What is the difference between a two-wire loop-powered transmitter and a three-wire powered transmitter, and how does this affect installation?

• A. Two-wire uses loop current for power and signal; three-wire uses separate supply for power and a separate signal return.
• B. Two-wire uses separate supply for power; three-wire uses loop current for signal.
• C. Two-wire uses loop current for power; three-wire uses digital communication.
• D. Two-wire is less flexible; three-wire is simpler.

Answer: (A)

The idea being tested is how power and signal share paths in two common transmitter designs, which changes how you wire and install them. In a two-wire loop-powered transmitter, the same two conductors carry both the supply current and the measurement signal. The transmitter taps power from the loop and modulates the loop current (typically 4–20 mA) to reflect the sensed value, so there’s no separate power wire. That means the loop must provide enough voltage to cover the transmitter’s power drop plus the load in the loop, and the total loop resistance has to stay within what the transmitter can tolerate. If the loop supply drops or the load is too high, the transmitter can’t operate properly, and you’re limited in how many devices you can power from the same loop.

With a three-wire powered transmitter, there is a dedicated power supply for the transmitter’s electronics. The transmitter still uses the loop current to convey the signal, but the power comes from separate conductors supplied by a dedicated source (often 24 V DC). The result is more flexibility: you can place the transmitter away from the loop’s power source, you have more headroom for power-hungry features, and you can troubleshoot more easily because the electronics have their own supply. Installation involves providing that separate power source, running the extra conductors, and ensuring proper grounding or isolation as required to avoid ground loops or interference.

So the difference is where the power is sourced and how the signal current is returned: a two-wire setup uses the loop itself for both power and signal, while a three-wire setup uses a separate power supply for the transmitter plus the loop current for signaling.