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Basic Electronics » Why TC negative connected to ground

March 15, 2012
by RogerFL
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Looking at the DS for MAX6675, regarding the T- thermocouple pin it reads:

"Lead of Type-K Thermocouple. Should be connected to ground externally."

The pin right next to it on the MAX6675 is GND, so do I really just connect these adjacent pins? Why wouldn't the IC just have one pin then for GND and T-?

March 15, 2012
by 6ofhalfdozen
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Hi Roger,

The short answer is that if there are no safety issues, yes, jumpering them is what is intended.

I am not sure exactly why they keep those two pins seperate (besides the fact that 7 pin dips look really WIERD! hehehe ) but there are three reasons I am aware of that would need a seperate ground for TC's; specially grounded/shielded thermocouples, wierd biasing schemes, and high voltage safety issues.

If you are using a "standard" ungrounded MgO thermocouple and you aren't expecting any crazy conditions where large voltages could be sent through the TC leads, the TC - should be connected to ADC ground. If you read the "Open Thermocouple Detection" paragraph on page 5 mentions grounding TC- as close to the ground pin as possible. I have run into a few people that put a 1k ohm resistor instead of a jumper to ground. I guess some kind of safety thing, though it could raise the delay of measuring a temperature change.

I have read several articles where people do "wierd" biasing schemes on TC's for various reasons; resolution improvement, cable length impedence matching(I think thats what they were doing), and noise reduction to name a few. I don't know a whole lot about those systems and what they were trying to do. They seemed really overly complicated and added voltage biasing, capacitors, inductors and other bits and bobs on the TC in/outs for something.

As for the high voltage safety, I have read of using special grounds and optocoupling when working with >600VAC systems. Once again I don't know much about it besides someone does it for reason, as I try to avoid anything that high if possible.

anyhow, despite the rambling, hopefully that helps.

March 15, 2012
by mongo
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Thermocouples come in four types where wiring is concerned. Some are grounded at the - lead, some are at the + lead. Then, there are some grounded right at the junction, and finally, completely ungrounded. Many times, it has to do with fault detection with the thermocouple. A lot of controllers are smart enough to tell you if it is open, shorted or out of range.

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