Answer / ranganadh

Long distance communication mA signal is preferable than
mV, V signals to avoid noise. It cann't be predictable with
0 mA, whether the loop was broken or the signal itself. So
to avoid loop break problems we consider 4mA as the lowest
signal. Moreover, it is recommended to put a 250 ohms
resistor at the input terminals of the instrument to get
1VDC for 4mA and 5 VDC for the 20mA and further these
signals will be amplified by instrumentation amplifier in
the instrument circuit.

Answer / jazz

4-20 mA signal differentiates live process 0% with 4mA
signal with a wire break (0 mA), current is also used for
transmission because even if your receiver or PLC/DCS card
is far away current signal sent proportionately by the
transmitter will be received as the same current at the
receiver/PLC/DCS card, although it is converted as a voltage
signal (1 to 5 VDC) by a 250ohm resistor, since
micro-processor process signals in voltage levels.

Answer / jagdish

bcoz span 16 is suitable for coding in plc logic and its use
for live zero detection and as well as plc operated at 1 to
5 volt so if 250ohm Resistance is there ,so, 4 to 20mA is
suitable.

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