Tech Note 115 Shunt Resistors

Occasionally a customer will ask us how to interface one of our Transmitters with a 4-20 mA output to a DC Input Field IO Card. What is important to understand is that even a 4-20 mA Input Card uses a shunt resistor to convert the 4-20 mA to a voltage usable by the card for measurement. If you have a DC Voltage IO Card simply install a shunt resistor across the input that will change the 4-20 mA to DC Voltage. A 250 Ohm 2% Resistor produces 1 to 5 VDC. Many of the IO Cards we have been interfacing with in the last few years use a 100 Ohm resistor which when installed across the 4-20 mA provides a 0.4 to 2.0 VDC signal (Ohms x Current = VDC). Remember to use your PLC's "Offset" setting to have 0.0 in/sec start at 4mA or the equivalent calculated voltage.