What is the typical amperage range for electronic control signals?

• A. 4 to 20 mA
• B. 0 to 20 mA
• C. 1 to 5 mA
• D. 10 to 30 mA

Answer: (A)

The idea being tested is why current loops in industrial controls use a milliamp range rather than a voltage signal. Using current makes the signal robust over long cable runs and in noisy environments because current is easier to preserve than voltage when wires have resistance or interference. The standard signaling range is 4 to 20 mA. The reason this is preferred is that a minimum current of 4 mA gives a live-zero: the loop is always producing some current, so a fault like a broken wire shows up as a zero current and can be quickly detected. The maximum current of 20 mA provides enough headroom to carry the full-scale signal and to power two-wire transmitters while still leaving margin for tolerance and system noise. In practice, 4 mA corresponds to 0% of the measured value and 20 mA to 100% of the span, with all values in between mapping linearly to the process variable.

Other options don’t align with common practice. Starting at 0 mA makes it harder to distinguish a real zero from a fault, since a break could look the same as a zero reading. A range like 1 to 5 mA is too small to maintain signal integrity over distance and with typical receiver inputs. A range of 10 to 30 mA pushes beyond standard input ratings and uses more power, reducing compatibility and margin for error.