for a short )Running knife between the pads fixes it ( you are on your own doing this , I accept no responsibility)
hammy:
It sounds as though you thermocouple is not isolated from the sheath. You can check that with a multimeter .
Bin it and buy an isolated one .
That's the fact that making me confused. I tested it with multimeter (both in continuity and resistance mode), between those terminals +ve and -ve and the metal sheathing...it doesn't connected at all! So i would assume it is ungrounded type K thermocouple.
Another weird cases is, my friend use the same thermocouple to interface with the industrial PID controller. It doesn't show 0.00 on the PID display when the metal sheathing touches the earth!!! What sorcery is this!!
hammy:
There is another issue relating to these boards - there is often a short between two of the pins on the chip, which can pull one side of the thermocouple to ground . This is worth checking . The boards are very poor quality , with the tracks coming out of the chip pins at an angle and not getting any clearance. I think it is between chip pins 1&2 or between 3&4 -those pins nearest the thermocouple connector anyhow ( check for a short )Running knife between the pads fixes it ( you are on your own doing this , I accept no responsibility)
I've checked my MAX6675 boards, it should be ok. The pin1 and pin2 on the MAX6675 Chip should be shorted, from the datasheet. And the board itself, between the GND and VCC show value on continuity, i guess it is due to the 0.1uf capacitor on board, *from the datasheet as well.
The capacitor on pin 1-4 needs to be there and it’s to clean up the power. Try a 0.01 uF capacitor on pin 2&3. Also do you know for certain you have. K type sensor and it is good? Do you have a picture of how everything is wired?
