Which statement is true about 4-wire transmitters?

Which statement is true about 4-wire transmitters?

• A. They are powered directly from the process loop.
• B. They use a separate power supply for internal electronics.
• C. They cannot provide analog outputs.
• D. They have only one output channel.

Answer: (B)

In a 4-wire transmitter, the sensing electronics run from a separate power supply, independent of the 4‑20 mA current loop. This design lets the transmitter power its internal circuitry without drawing power from the loop, providing isolation and enabling more capable or higher‑power electronics. The current loop still carries the analog 4-20 mA signal to the controller, but the electronics themselves aren’t powered by that loop, which is the defining trait of 4-wire devices. Why the other statements don’t fit: being powered directly from the process loop describes a 2-wire, loop-powered transmitter rather than a 4-wire one. The transmitter does provide an analog output—the 4-20 mA signal—so claiming it cannot would be incorrect. And having only one output channel isn’t the defining feature of 4-wire transmitters; the essential point is the separate power supply for the internal electronics.

In a 4-wire transmitter, the sensing electronics run from a separate power supply, independent of the 4‑20 mA current loop. This design lets the transmitter power its internal circuitry without drawing power from the loop, providing isolation and enabling more capable or higher‑power electronics. The current loop still carries the analog 4-20 mA signal to the controller, but the electronics themselves aren’t powered by that loop, which is the defining trait of 4-wire devices.

Why the other statements don’t fit: being powered directly from the process loop describes a 2-wire, loop-powered transmitter rather than a 4-wire one. The transmitter does provide an analog output—the 4-20 mA signal—so claiming it cannot would be incorrect. And having only one output channel isn’t the defining feature of 4-wire transmitters; the essential point is the separate power supply for the internal electronics.