Is there a need of250 Ohm resistor for control valve calibration using HART protocol? If so how and where to connect the250 Ohm resistor?

The resistance will be connected in series, from power supply +ve wire (via Ammeter) to your instrument.
The HART communicator will be connected according to prescribed procedure also to your instrument.
Now, why use of resistor? Here are some ideas: Since V=IR, now When I=4ma, V Should be1 V putting in V=IR we get r=250 ohms.
Same in case when I=20 ma and V=5V we get r=250 ohms.
Idea #2: Ohms law (V = IR) dictates that a resistor must be present with current to receive a voltage.
Most field devices transmit HART as +/-0.5mA imposed on the4-20mA current loop.
The4-20mA is the centerline of the1mA signal.
The resistance in the loop converts the current to a voltage which is used as a digital voltage signal.
The carrier detect is100mV nominally.
If there is no voltage, there is no carrier, and therefore no communications.
Good Luck.

u use the shunt250 ohm resistor if the cable resistance is lower than that

This video shows how and why a loop resistance is critically important for the proper operation of a HART "smart" transmitter to communicate.http://youtu.be/XJ4A-0kGGkI

U have to conect in the series of power supply there is not compulsory to connect250 ohm this is impedence matching resistance because in field if control valve or some other instrument connected with loop then there is no need to connect resistance and also u have to consider that the resistance of system loop is not just about same250 ohm this could b very with the manufacturer design .however if u want to calibrate control valve with power supply then u have to connect resistance either in series of -ve or in series of +ve u can also used180ohm resistance instead of250 ohm upto180 ohm hart works ,if not trust then u have to do a practical and reply on this site same forum.

hope for good experiment

Not required to coonect250ohm resistance to calibrate control valve as they are loop powered. ....so you can directly connect hart devices and calibrate valve. ... Just to check pins should be connected to test in HART.

no in latest type communicator no need to connect250 Ohm resistance. it is fefault with probes connecting point with communicator

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