Why is a 250 Ω resistor commonly used in HART loops?

Why is a 250 Ω resistor commonly used in HART loops?

• A. To convert the 4-20 mA signal into a readable 1-5 V signal and allow HART communication
• B. To protect against short circuits
• C. To calibrate the transmitter
• D. To provide a reference ground

Answer: (A)

In a HART loop, the information is carried by the loop current, 4-20 mA. To let a HART communicator read that data, you need a voltage signal that reflects the current so the receiver can sense the modulation. A 250-ohm resistor acts as a shunt that converts current to voltage using Ohm’s law: V = I × R. At 4 mA, you get about 1 V; at 20 mA, about 5 V. This 1-5 V range is ideal for most data acquisition and HART modem inputs, providing a convenient and compatible window for detecting both the fundamental analog current and the superimposed digital communication without requiring special high-impedance interfaces. The resistor is chosen so the loop still has enough headroom to operate from the supply, while delivering a readable voltage for the HART signaling. This purpose isn’t primarily about protecting against short circuits, calibrating the transmitter, or providing a ground reference. Those functions are handled by other design aspects of the loop and instrumentation, whereas the 250 Ω shunt specifically creates a practical voltage representation of the loop current for the HART communication path.

In a HART loop, the information is carried by the loop current, 4-20 mA. To let a HART communicator read that data, you need a voltage signal that reflects the current so the receiver can sense the modulation. A 250-ohm resistor acts as a shunt that converts current to voltage using Ohm’s law: V = I × R. At 4 mA, you get about 1 V; at 20 mA, about 5 V. This 1-5 V range is ideal for most data acquisition and HART modem inputs, providing a convenient and compatible window for detecting both the fundamental analog current and the superimposed digital communication without requiring special high-impedance interfaces. The resistor is chosen so the loop still has enough headroom to operate from the supply, while delivering a readable voltage for the HART signaling.

This purpose isn’t primarily about protecting against short circuits, calibrating the transmitter, or providing a ground reference. Those functions are handled by other design aspects of the loop and instrumentation, whereas the 250 Ω shunt specifically creates a practical voltage representation of the loop current for the HART communication path.