A resistor is needed only if you need to connect a HART communicator to the transmitter. Without a resistor, the HART handheld communicator will be unable to read the HART signal which gets filtered out in the power supply.
The transmitter will operate just fine connected directly to the power supply with or without a 250 ohm resistor in the circuit.
it is necessary to add the resistor in between the 4-20 mA signal and digital display mostly we use 250 ohm resistor, the aim of addition is to convert 4-20 mA to 1-5 Volt <pre>
let us use ohms law V=IR
4mA x 250 ohm=1 volt
8mA x 250 ohm=2 volt
12mA x 250 ohm=3 volt
16mA x 250 ohm=4 volt
20mA x 250 ohm=5 Volt</pre>
hope it will be helpful
Yes,you do have to include a resistor. There has been lengthy thread of debates previously on this topic. Basically, you need a resistor with optimum value in order to facilitate communication between HART and transmitter/or to convert the signal into voltage for DCS/PLC communication.I am assuming you are intending to use a HART.
Although, for the new DCS these days, you wont need this resistor as the facility to transform the current signal to voltage signal in inbuilt.
Generally we do not need to place a resistance but many a time circuit resistance is lesser than that of requirement for HART communication. So to establish the communication with HART communicator, we may need to place a resistor.
