Using 4-20mA sensors with the ADC Pi Plus or Zero
Created 02/04/2016 | Using commercial 4-20mA sensors with the Raspberry Pi
We often receive enquires for how to use commercial sensors which have a 4-20mA output with the Raspberry Pi.
As many of the commercial sensors operate from a 24 volt supply and need to be isolated from the Raspberry Pi it wouldn’t be possible to measure the output current across a load resistor directly on the ADC input as there would be the potential for the input voltage offset to be too high and damage the Raspberry Pi and ADC chips.
In order to resolve this, we found a lot of commercial modules which convert the 4 to 20mA signals to a 0 to 5V output which can then be read using the ADC Pi Plus or ADC Pi Zero boards.
The conversion modules are often marketed on sites such as eBay called Current to Voltage Transmitter Signal Module 4-20MA to 0-5V Linear Conversion or similar and only cost a few pounds.
The photo below shows an example of one of the modules we purchased to test with the ADC Pi boards.
We also purchased a PT100 Temperature Sensor Transmitter which outputs a 4-20mA signal from the PT100 sensor and we connected this to the signal converter. The module uses a 24 volt supply which allowed us to test for the voltage conversion.
The output from the current to voltage module is then connected to one of the ADC Pi inputs and you could use up to 8 of these current to voltage modules with a single ADC Pi Plus/Zero.
We do not have any affiliation to the eBay sellers on the above links, these are just examples of the modules available.