Answer / dinesh priyadarshi

1)1-5Vdc have voltage drop at longer distance. It is only
loop voltage for 4-20mA signal
2)4-20mA gives linear output
3)All the eletronics circuit have a leakage current range
between 0-3mA. so for perfact meausre of curent we start
from 4 mA
4)If we see the span of 4-20mA is 16 mA. We are using
digital eletronics, so we have achieve good resolution with
16 bit resoution with successive approxiation DAC. We can
say i the step of 65,353 in terms or digital count for
processor.
5)4 mA is also called live zero. If cable may cut or damage
transmitter shows you 0 mA. So we cannot differentiate
between our signal so we used 4mA as live starting signal.
So if cable cut or damage we get 0 mA.
6)We use 20mA as maximum current because between 4 to 20 mA
we have get linear output.

Answer / sikandar mehra

to differentiate live and dead zero.........

Answer / adrianmlopez

When we use 1-5V in the loop, it may have some loses to the
destination point since voltage dissipated when the
conductor is very long.When we applied current, it can't be
dissipated even a kilometers away from the source to the
destination point. Since we do an instrumentation,
instrument or equipment needs an accurate value,we do not
use voltage but 4-20mA instead.But it may depend on some
other applications,for instance if your output variable
needs voltage,why used 4 to 20mA, right.

Answer / amit

because if wire is broken or signal not come so we are
check that cable working or not so we are using 4-20ma.

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