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I'm trying to build a basic circuit that can monitor a thermocouple, compare the resistance to a known source and then adjust a valve to maintain a certain temperature.

I have designed and tested the circuit to compare the thermocouple with the known voltage and have an output of between -5 and +5v. Ideally I'd like to take that variable voltage and simply add it to the voltage currently going to the valve so the final voltage will either go down, stay the same or go up.

Is there a way I can use an op amp to monitor the final voltage and the sum it to the variable one? I've attempted to use a capacitor as a 'memory' for the final voltage, but I'm not getting anywhere.

As a side note, I'm running at 24V with a virtual ground to create +-12V. The thermocouple and known source are both variable resistors with their outputs going through a differential amplifer to give the -5V to +5V range. The valve takes a 0-20ma signal which I'm generating from 0-5V.

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    \$\begingroup\$ Welcome to EE.SE. A schematic is better than words. You can add one in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$
    – Transistor
    Commented Jul 25, 2020 at 13:51
  • \$\begingroup\$ First, you must spec or define the proportional gain from R in ( or temp range) to Vout = 0~5V . Offset voltage is the setpoint which must have a range. (Always learn to define specs before a design) \$\endgroup\$ Commented Jul 25, 2020 at 13:55
  • \$\begingroup\$ What you need is a PI op amp circuit to do PI control. You can google "pi op amp circuit" and get the circuit. \$\endgroup\$
    – scorpdaddy
    Commented Jul 25, 2020 at 14:08
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    \$\begingroup\$ A thermocouple is not a variable resistor. Are you sure you’re using a thermocouple and not a thermistor? \$\endgroup\$
    – The Photon
    Commented Jul 25, 2020 at 14:13

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Something along these lines is probably what you want.

RV1 adjusts the proportional gain. A PB of around 3% usually works well and is stable, if the system is well designed. Badly designed systems can still oscillate, but proportional controllers are inherently more stable than PI, and can hold the temperature to within a fraction of 1% in many practical circumstances.

RV2 is "manual reset". Adjust it so the valve is approximately in the correct position for the setpoint with no error. Alternatively, wait for it to settle then adjust it to eliminate any offset under typical operating conditions.

schematic

simulate this circuit – Schematic created using CircuitLab

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