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why do we use 4-20mA as o/p and why don't use 0-20mA or 5-
21mA?
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Answer Posted / carl_ellis
Question:
why do we use 4-20mA as o/p and why don't use 0-20mA or 5- 21mA?
There are systems that use 0-20mA as an output from a
controller. Those are active', powered outputs, not
signals from 2 wire, loop powered field transmitters. 2
wire, loop powered instruments use about 3.5mA to power the
transmitter's electronics, so 2 wire, loop powered
instruments can never go below 3.5mA without losing the
current needed to power the instrument.
An active analog output from a controller that uses the
controller's power from the controller to drive the current
signal can be a 4-20mA signal or a 0-20mA signal.
People in the process industries use the 4-20mA output
because it is a 'live-zero' that immediately indicates a
broken wire or an open circuit if the loop current falls to
0.0mA. Those using 0-20mA probably have a 0-10Vdc or 0-5Vdc
analog input and want to use the entire span with a 500 ohm
or 250 ohm resistor.
As to why 5-21mA is not used, there are conventions used
that make systems maintainable over time. If every analog
output used oddball ranges or spans, it would be near
impossible to maintain the system over time. Given that
many control systems operate in excess of 10 years, it is
important to use standard analog signals in order to keep
the system maintainable.
The convention is 4-20mA, established by the ISA SP-50
standard in 1967. Deviations are punishable. Be warned.
Carl Ellis
Measure First
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