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Amplifying a thermocouple (for learning)

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solderjs:
Newbie here, trying to learn.

I have multimeter that came with a thermocouple. I was curious and decided to measure it on the voltmeter setting and found that seems to change maybe 0.2mV per degree above room temperature.

Then I thought "wouldn't it be nice to be able to amplify this?" and so after watching Dave's video I went down to the engineering building of the local university and checked their vending machine for an op amp. I found an LF347N (similar to a LT074).

After much trial and error |O I was eventually able to get a capacitor with 1v charge to read 3.55v using a 5k1 as RF and a 2k as R1. This checks out (3.55 = 1 + (5.1/2)).

Then I tried replacing the 5k1 with a 10k and the 2k with a 1k expecting to be able to see something in the range of 2mv - 10mv when holding my finger to the thermocouple, but instead I saw a reading of 1.5v and it didn't seem to vary at all in relation to the temperature of the probe.

Then I smelled the magic smell and got that sinking feeling that I'm going to have to make a visit back to the vending machine.


A couple of questions:

Since the voltage of the battery is only in reference to its internal mechanism (i.e. neither the positive nor negative terminals have any voltage in reference to any other "ground"), and considering that the thermocouple likewise only really has voltage relative to itself, can I consider the negative end of the type K "connectable" to the "ground" of the battery? Should I be using two of the LF347s op-amps to amplify each individually (like this https://www.analog.com/-/media/analog/en/products/image/functional-block-diagrams/ad8494-8495-8496-8497-fbl.png?h=270&hash=D6B26C5A2AFAB2DCB6D79A3A465D0F9E3016ACB7)  so that it's relative to itself instead of the battery?

I thought that the thermocouple might be generating a low enough voltage that it could just be hooked up to the in+ and in- directly, but with between 25v/mv and 100v/mv gain I don't think it would, and my tests seemed so wildly different different that I wasn't able to draw any conclusion from them (which makes sense considering a single degree C would result in somewhere between 5v and 20v of change). Is the feedback loop approach the correct approach?

Why did the thing burn up? What did I do wrong?

tpowell1830:
First, welcome to the forum. Second, as always, when trying to get help on the forum with a specific circuit, always post your schematic and a couple of pictures of your setup.

No one can guess what happened to your circuit if we don't know what it is.

Third, if you do a google search for "thermocouple op amp circuit", you will get a boatload of examples.

Hope this helps...

kosine:
The first circuit on this page using a 358 (or equivalent) is about as simple as you'll get:

https://www.electroschematics.com/12610/how-to-play-with-thermocouples/

Not the best way to accurately measure the temp as explained later on the page, but it should get you started.

Note the Seebeck chart just above the circuit diagram shows that above 0C the thermocouple output voltage changes by just 40uV per degree. Not completely flat, but good enough for many simple applications.

jysd:
Just want to add that for cold junction compensation you should not just add the (cold junction) temperature to the hot junction reading. This works reasonably well for K type as it’s reasonably linear, but for other types it can put your measurement way off.

Instead read the temperature at the cold junction and convert that to the equivalent Emf voltage for that specific temperature (lookup tables available). Add this voltage the voltage read at the hot end, then convert that to degrees. (If you’re amplifying the hot junction reading you could either first divide that reading by the op gain then add the CJC voltage, or, multiply the CJC voltage with the op gain, add it to hot reading and then divide the sum with the gain)

soldar:

--- Quote from: solderjs on March 14, 2019, 09:53:13 am --- I have multimeter that came with a thermocouple.
--- End quote ---

I would suggest you keep that thermocouple safe and buy another one to experiment with. They are very inexpensive on ebay or other sites.

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