Electrical Circuits for RTDs

Since RTDs vary the amount of their resistance when they are heated, they must be connected to a resistive-sensitive circuit like a Wheatstone bridge to produce a usable signal as a sensor. ill. 1 shows an example of a two-wire RTD connected to a bridge circuit. The two terminals in the middle of the bridge can be connected to a digital voltmeter (DVM) or they can be connected to a controller if the RTD is used as the temperature sensor for the system. Since the RTD can only change resistance, the bridge circuit must have an external power supply. In some applications the RTD must have additional wires connected to it to provide temperature compensation. Temperature compensation is necessary in some applications because of the difference in temperature between the point where the RTD is mounted and the location where the transmitter or controller is converting the value. When the two-wire RTD is connected in the Wheatstone bridge circuit as in ill. 1, it becomes one of the "legs" of the bridge. In this configuration, the RTD does not have any means of providing temperature compensation. This configuration is the most common circuit for the RTD.

Above: ill. 1 Electrical diagram of a two-wire RTD connected to a Wheatstone bridge circuit.

The circuit in ill. 2 shows a three-wire RTD. In this circuit notice that the third wire for the RTD is connected at the same terminal as one of the original two leads. This extra lead can be used in a wide variety of circuits to cancel the effects of unwanted temperatures so that all changes in resistance to the bridge come from the RTD sensor. Unwanted temperatures can come from the wire heating slightly due to current flowing through it, or from temperature changes of the air that is near the sensor and its wires. The compensation occurs because the same amount of current will flow in the compensation lead as in the original lead and the design of the circuit causes the voltage drop across each set of terminal wires to be the same, which effectively allows them to cancel each other.

Above: ill. 2 Diagram of a three-wire RTD connected to a Wheatstone bridge.

ill. 3 shows a diagram of a four-wire RTD. In this configuration, one additional wire is connected to each end of the original two-wire RTD. These additional wires provide another way to compensate for unwanted changes in resistance. The two-wire RTD is adequate for the vast majority of temperature sensors. The three-wire and four-wire RTDs are available for applications where the RTD must have greater accuracy.

Above: ill. 3 Electrical diagram of a four-wire RTD.