Thermocouple amp with one stage and single rail?

Discussion in 'General Electronics Chat' started by prometei, May 1, 2015.

  1. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
    56
    0
    Hi,

    I've been trying to put together a simple one stage amp for a K-type thermocouple on a breadboard, without success. I've tried a TL072 and a TL082 with the 200mV setting on the oscilloscope but it only shows noise. I'd like to have something like 10mV per 1 degree C.

    Is it it possible to do this with just one amp stage or do I need two or more? Should I use inverting or non-inverting configuration? Can it be done with just a single rail power supply?

    If someone has a circuit for this purpose above mentioned op-amps I'd appreciate if you post it here.

    Thanks!
     
  2. #12

    Expert

    Nov 30, 2010
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    • LM35.pdf
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  3. Reloadron

    Active Member

    Jan 15, 2015
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    What #12 said.

    Also if you are set on type K thermocouple (or any thermocouple) there are several important considerations. As to an amplifier? You can start with the AD595 Type K Thermocouple Amplifier with CJC (Cold Junction Compensation). You really want to pay attention to that last part about CJC. You need an instrumentation amplifier type design, just trying to amplify a K thermocouple output using a simple operational amplifier like those you mentioned will not work. Even when using an amplifier like the one I linked to the board layout is critical.

    Next, depending on your range of measurement there are other options, among them are Thermocouple Transmitters which make for easy and do all the work including CJC and thermocouple linearization. As can be seen these devices are available from a host of manufacturers with a wide range of cost. Just a matter of what your project is and what you need. I will suggest that if you have any plan to roll your own you study CJC as it becomes very important.

    Ron
     
    Dodgydave and #12 like this.
  4. DickCappels

    Moderator

    Aug 21, 2008
    2,664
    634
    Below is the schematic and parts list of the Fluke TK-80 thermocouple adapter. It uses a single opamp and a single 9V battery.

    It was state of the art 25 years ago. I suspect that today they would use one of the thermocouple amplifier integrated circuits such as those mentioned by Reloadron.

    The schematic is best viewed by viewing the image separately.

    Given that the input signal is microvolts, using one of those plastic plug-in breadboards might be a way to see if an idea works, but it really is not good for small signals. Remember, every junction of dissimilar metals is another thermocouple to worry about.

    Fluke-TK-80.jpg
     
  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
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    yep the AD595 gets my vote too, built an oven thermostat with it ,10mV per deg C,accurate to 1deg C easy to make...
     
  6. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
    56
    0
    hi guys,

    thanks for the input. I should have mentioned that it is a thermocouple in a soldering iron heating element. I'm trying to put together a DIY soldering station with temperature control. I found some circuits online, one uses an LM385 and another that uses a TL072.

    this one uses LM385's. http://d-serviss.lv/uploads/files/1428938349.png
    IC1A is connected to a thermocouple. I copied the circuit but instead of using a LM358 stage I used a TL082/72 stage.

    this circuit uses TL072:
    http://sc0ty.pl/wp-content/uploads/2012/10/soldering1.png
    Hooked up this one on a breadboard with a TL072. I only tried the first op-amp stage IC1A, measured the output and wasn't getting any expected reading while heating the thermocouple with a lighter.

    I think I might have been powering the op-amp incorrectly. Gonna redo the wiring today and check the quality of the power.
     
  7. Reloadron

    Active Member

    Jan 15, 2015
    969
    235
    I looked at the circuit for the TL072. It is not a very well thought out circuit for measuring a thermocouple. There was good reason I suggested you read a little on CJC earlier. OK, lets say my room temperature is 70 F. At 70 F. a type K thermocouple outputs about 0.843 mV.

    Above Dick mentions:
    Your thermocouple likely just connects to your amplifier without any regard for CJC. What about those junctions? You have thermocouple alloys connecting likely to Cu (Copper) making for an additional 2 junctions. In effect these junctions at 70 F. will cancel out your thermocouple resulting in a thermocouple output voltage of Zero or pretty close to zero. The circuit using the TL072 is garbage. They not only use a TL072 to try to amplify a thermocouple output but neglect CJC. Then the op amp out is passed on to a TL072 acting as a comparator. Why not use a comparator chip designed to be a comparator rather than an operational amplifier? Once again I suggest you read a little about CJC, while not great this is an OK read about thermocouples and includes an explanation of CJC.

    Ron
     
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