What advantage do 4-wire transmitters provide regarding loop load?

What advantage do 4-wire transmitters provide regarding loop load?

• A. They require less power.
• B. The signal is independent of loop resistance.
• C. They can only drive low impedance loads.
• D. They increase the loop resistance.

Answer: (B)

A four-wire transmitter uses a separate power supply for the transmitter electronics while the current loop is driven by its own controlled output. This lets the transmitter regulate the loop current without being affected by the loop resistance, as long as the required voltage to push that current through the load stays within the transmitter’s compliance voltage. In other words, the signal stays at the commanded 4–20 mA regardless of how the loop resistance changes. This independence comes from having a dedicated supply for the transmitter, unlike a two-wire setup where the loop supply and current regulation share the same source, making the current more sensitive to load changes. The other statements aren’t correct: power needs aren’t inherently reduced by using four wires, four-wire systems aren’t limited to low-impedance loads, and they don’t inherently increase the loop resistance—the load and wiring determine resistance within the transmitter’s voltage headroom.

A four-wire transmitter uses a separate power supply for the transmitter electronics while the current loop is driven by its own controlled output. This lets the transmitter regulate the loop current without being affected by the loop resistance, as long as the required voltage to push that current through the load stays within the transmitter’s compliance voltage. In other words, the signal stays at the commanded 4–20 mA regardless of how the loop resistance changes.

This independence comes from having a dedicated supply for the transmitter, unlike a two-wire setup where the loop supply and current regulation share the same source, making the current more sensitive to load changes. The other statements aren’t correct: power needs aren’t inherently reduced by using four wires, four-wire systems aren’t limited to low-impedance loads, and they don’t inherently increase the loop resistance—the load and wiring determine resistance within the transmitter’s voltage headroom.