Why do analog signals use 4-20mA current transmission?

In view of this problem, Jiangsu Sifang boiler suggests:

1) When the distance from the site to the control room is long and the resistance of the connecting wire is large, if the voltage source signal is used for remote transmission, a large error will occur due to the voltage division between the wire resistance and the input resistance of the receiving instrument. If the constant current source signal is used for remote transmission, as long as there is no branch in the transmission circuit, the current in the circuit will not change with the length of the wire, thus ensuring the transmission accuracy;

2) The selection of the maximum current of 20mA is based on the consideration of safety, practicality, power consumption and cost. The safety spark instrument can only use low voltage and low current. The spark energy caused by the on-off of 20mA current is not enough to ignite the gas, which is very safe;

3) The transmitter circuit will not work without static working current. The signal starting point current of 4mA is the static working current of the transmitter. Meanwhile, the electrical zero point of the instrument is 4mA, which does not coincide with the mechanical zero point. This "active zero point" is conducive to identifying faults such as power failure and disconnection.

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The above is the answer of Jiangsu Sifang boiler to this question. If you have any questions about the boiler, please consult the online customer service of Jiangsu Sifang boiler or call 0516-85871846 and 0516-85532705. The professional and technical personnel of Jiangsu Sifang boiler will answer your questions in time.