A 1-5 VDC analog device can be used in a 4-20 mA DC loop circuit if a 250 ohm resistor is connected across the input to the device and the 4-20 mA DC current is routed through the resistor. The current signal produces the proper voltage to operate the 1-5 VDC device. (True/False)

• A. True
• B. False
• C. Not Stated
• D. Not Applicable

Answer: (A)

Converting a 4-20 mA signal to a voltage signal is done with a shunt resistor. The device with a 1-5 V input will read a voltage that is proportional to the loop current, as long as the current flows through the resistor and the device measures the voltage across it. With a 250 Ω resistor, the voltage across it is V = I × R. At the minimum loop current of 4 mA, V = 0.004 A × 250 Ω = 1 V. At the maximum of 20 mA, V = 0.020 A × 250 Ω = 5 V. That yields a 1–5 V range exactly matching the device’s input, so the device can operate correctly when placed across the resistor in the 4-20 mA loop. The key is that the device’s input must present a high enough impedance (so it doesn’t disturb the loop) and simply reads the voltage across the resistor.