IO Modules - Inputs - Current measurements using universal inputs

Introduction

This guide addresses the need for an additional component, specifically a 200 Ohm resistor, to configure current measurements using Universal Inputs. Since straightforward measurement of current signals is not directly possible, this resistor is crucial for converting current signals (0-20 mA or 4-20 mA) into usable voltage outputs (0-4 VDC)

Good Practice

Setting Up for 0-20 mA Measurements

Use a 200 Ohm resistor with Universal Inputs to convert a 0-20 mA current signal into a 0-4 V voltage signal. This setup allows for straightforward voltage measurement directly from current inputs without additional configuration.

Configuring for 4-20 mA Signals

To handle a 4-20 mA current signal, the same 200 Ohm resistor setup applies. This configuration will adjust the output voltage to a range of 0.8-4.0 VDC. After this conversion, it’s essential to configure the input type to Voltage measurement in the UI settings. Appropriate configuration of facets and linear conversion in the system (such as Niagara) will allow the client to recalibrate the signal

In MAC36's Universal Input configuration, to ensure accurate readings, apply the following linear conversion formula: y = ax + b , where x is the read input value, a is the scale (set to 1.0 as default), and b is the offset (set to 0.0 as default). This will correctly interpret the 4-20 mA signal as a 0.8-4 V output according to Ohm's Law.

While a 500 Ohm resistor can also be used to convert the 4-20 mA signal into a 2-10 V range, using a 200 Ohm resistor is recommended due to better linearity in voltage measurement (for voltages above 4V, the input impedance of the universal input (UI) is nonlinear, which can lead to measurement errors.), especially in higher ranges, ensuring more accurate readings.