Resistance Temperature Detectors

[pinkcashmere]

Can someone explain three wire and four wire RTD circuits in terms of how the lead resistances affect the voltage output

$V_o = \frac{R_3 V_s}{R_2 + R_3} - \frac{R_4 V_s}{R_1 + R_4}$

Also, in the case of a three wire, what is the purpose of Lead C?

 

[meBigGuy]

First you need to understand Kelvin Contacts:
https://en.wikipedia.org/wiki/Four-terminal_sensing

Then,the Kelvin Bridge.
https://en.wikipedia.org/wiki/Kelvin_bridge

Then either you will have your answers or more questions.

The whole goal is to eliminate errors caused by measuring the voltage drop across the current carrying wires that are feeding the sensors.

Maybe you understand that already, but I think you just need to work through the math of the two circuits to see how the errors cancel.