Glossary · Process instrumentation and analytical measurement
Three-wire transmitter
Also known as: 3-wire transmitter
German: Dreileiter-Messumformer
In instrumentation, a three-wire transmitter is a transmitter connected with three wires: supply voltage, common reference (0 V) and signal output, with the signal referenced to the common wire.
- Process instrumentation
- Process industry
In one sentence
A three-wire transmitter uses separate supply and signal wires with a shared 0 V reference, allowing more device power than a two-wire loop.
Example
A three-wire ultrasonic level sensor receives 24 V DC and 0 V and outputs its 4–20 mA signal on the third wire to the PLC input.
How it applies
- Engineering: The separate supply allows higher power consumption than a two-wire loop and supports voltage outputs as well as current outputs. Because the signal shares the 0 V wire, there is no galvanic isolation between supply and signal.
- Installation: Wire the common reference correctly; voltage drops on a shared 0 V wire can offset voltage signals. Signal isolators help when grounds differ.
- Commissioning: Check that the input card is set for an active current or voltage source, not for a two-wire loop.
- Documentation: State the wiring type and the output type clearly in device lists and circuit diagrams; mixing up two- and three-wire connections is a common commissioning error.
Three-wire vs. two-wire transmitter
A Two-wire transmitter shares power and signal on one pair and must work with less than 4 mA. A three-wire transmitter has its own supply line and therefore more power available.