The 4-20 mA current loop is one of the most robust sensor signaling standard. Current loops are ideal for data transmission because of their inherent insensitivity to electrical noise.
In a 4-20 mA current loop, all the signaling current flows through all components; the same current flows even if the wire terminations are less than perfect.
4-20 mA Current Loop
All the components in the loop drop voltage due to the signaling current flowing through them.
The signaling current is not affected by these voltage drops as long as the power supply voltage is greater than the sum of the voltage drops around the loop at the maximum signaling current of 20 mA.
Transmitting sensor information via a current loop is particularly useful when the information has to be sent to a remote location over long distances (500 meters, or more).
The loop’s operation is straightforward: a sensor’s output voltage is first converted to a proportional current, with 4 mA normally representing the sensor’s zero-level output, and 20 mA representing the sensor’s full scale output.
Then, a receiver at the remote end converts the 4-20 mA current back into a voltage which in turn can be further processed by a computer or display module.