What is current transmission?

• A. A system where the transmitter regulates voltage.
• B. A system where the transmitter uses digital data.
• C. A system where the transmitter regulates current in a loop.
• D. A system where the transmitter modulates frequency.

Answer: (C)

Current transmission in a 4–20 mA loop means the signal is carried by the loop current, not by the voltage. The transmitter sets and maintains a current that varies from about 4 mA to 20 mA to represent the measured process variable. The power supply simply provides the voltage to push that current through the loop, while the receiver reads the current (often by measuring the voltage across a sense resistor) to determine the value.

This approach is favored because current is less affected by wiring resistance and noise over long distances, making the signal reliable for process control. So the core idea is that the transmitter regulates the current in the loop to convey the measurement. (Digital data can be overlaid on top of this analog current, but the fundamental signal is the current itself.)