4–20 mA Current Loop
It is the industrial signal standard that carries analog information as current rather than voltage; 4 mA means the zero command and 20 mA full command. Because the current stays constant along the line, the voltage drop caused by cable resistance does not corrupt the signal, and thanks to the 4 mA zero offset a broken line (0 mA) can be distinguished as a fault.
In a current loop the transmitter keeps the current through the circuit constant whatever the line resistance may be. The receiving side typically converts this current to a voltage across a shunt resistor of 100–250 Ω: with 250 Ω, 4–20 mA gives exactly 1–5 V, which suits a 0–5 V analog input directly.
- Distance: no error occurs even over hundreds of metres of cable; it is preferred where a 0–10 V signal struggles.
- Live zero: because the lower bound of a valid signal is 4 mA, a reading of 0 mA can only mean a broken cable or a failed transmitter. The driver can then move to the safe side (stop).
- Compatibility: the analog inputs of DC drivers in this class are generally of the voltage type; to connect a 4–20 mA source you need to place a shunt resistor at the end of the line and convert the current to a voltage.
Place the shunt resistor as close to the driver as possible, between the input and chassis; if the resistor is far away, the chassis drop again enters into the error.
Context where this term is used: DC Motor Driver Selection Guide