RTD 2, 3, 4 Wire Sensor Connections

The resistance temperature devices (RTD) can be connected directly to the controller peripheral amplifiers using 2-wire, 3-wire or 4-wire lead configuration. The wires in all these connections are in shielded cables.

The RTD is usually driven from a constant current source (I) and the voltage drop (V) across the RTD is measured.

RTD 2-wire connection

The 2-wire connection is illustrated in figure 1(a) below:

The RTD 2-wire connection is the simplest and cheapest lead configuration. In this case of 2-wire connection, the voltage drop is measured across the lead wires as well as the RTD; the resistance in the two-lead wires can be significant giving a relatively high degree of error.

RTD 3-wire connection

The 3-wire lead connection is illustrated below:

The RTD 3-wire connection is a compromise between the cost and accuracy. In this connection, a direct return lead from the RTD to the voltmeter is added as shown in figure 1(b) above. The voltage drop δV between the ground connection and the lower RTD connection as well as the voltage drop V between the current source and the lower RTD connection can be measured. If the resistance in each lead to RTD is presumed to be the same, the voltage across the RTD is V –δV correcting for the error caused by the common lead wire. In most cases each lead wire will have the approximately the same resistance, hence this method is accurate enough for most applications.

Related: Temperature transducers

RTD 4-wire connection

4-wire connection is illustrated in figure 1(c) below:

This RTD 4-wire connection is the most expensive but most accurate lead configuration. In this lead configuration, the voltmeter is connected directly to the RTD as shown in figure 1(c) above and since no current flows in the leads to the voltmeter there is no voltage drop in the measuring leads and an accurate RTD voltage is achieved.

You can also read: Types of Sensors used in Measurement and Process Control