Why do you use 4 - 20 mili ampere?

Answer 1

A 4-20ma current loop is often used in process control and monitoring systems to signal a value from one point to another. It operates using Kirchoff's current law, in that the transmitter can monitor the current on the loop, knowing that will be the same current at the receiver. Any deviations in reference voltage or ground differentials between transmitter and receiver can then be compensated for at the transmitter. The signal range starts at 4ma, so that the receiver can detect the 0ma condition representing a break in the wire.

You can also have multiple receivers on the loop, all in series, so long as the delta voltage at each receiver does not contribute too much voltage to the overall loop. Each receiver will see the same current, again due to Kirchoff's current law.

Answer 2

4 - 20 milliampere is a common range used for industrial process control instrumentation because it provides a clear signal while minimizing the risk of false readings due to noise or power supply fluctuations. This current range is also well-suited for long-distance transmission with minimal signal loss, making it practical for various applications in industries like manufacturing, water treatment, and oil and gas.