Which statement about the 4-20 mA current loop is true?

Which statement about the 4-20 mA current loop is true?

• A. The loop uses a voltage signal instead of current to encode measurements.
• B. The transmitter generates a fixed 20 mA current regardless of input.
• C. The loop keeps a constant current while voltage across the load varies.
• D. The loop requires a supply voltage to power the transmitter and load, and the current encodes the measurement.

Answer: (D)

The main idea is that a 4-20 mA loop uses current as the signal. A dc supply powers both the transmitter (sensor) and the load in series, and the transmitter modulates the loop current in proportion to the measured process variable. The current ranges from about 4 mA to 20 mA, so the reading is obtained by measuring the loop current (often with a resistor that converts it to a voltage for a meter or controller). This setup is robust over long cables because the same current flows through everyone in the loop, while the voltage across the load varies with that current. Therefore, the statement that the loop needs a supply voltage to power the transmitter and load, and that the current encodes the measurement, is correct. The other descriptions—signaling with voltage instead of current, a fixed current regardless of input, or maintaining constant current while the load voltage changes—do not reflect how a 4-20 mA loop operates.

The main idea is that a 4-20 mA loop uses current as the signal. A dc supply powers both the transmitter (sensor) and the load in series, and the transmitter modulates the loop current in proportion to the measured process variable. The current ranges from about 4 mA to 20 mA, so the reading is obtained by measuring the loop current (often with a resistor that converts it to a voltage for a meter or controller). This setup is robust over long cables because the same current flows through everyone in the loop, while the voltage across the load varies with that current.

Therefore, the statement that the loop needs a supply voltage to power the transmitter and load, and that the current encodes the measurement, is correct. The other descriptions—signaling with voltage instead of current, a fixed current regardless of input, or maintaining constant current while the load voltage changes—do not reflect how a 4-20 mA loop operates.