generally in a transister some amount of voltage is
required for turning it on. this voltage is the cutin
voltage. up to this voltage the exists a nonlinerity in its
charecteristics. beyond this cutin voltage the char. of
transister is linear. this nonlinear region lies between 0-
4ma,beyond 4ma it linear. this is why 4-20 ma range is used.
the signal are transmitted from the field to the
controlroom and vseversa through cables. for long
transmission the cable resistance increases.thus if 0-10v
are transmitted over this long distance,due to the
resistance of cable there will be drop in the voltage and
the signal received at the controlroom will not be
accurate. to overcome this difficulty 4-20ma signal is used
for transmission.
If 0-10 mA is used instead of 4-20mA then there will be
difficulties in error detection.Such as if there is any
fault is the transmission lines..we will never know,wheather
the indicated 0mA current corresponds to the initial
value(calibrated) of the measurement or it is due to fault
in transmission lines.
0-10V :-
1. Voltage droup occurs.
2. Not able to find open circuit(Normally the circuit in
open condition o/p is "0", In 0-10V signal level starts
from 0V itself, so that can not able to find open circuit
easily)
3. Noise interference is high.
4-20mA :-
1. Easy to find the open circuit(because this signal level
starts from 4mA, if the circuit in open the output shows
0mA, so we can find easily the open circuit) .
2. Less noise interference.
3. Current measures remain same in any point of the signal
line.
In case of voltage transmission(here 0-10V)
1.there will be voltage drop along the transmission wire.
2.if we use 0 volt then we will be unable to find whether
the obtained 0 volt output is really the system output or
its due to instrument error.So for "live zero" concept we
need to give a particular value(other than zero) in the
lower range value.
3.Capacitive and magnetic effect due to voltage
transmission will be more than that in case of current
transmission.
4.2-wire transmission is not compatible with voltage
transmission.
But in case of 4-20 mA signal the above problems are solved.
1.In series transmission same current flows through the
ckt.So no current drop occurs.
2.Here the lower range value is 4 mA.So we can easily
detect after looking at the o/p whether the instrument
error occurs or not.In case of instrument error zero
reading will be shown.
3.2-wire transmission is compatible with it.
In industry, the distance from controlroom
and the field is too long. If we use 0-10v signal
for feedback means voltagedrop will occur because of the
resistivity. So, 4-20mA is preferred instand of 0-10v
signal.
in 4- 20
we can find if the process variables give zero because zero
is indicate 4ma
but in 0 - 10 if the process give zero we can not know
because the signal also 0v
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