Using arduino to simulate a pt100 temperature sensor

Question

I want to simulate a temperature PT100 sensor using arduino. What I did till now is on that draw that I attach here. The components were determined by experiments so I guess if there can be some calculations maybe it will improve.

I have a device that reads a PT 100 sensor and if instead the sensor (2 wires model) I wire my device I am able to simulate different temperatures and also I am able to make some sort of calibrations means for example at pwm 163 I get 100 degrees on the device.

My problem is that at some point in time that temperature varies without touching anything. Also there were cases like from a day to another to measure some different temperatures with same setup.

Any idea or hint will be much appreciated

Thanks

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A modified circuit for simulation. Note that Q1 is the opto-isolator transistor but has its base connected through as the CircuitLab simulator doesn't have an opto-isolator.

How it works:

• The PWM turns D1 fully on or off.
• Q1 turns hard on or off. In general opto-isolators do not do analog transmission well. Much more reliable results are obtained with digital transmission.
• R3 is set at 250 Ω which is about 400°C on a Pt100. With Q1 turned off you should get a reading of 400.
• With Q1 on all the time (PWM = 100% duty) R2 will be in parallel with R3 and, neglecting the voltage drop across Q1, the parallel combination of R2 || R3 will be 94 Ω which is a little below 0°C.
• C1 smooths out the ripple due to the PWM. Bigger is smoother. Higher frequency is smoother.

Figure 2. The simulation result for the values shown in Figure 1.

You can play with the CircuitLab simulation by editing my post and "edit the above schematic". Don't save your changes!

Answer 2

Somewhat ironically, your circuit may be sensitive to temperature.

If you look into the EL817's datasheet (the optocoupler's), page 4, figure 3, you'd see a graph of the optocoupler's Current Transfer Ratio (CTR) varying with temperature, and somewhat dramatically at that.

Your circuit relies on the CTR being constant.

Answer 3

The simplest way would be to use a potentiometer to simulate it.

Look up the datasheet of your PT100 and look at the resistance of the sensor at different temperatures. Then you just simply adjust the potentiometer to those resistances. Easy!

If you want to have the simulation controlled by the Arduino, you can always use a digital pot.

Control a digital potentiometer with Arduino