The most widely used standard analog quantity electrical signal in industry is to transmit analog quantity with 4~20mA DC current. The reason for using the current signal is that it is not easy to be interfered with, and the internal resistance of the current source is infinite. The wire resistance series in the loop does not affect the accuracy, and can be transmitted for hundreds of meters on the ordinary twisted pair. The upper limit of 20mA is because of the explosion-proof requirements: the spark energy caused by the current break of 20mA is not enough to ignite the gas. The reason why the lower limit is not set as 0mA is to detect the broken line: it will not be lower than 4mA in normal operation. When the transmission line is broken due to fault and the loop current drops to 0, 2mA is often set as the broken line alarm value.
Post time: Aug-29-2022