Electronic – How to make a voltage controlled adjustable resistor

resistorsrtdthermistorvoltage

I need it for simulating PT100, PT500, PT1000 RTDs, and thermistors in a temperature range of 0-200 °C.
What I want to do is for example: enter 50 °C using a microcontroller, and through the DAC output, it generates me a specific voltage, which I can use to adjust a resistor.
I have to connect it as a two-terminal to devices, and it has to behave like the temperature is 50 °C.

Best Answer

This is presented as an incomplete answer.

Most Pt100 circuits work by feeding a constant, known, current in the order of 1 mA through the sensor and by measuring the voltage developed across its terminals to either calculate or look up the temperature. By replacing the Pt sensor with a calibrated resistor we can easily simulate the 0°C condition. Any temperatures higher than zero can be simulated by pushing additional current from an external source through the resistor.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Pt100 / 500 / 1000 simulator.

If the sensor current is known and is exactly the same on every device to be tested the circuit of Figure 1 might go some way to providing a solution. For example, assuming 1 mA from the Pt100 source and SW1 closed and "resistances" of 100 Ω at 0°C and 119.25 Ω at 40°C (based on 0.385 Ω/K, which I have not checked):

  • Voltages required across R1 will be 100 mV and 119.25 mV respectively.
  • Setting DAC voltage to 100 mV should result in the op-amp contributing nothing as the voltage drop from the Pt100 current source will provide the required voltage drop and the op-amp input terminals will have equal voltage and will be "happy".
  • Setting the DAC voltage to 119.25 mV will cause the op-amp output voltage to increase, driving additional current through R1 and causing its voltage to rise unitl then non-inverting input is also at 119.25 mV. This should result in a 50°C reading on the instrument under test.

"Ah," I hear you cry, "but we don't know the source current!"

Well then we'll have to measure it.

schematic

simulate this circuit

Figure 2. Pt100 / 500 / 1000 simulator with sensor current measurement.

By reducing the DAC output to zero the op-amp output will go to zero. D1 prevents the op-amp from loading R1. By reading the resultant voltage across R1 we can calculate the current and scale the DAC output to suit. D1 should be low reverse leakage type.

Note that the circuit must be floating and not have any other connections to the device under test. Reverse connection protection is advised: a diode from op-amp output to V+ and another from GND should do the trick.

Several design challenges remain:

  • Adequate resolution on the DAC output. This may be possible by using the full DAC output range and using a further op-amp circuit to amplify (gain < 1) and offset the control signal.
  • Adequate resolution on the ADC input. As above but remove offset and scale up.
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