The resistor value used to interface a 4-20 mA loop with a 1-5 VDC device is 250 ohms. Which option matches this value?

• A. 100 Ohms
• B. 250 Ohms
• C. 500 Ohms
• D. 1000 Ohms

Answer: (B)

Converting the loop current to a voltage with a resistor lets a 4–20 mA signal be read by a 1–5 V input. The device voltage is V = I × R, so you want 4 mA to produce about 1 V and 20 mA to produce about 5 V. Solving for R: R = 1 V / 0.004 A = 250 Ω. This same resistance gives V = 0.02 A × 250 Ω = 5 V at the top end, matching the input range exactly. Other resistances either undershoot the minimum voltage or overshoot the maximum voltage for the 1–5 V input, so they don’t map the current properly. 250 Ω is the value that makes the 4–20 mA current loop translate cleanly to 1–5 V.