Answer / carl_ellis

1-5Vdc is not a standard, it is a widely used convention for 4-20mA current loops.

A 250 ohm resistor drops a 1-5 Vdc voltage proportional to the 4-20mA current.

DCS analog inputs historically used 250 ohms for 4-20mA analog input resistance and over time that value has become quite common.

But I have seen 1 ohm, 4.7 ohms, 10 ohms, 12 ohms, 24.9 ohms, 50 ohms, 62.5 ohms, and 100 ohm resistances for 4-20mA current input.

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