What is a loop-powered transmitter and a typical application?

What is a loop-powered transmitter and a typical application?

• A. A transmitter powered by its own supply; used in lab calibration
• B. A transmitter powered by the 4-20 mA loop; common in remote sensing where separate power is unavailable
• C. A transmitter that requires AC mains power
• D. A transmitter that only outputs digital signals

Answer: (B)

Loop-powered transmitters draw their operating power from the 4-20 mA control loop. They piggyback on the current flowing in the loop: the transmitter uses a portion of that current to run its electronics and then modulates the current to represent the measured process variable. Because power and signal share the same two wires, there’s no need for a separate power supply at the sensor location. This makes loop-powered devices ideal for remote sensing or field installations where a separate power source isn’t available or practical. The other descriptions don’t fit this common setup: a transmitter with its own supply needs extra wiring for power, a mains-powered unit requires AC electrical supply, and a device that outputs only digital signals wouldn’t provide the analog 4-20 mA signal used in many analog control systems.

Loop-powered transmitters draw their operating power from the 4-20 mA control loop. They piggyback on the current flowing in the loop: the transmitter uses a portion of that current to run its electronics and then modulates the current to represent the measured process variable. Because power and signal share the same two wires, there’s no need for a separate power supply at the sensor location. This makes loop-powered devices ideal for remote sensing or field installations where a separate power source isn’t available or practical.

The other descriptions don’t fit this common setup: a transmitter with its own supply needs extra wiring for power, a mains-powered unit requires AC electrical supply, and a device that outputs only digital signals wouldn’t provide the analog 4-20 mA signal used in many analog control systems.