Looppowered

Loop-powered refers to devices that derive their operating power directly from a two-wire signal loop, most commonly the 4-20 mA current loop used in industrial process control. In a loop-powered arrangement, the device is inserted in series with the loop’s current source and the receiving equipment, and it encodes a measurement by modulating the loop current. The loop current normally ranges from 4 mA (minimum) to 20 mA (maximum). The receiving instrument or input module converts the current back to a voltage or digital value.

Power is drawn from the loop itself; the device uses the loop supply voltage and the series load to power its electronics, without a dedicated power supply. Because the loop’s current must be regulated, the device must operate within a defined power budget set by the loop resistance and the supply voltage. If the loop impedance is too high or the supply voltage too low, the transmitter cannot draw the required current. Conversely, loop conditions outside these limits can impair accuracy or prevent operation. Some loop-powered devices incorporate low-dropout regulators or energy-saving techniques to maximize the headroom.

Applications include sensors and actuators in process industries such as chemical plants, oil and gas facilities, and water treatment. Two-wire loop-powered devices simplify installation by reducing wiring and eliminating a separate power supply, while enabling loop-powered signals to travel long distances. Many loop-powered transmitters also support digital signaling standards such as HART while maintaining a 4-20 mA current loop. Limitations include a fixed current range, a limited power budget for complex electronics, and potential calibration constraints due to shared power. They remain common in both legacy and new installations where simple, robust instrumentation is required.