Measuring 4-20mA current using the 4 20ma R Click module
*Figure 1. Extract from 4-20mA R click.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. Redrawn schematic showing current flow from top to bottom.

As shown more clearly (I hope) in Figure 2, the R-click internal 16 V supply powers your 4-20 mA sensor. You should not connect an external supply to the current loop between the terminals on CN1.

How it works:


The sensor draws current in the range of 4 - 20 mA from the CN1 + terminal and returns it to the power supply via the GND terminal.
R3 will create a voltage drop between the INA196 VIN+ and VIN- terminals. From Ohm's Law we can calculate this as being between 20 mV at 4 mA and 100 mV at 20 mA.
The INA196 is one of the range of INA19x Current Shunt Monitors. One of the features of this chip is that it can monitor current on the "high side" of the supply rather than on the ground return which, without this chip, is generally simpler.


To test the circuit:


Measure the open-circuit voltage across CN1. It should be about 16 V.
If we connect \$ R = \frac {16 \ \mathrm {V}}{4 \ \mathrm {mA}} = 4 \ k \mathrm {\Omega} \$ the Arduino should report the correct minimum value.
If we connect \$ R = \frac {16 \ \mathrm {V}}{20 \ \mathrm {mA}} = 800 \mathrm {\Omega} \$ the Arduino should report the correct maximum value.




To understand how the INA196 works have a look at my answer to Why does current flow into the transistor in a current shunt monitor?.