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problem with ina 333 - thermocouple type J

Luis Martin
Prodigy 80 points
Other Parts Discussed in Thread: INA333, OPA333, LM358

Hi everybody.


I have a problem using a INA333.

I'm using it for the measurement of a type J thermocouple for a project.

The temperature range my thermocouple will be on is between 0 and 100 °C, i'm gonna read the value on a microcontroller and do the cold junction compensation by the software method using another temp sensor which will be sensing the temp at the board or junction area.

my problem is that using the datasheet instructions for the right connections, i have no voltage (zero) at the Vout pin.

I used different gains, tested if my thermocouple is ok, used the special conectors for the type J thermocouple, and nothing happened.

I use a single voltage supply of 5v.

Could anyone please show me any circuit using what I mentioned before?......i think i'm missing something.

If you need additional info please let me know.

  • 0
    Expert 1130 points

    Try reversing the thermocouple connections. The output cannot swing negative.

    So if the input voltage is of wrong polarity, it will limit to zero.

  • 0 in reply to Nikhil Kant
    Prodigy 80 points

    hi, nikhil kant.

    thanks for answering so soon.

    i already did it , and nothing changed...i think the circuit i i'm using is incorrect, but i just followed the datasheet instructions.

         

  • 0 in reply to Luis Martin
    TI__Intellectual 2770 points

    Hi Luis,

    Here are a couple of things I would try to narrow down the problem:

    • Measure the Voltage at the V_IN+ and the V_IN- pin with a good multimeter. With a hot air fan at the end of the thermocouple you should get a couple of mV of differential input voltage.
    • Calculate the expected output voltage. Ask yourself if both the expected input and the output voltage is in the allowed common mode range (see page 8 of the datasheet).
    • If everything looks good up to this point and you still get no output: Try building the simplest circuit you can imagine with the INA333 to verify that it is hooked up correctly. A gain of 1 configuration with a signal generator at the input might be a good idea. When you do this verification: Make sure that you don't violate the input common mode range!
    • If you don't get a differential voltage at the input of the part: Something might be wrong with the way you have hooked up the sensor. Make sure to verify this part of the circuit again.

    These are just a couple of ideas. It would be more in interesting to sit together with you in the lab and play a little bit with the circuit...

  • 0
    TI__Genius 12345 points

    Luis,

    I have a feeling that you are using the INA333 in a high gain configuration and have the reference pin grounded as well as the input referenced to ground (0V). This configuration will only work if you are using dual supplies (+/- 2.5V). The schematic in the datasheet is simplified and is really only meant to remind the user to provide an input bias current pathway (10kOhm resistor) when connecting a thermocouple.Try placing a bias voltage at the reference pin and at your input as shown below:

  • 0 in reply to John Caldwell
    Prodigy 80 points

     

    John thank you very much for the answer.

    I'm using the INA333 just as you showed me.....before looking at your schematic, I had the same circuit, but the 10k resistor was between the thermocouple pin + and GND, and the Vref pin was connected to GND.

    Then I used your circuit, but I'm using a LM358 instead of the OPA333 and I have 2.5V using two 1k resistors.

    I followed your instruction, so using a hot air fan and measuring the input voltage at the INA333 I have readings between 0.9 mV and 3.3 mV....which i thing are ok looking at the type J thermocouple reference table.

    But now the problem is....no matter what voltage I have in the input....I always have the same 2.5V at Vout pin.

    I realized that when I connect the power supply and measure continuity between Vref and Vout, there is a short....is that ok?.....when the device (INA333) is not connected, no short is heard.

     

  • 0 in reply to Luis Martin
    TI__Genius 12345 points

    Luis,

    Any time you take a measurement, it is good to consider how the piece of equipment is performing this measurement and how the circuit you are testing will interact with it. For a continuity measurement, your meter is most likely injecting a small current through one of the test leads and measuring the voltage difference between the two test leads. When there is continuity between the test leads, there should be no, or a very small, voltage difference between the two test leads. The INA333 in this case is tricking your meter, it is maintaining its output voltage (the reference voltage) by sinking the current injected by the meter, hence why the meter believes there is continuity between the output and the ref pin.

    This doesn't eliminate the potential that your INA333 is damaged however. One of the best ways to start the debug process is to examine the amount of current that the part is drawing from its power supply, is it close to the value given in the datasheet?

    I simulated your circuit a bit and will modify what I said in the previous post, the reference pin can be tied to ground (to maximize the output voltage swing) but the input must still be bias to mid-supply. What is the value you are using for the gain resistor RG?

  • 0 in reply to John Caldwell
    Prodigy 80 points

    Thank you again John....

    I'm using a 1k resistor RG for a 100 gain...I'm going to measure the current that the part is drawing from it's power suppy right know.

    i'll tell you what i found :D

  • 0 in reply to Luis Martin
    Prodigy 80 points

    Well i measured the current...and i had 3.64mA.

     

    With the circuit you just showed me, with an input of 0.8mV, the vout pin is at 3.4mV using a 1k RG resistor for the gain

  • 0 in reply to Luis Martin
    TI__Genius 12345 points

    Luis,

    If you were measuring the total supply current that would be about right, 2.5mA for the voltage divider, ~1mA for the LM358. The INA333 only consumes around 50uA on its own.

    Try reversing the polarity of the thermocouple and see if the output voltage increases or decreases.

  • 0 in reply to John Caldwell
    Prodigy 80 points

     

    John thank u again.

    I measured the current for the voltage divider: 2.5mA, for the LM358: 0.7mA and the INA333 42mA.

    By reversing the polarity of the thermocouple the output voltage decreases

    I have 3 different INA333 devices and all measures were the same.

  • 0 in reply to Luis Martin
    TI__Genius 12345 points

    42mA for the INA333? If that is the case the part is definitely damaged.

  • 0 in reply to John Caldwell
    Prodigy 80 points

     

    Yes I think it is.....the devices I used were used by other people before.....I'm gonna weld a new INA333 and see what happen.

    I really apreciate your help thank you so much......if everything goes well...i let you know.  

  • 0 in reply to Luis Martin
    Prodigy 80 points

     

    John Thank you very much for your help.....after a lot of work and time playing with the circuit.....I realized that the INA333 was not damage.

    The problem was board conections.....I decided to re build all the circuit and thank God everything is fine.....the last  schematic you showed me works excellent!!!

    TI support is awesome....I really apreciate the time and attention you had with me.!!

    God Bless you!!

  • 0 in reply to Luis Martin
    TI__Genius 12345 points

    Hi Luis,

    I'm really glad to hear the circuit works properly now and I hope your project is a success!