Glossary · Process instrumentation and analytical measurement
Loop-powered transmitter
Also known as: Loop-powered device, Loop-powered instrument
German: Schleifengespeister Messumformer
In process instrumentation, a loop-powered transmitter is a transmitter that draws its operating power from the 4–20 mA signal loop it controls, so no separate power supply is needed at the device.
- Process instrumentation
- Process industry
In one sentence
A loop-powered transmitter draws its operating power from the 4–20 mA loop it controls and needs no separate power supply.
Example
A loop-powered pressure transmitter operates from the 24 V supplied by the analog input card of the control system through its two signal wires.
How it applies
- Engineering: The device must operate on less than the minimum loop current, and the loop supply must cover the device's minimum terminal voltage plus the voltage drops across input resistors, barriers and cables. A load calculation confirms this.
- Installation: Loop-powered devices simplify field wiring and suit intrinsically safe circuits because of the low energy.
- Troubleshooting: A too-low supply voltage causes the signal to saturate below 20 mA at high values, which looks like a process limit rather than a fault.
- Documentation: Include the load calculation or maximum loop resistance in loop documentation, especially when devices such as indicators or barriers are added later.
Loop-powered vs. two-wire transmitter
In practice, the terms describe the same device class. Loop-powered emphasizes the power source; Two-wire transmitter emphasizes the wiring. Some loop-powered displays are not transmitters, so loop-powered is the broader description.