What does a passive transmitter do with current?

• A. It sources current to drive the loop.
• B. It amplifies current while supplying it.
• C. It converts current to voltage only.
• D. It modulates the current but does not supply it.

Answer: (D)

In a 4-20 mA loop, a passive transmitter doesn’t have its own current source. It relies on the loop’s power to operate. What it does is change how the loop current flows to encode the measurement. By varying its input impedance as the process variable changes, it causes the existing current in the loop to rise or fall, without providing that current itself. So the device is modulating the current that’s already being supplied by the loop power.

That’s why it isn’t describing itself as sourcing current to drive the loop, nor as amplifying current while supplying it. And while some devices may involve converting sensed current to a voltage internally, the defining behavior of a passive transmitter is altering the loop current to convey information, not generating or boosting the current.