What is the role of a resistor in the 4-20 mA loop when measuring current?

• A. It converts the loop current into a voltage that can be measured by a controller or meter; larger resistor generates larger voltage for the same current.
• B. It stores energy
• C. It increases current flow
• D. It filters noise completely

Answer: (A)

In a 4-20 mA loop, the current carries the measurement information, so the resistor acts as the load that translates that current into a voltage the controller can read. With V = I × R, the voltage across the resistor increases proportionally as the current goes from 4 mA to 20 mA. A larger resistor therefore produces a larger voltage for the same current, improving signal amplitude and readability, but it also uses more voltage headroom in the loop, so the total drop must stay within the supply and the transmitter’s minimum voltage requirements. The resistor doesn’t store energy, doesn’t increase the loop current, and doesn’t completely filter noise—those would require other components or circuit features.