Why would a 250 ohm resistor be in a loop?

• A. To stabilize voltage
• B. To limit current
• C. So you can use a HART communicator
• D. For grounding

Answer: (C)

In a 4-20 mA control loop used with HART, you place a resistor to turn the loop current into a voltage that the HART modem can read. The 250-ohm value is chosen so that 4 mA produces about 1 V and 20 mA produces about 5 V. That 1–5 V range provides a convenient, readable signal for the HART communicator to superimpose and decode the digital communication on top of the analog current. So the resistor’s role is to create a suitable voltage signal for HART, not primarily to stabilize voltage, limit current, or provide grounding. If the loop used a much different load, the voltage range might be unsuitable for the HART equipment or the modulation would be harder to detect.