Two-Wire, Three-Wire, and Four-Wire RTDs

ill. 1 shows examples of two-wire, three-wire, and four-wire RTDs. The symbol for the RTD is a resistor with an arrow through it indicating it's a variable resistance. The resistor is also identified by R1 to indicate this resistor is temperature sensitive.

ill. 1a shows a two-wire RTD; ill. 1b shows a three-wire RTD with terminals 2 and 3 connected to a common point. The extra wire is used to provide temperature compensation and the typical circuit for this is shown in ill. 2. ill. 1c and 1d show two configurations for a four-wire RTD. ill. 1c shows a second wire R2 that is connected to R1, and R3 connected to R4. Since terminals R1 and R4 have extra wires connected to them, they both can be used in the circuit to provide junction temperature compensation. ill. 1d shows a second way to manufacture a four-wire RTD. In this application a two-wire RTD is used with two additional wires added as a lead resistance loop at terminals R3 and R4. The lead resistance loop is used in bridge circuits to provide temperature compensation.

Above: ill. 1 a-d Examples of two-wire, three-wire, and four-wire RTDs.

Above: ill. 2 Electrical diagram of a two-wire RTD connected to a Wheatstone bridge circuit.