Glossary · Process instrumentation and analytical measurement
Four-wire transmitter
Also known as: 4-wire transmitter, Separately powered transmitter
German: Vierleiter-Messumformer
In process instrumentation, a four-wire transmitter is a transmitter with separate pairs of wires for power supply and output signal, used when the device needs more power than a current loop can provide, for example analyzers or flowmeters with heated or excited sensors.
- Process instrumentation
- Process industry
In one sentence
A four-wire transmitter has separate wire pairs for power and signal and is used for devices that need more power than a current loop provides.
Example
A magnetic flowmeter is powered with 230 V AC and provides its 4–20 mA output on a separate pair of wires as a four-wire transmitter.
How it applies
- Engineering: Separate supply allows higher power consumption and outputs that are either active (the transmitter drives the current) or passive. Output type and input card must match; otherwise the loop is either unpowered or double-powered.
- Installation: Mains-powered transmitters need their own protection, disconnection and, in hazardous areas, a suitable type of protection. Galvanic isolation of the output avoids ground loops.
- Maintenance: Before working on the device, both supply and signal circuits must be considered; switching off the control system input does not de-energize the device.
- Documentation: Show supply and signal circuits in the loop diagram and state whether the output is active or passive.
Four-wire vs. two-wire transmitter
A Two-wire transmitter gets its power from the signal loop. A four-wire transmitter has separate supply, which costs wiring but removes the power limit.