Which statement best describes a passive input device?

Which statement best describes a passive input device?

• A. It does not require its own power supply to drive current through the loop.
• B. It requires its own power supply to drive current through the loop.
• C. It actively sources current into the loop.
• D. It converts current to digital signal independent of loop.

Answer: (A)

Passive input devices in a 4–20 mA loop are powered by the loop itself; they do not have their own power supply. The loop current provides the energy, and the device converts the sensed value by changing its impedance, so the current flowing through the loop reflects the measured process variable. Because it does not supply power, it cannot drive current into the loop—the loop transmitter controls the current. An active device would require its own power and could source current into the loop. The idea of converting current to a digital signal independent of the loop describes a function some devices perform, but it doesn’t define a passive input device—the defining trait is the absence of an internal power source to drive current.

Passive input devices in a 4–20 mA loop are powered by the loop itself; they do not have their own power supply. The loop current provides the energy, and the device converts the sensed value by changing its impedance, so the current flowing through the loop reflects the measured process variable. Because it does not supply power, it cannot drive current into the loop—the loop transmitter controls the current. An active device would require its own power and could source current into the loop. The idea of converting current to a digital signal independent of the loop describes a function some devices perform, but it doesn’t define a passive input device—the defining trait is the absence of an internal power source to drive current.