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Reading Temperature From PT100 | Electronics Lab

Reading Temperature From PT100
Version 1 - Last update: Nov 26, 2016

Reading Temperature From a PT100

** Arduino not in circuit as it didn't have required library and slowed down simulation,
** Arduino code can be found here and my instructables post about this can be found here. The output of the differential op amp is connected to A0 and make sure the GND pin of the Arduino is connected to the GND of the circuit

The circuit is being used to simulate reading the temperature from a PT100 which is a resistive temperature detector(RTD). As the PT100 changes resistance with temperature, we need to convert this resistance to a voltage that can be read in to the Arduino then a temperature can be calculated. Using a wheatstone bridge the voltage above a offset resistance set by R2 can be amplified and read by the Arduino. When choosing the top resistors you want to limit the current flowing through the PT100 as this will heat it up and give wrong readings, but you don't want to increases this resistance so much that the voltage change from the temperature change is so small you can't amplify it. The offset resistant R2 will be the minimum resistance you expect the PT100 to go to i.e. 80 ohms for -50°C.
First the output of the bridge can be calculated as follows:
Vb1-Vb2=Vin(Rx/(R1+Rx)-R3/(R3+R4))
where Rx is the PT100 resistance, Vb1 is the voltage at the PT100 and Vb2 is the voltage at R4.

This could be be put straight into a differential amp but there's problems with this such as loading and needing to change two resistor to vary the gain, instead we'll go into an instrumentation amp which we can change the gain with only one resistor and doesn't load the bridge. The gain of the instrumenation amp is as follows
(V1-V2)=(Vb1-Vb2)(1+2Ri/RG)
where V1 is the voltage at RA1 and V2 is the voltage at RA2. Changing the value of the pot RG will vary the gain.

After the instrumentation amp, the signal is passed through a differential amp which will subtract V1 and V2 and amplify this voltage by the follow:
Vout = (V1-V2)RB/RA
You will have to change both the RA's and RB's to change the gain and they have to be the same. Because of this we use a pot on the instrumentation amp it only needs one resistor to be changed the vary the gain. My current setup uses a offset resistance of 80 ohms(-50°C) and uses a pot to change the resistance to 150ohms(130°C). Now you can read this voltage into something like an arduino and calculate the temperature using the formula below
Rx = R0(1+A
T)
Rx = PT100 Resistance
R0 = Resistance @ 0°C = 100ohms
A = Temperature coefficient of resistance = 0.00385

For more accurate temperature calculation you use a formula found here. Or if you want to visit my website which is where I post the projects I make, you can find that here.

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