Why would a 250 ohm resistor be included in an instrumentation loop?

• A. To use a HART communicator
• B. To limit current
• C. To calibrate the transmitter
• D. To provide power supply

Answer: (A)

In an instrumentation loop, a burden resistor converts the loop current into a voltage that measurement and comms devices can use. For a 4-20 mA loop, a 250-ohm burden produces about 1–5 volts across it, which is a convenient range for typical inputs and for detecting the digital data superimposed by HART on top of the analog current.

HART communication overlays high-frequency digital signals on the same 4-20 mA loop. The HART modem senses and modulates this voltage across the burden resistor, so having a standard 250-ohm load ensures the digital data can be transmitted and read reliably without disturbing the analog current range. That’s why this resistor is commonly chosen when a HART communicator is involved.

It isn’t primarily there to limit current, calibrate the transmitter, or power the loop; the transmitter sets the current, and the resistor simply provides the appropriate load and voltage for HART signaling.