Which device type is capable of sourcing its own power to drive the 4-20 mA signal?

• A. Active output device
• B. Passive input device
• C. Passive transmitter
• D. Passive output device

Answer: (A)

In a 4-20 mA loop, the current is the signal, and devices can either be powered from the loop or have their own power source. A device that can source its own power to drive the 4-20 mA signal uses its own supply to actively regulate and push current into the loop. That ability to supply current independent of the loop means it’s an active output device. Passive devices, on the other hand, rely on the loop to provide power and do not source current themselves; they either draw power from the loop or simply sense it without driving the current. A self-powered active transmitter is the clear example of the capability described.