Describe the 4-20 mA current loop used for transmitting sensor signals, including how the loop encodes the measurement and the role of supply voltage.

• A. The transmitter modulates the current through the loop, with 4 mA representing minimum/zero and 20 mA maximum; the current flowing through a resistor produces a voltage read by the receiver; a sufficient supply voltage is required to drive the transmitter and load.
• B. The transmitter modulates current through the loop as a fixed 12 mA; the load is powered by separate supply.
• C. The loop uses PWM to encode the measurement and requires no dedicated supply.
• D. The transmitter encodes measurements in a voltage signal transmitted over the loop.

Answer: (A)

In a 4-20 mA loop, the signal is carried by current, not by voltage. The transmitter modulates the loop current to represent the measured value, with 4 mA corresponding to minimum (or zero) and 20 mA to full scale. A resistor in the receiver converts that current into a voltage (V = I × R) that the controller or display reads. The supply voltage must be high enough to power the transmitter and provide the headroom for the load’s voltage drop; if the supply is too low, the loop cannot reach the required current levels or accurately reflect the measurement. This description matches how the 4-20 mA loop works, unlike options that describe a fixed current, PWM encoding, or pure voltage signaling over the loop.