What is the purpose of a series resistor in a 4-20mA circuit? 2

Are you sure it's a "series" resistor, or is the PLC card measuring the voltage across the resistor? May the analog card of your PLC reads only voltage, not current.

It can be used to limit voltage to the transmitter or so you can measure the voltage across the resistor and calculate the current in the loop when troubleshooting.

1) Analog input cards are voltage sensing. So when the signal is current, a dropping resistor develops the IR drop which the analog input then 'sees'. This is usually a precision resistor (0.1-0.05%), because any deviation from the ideal is a proportional error.

Sometimes the dropping resistor is built into the analog input, other times it is added to the input terminals externally.

2) Since current measurement requires breaking the circuit to insert an milliampmeter, some people install an external resistor at some point across which they can measure the voltage drop for troubleshooting purposes (like badservo says).

3) Some current loop analog input resistors have a resistor value that is less than the minimum 230 ohms required for digital HART communications (used to configure field devices). For sites that frequently use HART, they'll install an additional resistor (not across the (+)/(-) terminals, but elsewhere in the circuit to get the minimum loop resistance needed for HART. These are not precision resistors.

Back in the day, we used to put a 1 ohm series resistor at the terminal block in all 4-20mA loops. This is allows for a quick voltage check with a voltmeter to determine the current in the loop. Makes a nice independent sanity check when nothing is making sense.

A resistor in series can also protect the power supply from shorts that can occur in the field wiring. This would also keep the other sensors "alive" if the wiring to one sensor shorts out. The supply is often has a much higher voltqage that is needed given the minimum voltage of the sensor and the reading electronics. With a normal current draw of 20ma it is easy to put a resistor in series that would limit the maximum short circuit current to 500ma without changing accuracy. And as said before it will also provide a test voltage for verification of current.

I agree with Operahouse.
I often add a series resistor if there is a possibility that the passive 4-20mA source could become shorted.
For example, we do a lot of pump controls where 24V is fed out to a pressure transducer and then on to a 4-20mA input on a PLC card.
If the installer decides to swap the wires on the transducer, because they gott them reversed on installation, they will invariably create a temproary short dumping 24V across the 4-20mA input and blowing the internal resistor.

Mark Empson
Advanced Motor Control Ltd