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in current transmitter why we use 4-20ma ???
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Answer Posted / carl_ellis
1) A current signal has the same current in all parts of the loop, unlike a voltage that drop voltage over distance, which provides reliability that a voltage signal does not.
2) A current signal is more resistant to noise pickup than a voltage signal is.
3) A live zero at 4mA allows for '2 wire operation', where the power for the loop uses the same two loop wires as the signal, a critically important feature for geographically dispersed industries like oil & gas, steel, paper and mining. 2 wire loop powered transmitters use about 3.5 or 3.6mA of current to operate the transmitter.
4) A live zero is a diagnostic detection of an open circuit
5) 4-20mA is a low enough current with limited energy that transmitters can be designed to operate intrinsically safe in hazardous areas with the appropriate barrier protection from faults in the safe area.
6) The DC current signal is suitable for superimposing a modulated FSK signal used by HART for digital communications that allow for configuration, troubleshooting, diagnostics or multivariable data transmission, all while retaining an analog primary variable
7) 4-20mA drives considerable sized loads at fairly low DC voltages, allowing for a relatively long distance signal path (1-2km depending on wire size). 20mA has a relatively low IR drop.
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