What is the fundamental principle of a 4-20 mA current loop transmitter?

• A. It modulates voltage from 0 to 24 V to represent the process variable.
• B. It modulates current from 0 to 20 mA to represent the process variable.
• C. It generates pulses proportional to flow rate.
• D. It modulates current from 4 to 20 mA to represent the process variable.

Answer: (D)

In this signaling approach, the information about the process variable is carried by the current, not by voltage. The transmitter adjusts the loop current to reflect the measured value, and a receiving instrument reads that current to determine the process variable. The range from 4 mA to 20 mA is used so that there’s always a small, nonzero current even at the low end (which helps power the transmitter and avoids confusion with a broken or open loop), and the full-scale value at 20 mA represents the maximum measurement. This current-based scheme is also robust over long cable runs because current is less affected by voltage drops along the wiring than voltage signals are. So the fundamental principle is modulating loop current from 4 to 20 mA to represent the process variable.