How to connect a 4-20mA sensor to the iSMA devices

All iSMA Universal Inputs cannot read a 4-20mA signal directly, so to read a 4-20mA sensor you will find in the box of every iSMA device (with Universal Inputs) a set of 200 Ohm resistors.


Those resistors will convert the Current (mA) into a readable Voltage for the inputs:

4 mA will translate to 800 mV (0.8V)           -                 4mA*200Ohm=800mV

20 mA will translate to 4000 mV (4V)           -                 20mA*200Ohm=4000mV


The scaling from Volts to temperature/humidity/CO2/pressure (depending on the sensor that it is connected) will need to be made in the BMS controller.

In the following example, I have converted the linear signal from a 4-20mA temperature sensor into degC. The iSMA modules are reading mV that is why I have filled the Source field with mV values.



In case you want to read a sensor that has only 2 wire connection for both powering and signal, it is possible to connect it as shown in the picture below. You must remember that G0 from the Power Supply has an inside connection to G0 in Universal Inputs in all iSMA Modules, so you can use the same power supply for device and sensor, if the sensor requires 24 V power supply.