Answer included by Abdul Rauf Nighyal, Unit Manager Instrumentation & regulate , Pakarab fertilizers Ltd6 years ago

It is simply conversion of existing signal right into voltage signal follow to OHMS LAW, V=IR By placing the current values4mA -20mA(4,8,12,16,20mA) main point by250 Ohm.You will get1v,2v,3v,4v,5v respectively.Now this voltage signal cab be supplied for Display1V together zero2V for25%3V for50%4V for75% and5V as100%, i think this will clear advantage of conversion.

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to get a1v to5volts dc signal come be used in transforming analog signal to digital signal. Ours signal is4 to20mA so if we put250ohms resistor we can get1 to5 volts signal which is needed in ADC analog digital converter 


Answer added by Irshadahmed Syed, electric Site Cum Procurement engineer , Mesmack building and construction &Development agency Since5 years back

only4- mA HART Protocol require Ohm Resistor in loop come convert4- mA in to1-5 V which compelled for DCS or PLC to work Which are field devices..



Answer included by Sakthi Venkatesh Kumar Kalimuthu Chellamariappan, Sr. Instrumentation technician , Kazmunai India Pvt. Ltd6 years ago

The resistor"s position in the loop is in between the Transmitter and the DC strength supply. Together we attach the communicator it should be tap between the resistor and the transmitter or directly parallel to the resistor. Why? since the DC power supply has actually a LOW pass CAPACITOR FILTER that basically squelches analog signals like ripples. Thus, if there is no resistance in between the PS and the TX the analog signal comes from the HART gadgets will be filtered out.

The dc strength supply we usage in the loop has low happen capacitor filters, and in lack of a resistor, the filter is in parallel with the Transmitter, thereby bypassing the digital information, and including a resistor just provides impedance in between the Transmitter and filter circuit

While act bench calibration if you attach it directly throughout a24Volt power supply it will be quite happy due to the fact that the4-20mA current signal i will not ~ be disturbed, but you will certainly not be able to communicate with it using HART Communicator. You need some impedance so the HART communicator deserve to modulate the lines.

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The250 ohms resistance in the loop serves two purposes:-1) The HART Communicator communicates to the transmitter by FSK method. The capacitor in the power supply because that the transmitter distorts this HART signal. Thus the resistor serves together an help to acquire the signal back, for the communicator.2) DCS, PLC needs voltage (1 to5V) signal to know the signal from the transmitter. The resistor again offer to convert this present signal into voltage signal because that the DCS/PLC.