Answer / sivakumar d

Its Ohms's Law:

We need output: 4 to 20mA. corresponding voltage level is 1-5V

when Min Transmitter O/P

I=V/R =1/250 =0.004 =4mA

When Max Tranmitter O/P
I=V/R =5/250=0.02 =20mA.

So to get Standard Current Signal We need 250 ohms resistor.

Answer / anees

Because that Hart Calibrate need proper loop resistance
there for when we use in lab/Bentch calibration we need
it,otherwise in field calibration we doesnot needed

Answer / vinodkumar

when calibrating an hart transmitter in lab, we need a loop resistance between 250 ohms & 600 ohms, because without a resistor the HART communicator did not communicate with the transmitter. Because transmitter is a high impedance circuit with respect to the HART communicator. When HART communicator try to communicate with transmitter by producing a digital signal, the signal gets grounded through power supply and thus the signal didn't enter the transmitter without the resistor. When resistor is added, the transmitter become a low impedance circuit with respect to the resistor, so the signal produced by the HART communicator SHOULD have pass through the transmitter and so does the communication between HART communicator and HART Transmitter. Without the resistance between 250 ohms and 600 ohms the HART communicator did not communicate with Transmitter.
But in analog transmitters exact 250 ohms is used to convert 4 - 20mA to 1 - 5V respectively. For this only and exact 250 ohms is needed

Answer / md. afroz alam

To increase the loop of Resistance we use 250 ohms Resistance with smart transmitter in bench or lab calibration

Answer / g.ramesh

because of the load impedence it should need the 250
resistor at time only we are able to get the frequency
of 1200hz to 2200 hz

Answer / imran

According to Ohm Law V=IR .we need resistance for voltage
drop across Hart as current is there to energize Hart
circuit.and due to hart hat low impedence w.r.t XTMR .DCS
Cards hav builtin resistance that s why no need for
resistance in field

Answer / sandip

we r using 250 ohm resister to convert 4-20 ma output.

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