Answer / sam daniel, marthandam, kanyak

To operate a Digital signal we need 1-5Vdc. For Analog
Instruments we are using 250 ohms as load resistance. When
we converting this signal into current By Ohm's
Law V=IR
I=V/R
Put, V=1V Then I=1/250=4mA
Put, V=2V Then I=2/250=8mA
Put, V=3V Then I=3/250=12mA
Put, V=4V Then I=4/250=16mA
Put, V=5V Then I=5/250=20mA

*Also, since this is linear region
*Not possible to cause spark
*Since it is a current signal we can transmit it into long
distance.
*Suppose if the instrument is dead we can identify it.

Answer / jayaraj

1}///in earlier days we had used BJT transister in signal
conditioning circuits for amplification purposes so in BJT
transfer characteristics 4 to 20 milli amps is a linear
region,....but now-a-days we re using cmos only.... as a
convention only we are following this 4-20 range
2}///another reason...for not using 0 milli amp
coz if we use like that we cannot distinguish between true
signal zero and dead zero(break in wires)

Answer / nileshkumar

1) 4 - 2 ma signal can transmited long distance without
signal drop in cable.

2) To operate a Digital signal we need 1-5Vdc. For Analog
Instruments we are using 250 ohms as load resistance. When
we converting this signal into current By Ohm's
Law V=IR
I=V/R
Put, V=1V Then I=1/250=4mA
Put, V=2V Then I=2/250=8mA
Put, V=3V Then I=3/250=12mA
Put, V=4V Then I=4/250=16mA
Put, V=5V Then I=5/250=20mA
*when two connection short no sparking.
*Since it is a current signal we can transmit it into long
distance.
*Suppose if the instrument is dead we can identify it.

Answer / abhijith

in 4-20ma we can easily check the zero and wire breaking in
transmitter. Analog card accept 1-5v dc supply. thatsy V-IR

I=V/R
V=1V R=250OHMS

1/250 = 4MA

V=2V THEN I=2/250 = 8MA

V=3V THEN I=3/250 =12MA

V=4V THEN I= 4/250 =16MA

V=5V THEN I=5/250 = 20MA.

Answer / anish kumaran r

Live zero and minimum current required to keep alive the tx

Answer / suresh nagdeo

another reason from starting current from 4 ma is between two cables maximum induced current is always below 4 ma. if electrical and instrumentation cables running together this type problems arised.

Sir i am preparing for PSU exam for ONGC but i am not able to get the appropriate syllabus and previous year question papers. Kindly send me the same, that will be a great help from ur side. OR if anyone can plz send me the papers and syllabus please.

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