250 Ohms Resistor in a Transmitter circuit.

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The talking device transmits a current· The listening device picks up on voltageThe purpose of the ohm shunt resistor is to convert the transmitted current to a voltage the received can pick up. The ohm resistor converts the4- mA analog signals from the HART transmitter to a1-5 Volt drop to allow the Analog Input card of the DCS or PLC to read the analog signal. If you are sending signal-mA then you should useΩ Resistor to convert into voltage1-5 Volts.Because of ohms law V = IR1V=4mA,2V=8mA,3V=mA,4V=mA,5V=mA so what is the resistance?1V=4mA*R1V=0.A*R then (4mA =0.Amps)R=1/0.= ΩDCS, PLC needs voltage (1 to5V) signal to understand the signal from the transmitter. The resistor serves to convert this current signal into voltage signal for the DCS/PLC.

The resistor's position in the loop is between the Transmitter and the DC power supply. As we connect the communicator it should be tap between the resistor and the transmitter or directly parallel to the resistor. Why? Because the DC power supply has 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 that analog signal coming from the HART devices will be filtered out.The dc power supply we use in the loop has low pass capacitor filters, and in absence of a resistor, the filter is in parallel with the Transmitter, thereby bypassing the digital information, and adding a resistor just provides impedance between the Transmitter and filter circuitWhile doing bench calibration if you connect it directly across aVolt power supply it will be quite happy because the4-mA current signal won’t be disturbed, but you will not be able to communicate with it using HART Communicator. You need some impedance so the HART communicator can modulate the lines.