What is an active output device?

• A. A device that modulates current only.
• B. A device that requires external power supply to drive the loop.
• C. A device that supplies its own power to drive the 4-20 mA signal.
• D. A device that measures loop resistance.

Answer: (C)

In a 4-20 mA loop, an active output device is one that has its own power source and can source current into the loop to drive the 4-20 mA signal. That self-contained power lets the device generate and control the loop current without needing the loop supply to power its electronics. This is what sets it apart from passive, loop-powered devices that rely on the loop current to power them, and from measurement devices that simply sense or measure the loop (like loop resistance). So, supplying its own power to drive the 4-20 mA signal is the defining trait of an active output device.