Convert 4-20 mA to 0-10 V using a resistor

Written by admin

17 May 2022

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.

It is extremely simple to measure 0-20 mA analog signal from a sensor with a JACE that will measure only voltage inputs.

A JACE voltage input accepts a 0-10 V signal, but may not accept a 0-20mA signal directly.

Basically, Ohms law saying that voltage(V) is equal to the current(I) multiplied by the resistance(R) is used to calculate a resistor value in order to convert the 0-20mAsignal to a voltage.

Voltage (V) = Current (I) x Resistance (R)

0-20 mA to 0-10 VDC : 500 Ohms resitor

0-20 mA to 0-10 VDC : 500 Ohms resitor

0-20 mA to 0-5 VDC : 250 Ohms resitor

0-20 mA to 0-5 VDC : 250 Ohms resitor

With a passive 4-20 mA sensor and an external 24vdc power supply

With a passive 4-20 mA sensor and an external 24vdc power supply
iSMA connection at terminals

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 to) 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 which is why I have filled the Source field with mV values.

iSMA Voltage proxy Point

In case you want to read a sensor that has only 2 wire connections 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 a 24 V power supply.

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