In a 4-20 mA loop, which pairing of devices is allowed?

• A. Active to passive
• B. Passive to passive
• C. Active to active
• D. Passive to active

Answer: (A)

In a 4-20 mA loop, the loop current must be driven by a source. The transmitter is the active device that controls and sources the current (4–20 mA) proportional to the measured value. The receiver or indicator is a passive load that simply consumes that current to produce a signal or reading. Two active sources would fight each other for control of the loop current, two passive devices would have no source of current at all, and a passive device cannot drive current into an active device. Therefore, the standard and valid pairing is an active transmitter driving a passive receiver/load. For example, a loop-powered transmitter (active) feeding a PLC input or indicator (passive).