# Pt100 wires to make Wheatstone Bridge

I'm currently trying to build a circuit on a breadboard that, through a temperature sensor, gives off a voltage which I need to amplify. I'm using a Pt100 sensor with 3 wires (red, blue and yellow) connected in a Wheatstone bridge that I in turn connect to an instrumental amplifier and get an output voltage. I have built pretty much the entire circuit; the schematic is this:

The negative voltage input to the amplifier goes in pin 2 and the positive in pin 3. I'm using 100 Ohm resistors for the Bridge and a potentiometer for the Gain Resistance, but that's not the problem. I don't know how to connect the Pt100 wires in order to get my Wheatstone Bridge. I have thus far built the following circuit in my breadboard:

I have wracked my brains over this and although I'm sure the answer is right in front of me I can't for the life of me see it. My head hurts. I wouldn't know if you need any further information. I figure with the 100 Ohm resistances further up on the breadboard I'm getting the lower 2 that show up in the schematic. I'm not sure anymore though. Everything got messed up in my head somewhere along building it.

I greatly appreciate your help. I looked up how the wires work but didn't understand much. I guess the answer I'm looking for is an explanation of why there are 3 wires, how to connect them and breadboard-related, where exactly (row number, letter) to connect them. Or any corrections to the bridge.

• en.wikipedia.org/wiki/… Nov 3, 2014 at 0:17
• Two will be effectively shorted together, another will measure about 118 ohms to either of the other two. Nov 3, 2014 at 0:26

PT100 can be used as 3-wire or 4-wire based on the RTD you have. There are different options available and which one you need to use depends on the precision required.

As far as Wheatstone bridge is concerned, the idea lies between balanced and unbalanced bridge. Check wiki link for more details on bridge.

I don't know how to connect the Pt100 wires in order to get my Wheatstone Bridge.

The variable resistance part R2 shown below is your PT100

Image source: Wikipedia

You should use R1, R3 and Rx and 1% 100 ohm resistors for good results. The voltage change is measured along the terminals D and B. When the bridge is balanced you get almost zero voltage between terminals B and D. And when there is variation in the temperature, the resistance value of PT100 changes which in turn makes the bridge unbalanced and you get few mV between terminals D and B.

The important part here is the excitation i.e. the voltage applied at terminal A of the Wheatstone Bridge, it must be constant current source to give good and precise results, you can use darlington pair or opamps to do constant current source design. Refer this thread from TI which talks about constant current source and measurement techniques.

For a good and robust design and a signal conditioning circuit must come after the Wheatstone Bridge and ADC which converts you analog temperature to digital useful data. See here application note from Microchip which talks about signal conditioning for RTDs.

And even there are integrated options available from different Silicon manufactures for temperature sensing using RTD or thermocouple like LMP90080 and other are available from Analog Devices.

And at last please don't build sensing circuits on breadboard as it will add lead inductance to circuit and will surely affect performance.

Hope this helps you.

This is the normal connection of a 3-wire RTD in a bridge:

simulate this circuit – Schematic created using CircuitLab

The current through R1 and R2 are approximately equal (a bit less than 1mA). The branch with R1 and R4 balances off the 0°C resistance of the RTD and mostly cancels the error due to leadwire resistance (assuming they're reasonably well matched to each other).

You really can't use a solderless breadboard for this kind of instrumentation circuit and expect good results. The RTD is probably stable to a few milliohms, and the connections on the solderless breadboard won't be nearly that good.

• Ahh, the "third" wire is to compensate for changes in the resistance of the two "ground" lead wires with temperature. Nice. @Emilio Botero, rather than platinum, there are lots of RTD's with ~10k ohm of resistance at room temperature. Much easier to use if you don't need the calibrated response of Pt. Nov 3, 2014 at 13:44