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    Question about thermocouple circuit

    Hi there - I'm new around here and I'm looking for help with this circuit.

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    This is a thermocouple circuit which makes the cold junction compensation using the 1N4148 diode. I can understand almost everything about it but the function of the R4 resistor. Can someone help me with this? Thank you.

    Edit:

    I'm working on this project with a friend of mine and we already made some tests trying to figure out this. Today we measured the voltage on the thermocouple terminals with the hot junction at 0ºC and the cold junction at 30ºC. The read was -1.18mV which is fine. Then we took out the R4 resistor and our measurement was -1.16mV. We tested it three times. The point is that if the resistor is a load to the thermocouple, taking it out the voltage should go up.
    Last edited by Packo_z007; 25th January 2013 at 01:54.

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  2. #2
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    Re: Question about thermocouple circuit

    As R4 is exactly 10 X the value of R5 and is a .1% tolerance, it is probablely used to keep the compensation accurate over a wide temperature range.
    Frank



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  3. #3
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    Re: Question about thermocouple circuit

    I'm sorry I don't understand what do you mean. Why would improve the accuracy? Could you be more specific ? I noticed that the image quality is not the best. Probably by R5 you mean R3. You can find the circuit in this datasheet at page 9.

    We made some other measurements today. Maybe this helps someway. Removing R4 resistor the current through R5 and the thermocouple decreases. That explains why the thermocouple voltage increases. Thanks for your answer.



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  4. #4
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    Re: Question about thermocouple circuit

    Clever people do not add expensive resistors to their circuits without a good reason. So if you think that it is not necessary, plot a complete temperature run with it and without it, then you can find out for yourself what its effect is. It might not be required for your temperature range and accuracy.
    Frank


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    Re: Question about thermocouple circuit

    I think, R4 is intended to achieve safe behavior in case of wire break. I don't exactly understand what you intended by omitting it?

    The shown TC amplifier is less-than-perfect, because TC respectively wire resistance have a certain effect on zero and scale. It might be a problem either with long wires or very thin TCs that can have up to several 10 ohm series resistance.


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  6. #6
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    Re: Question about thermocouple circuit

    Thank you both for your answers. Frank, I don't think it is not necessary. This is part of a college project and I have to explain how the circuit works and this resistor is the only thing left. Well, I made a simulation with a wider temperature range and I didn't found any big difference.

    Quote Originally Posted by FvM
    I think, R4 is intended to achieve safe behavior in case of wire break. I don't exactly understand what you intended by omitting it?
    I omitted it trying to figure out what is it for. I was looking for any change in the circuit state that give me a clue. Anyway, I'm staying with that explanation. It have a lot of sense. Thank you both.



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