Interfacing to two-wire 4-20mA sensors

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Many 4-20mA analog sensors only has two wires connection and are designed to be powered by the 4-20mA output current that flow through it. These type of sensors can be interfaced easily to the 0-5V analog inputs of the PLC as shown in the following diagram:



The sensor output will be converted to a 1-5V analog voltage and can be read by the PLC using the ADC(n) statement which will return readings of between 820 and 4092.

If the ADC is of 0-1V type (such as ADC1 & 2 on T100MD1616+) then just change the resistor to 50 ohm.

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If the 4-20mA sensor has its own power supply, then the connection is just straightforward, as follow:



If the PLC analog input is 0-1V (as in ADC1 and 2 of T100MD1616+) then change the resistor value to 50 Ohm.

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PT100 sensors can be connected to T100MD1616+ ADC1 and 2 (0-1V range) easily as described in the pdf document as follow:

http://www.tri-plc.com/appnotes/PT100Interface.pdf


The circuit is not suitable for 0-5V ADC range since the self heating will be quite significant. You will need a signal conditioner or build an instrument amplifier to amplify a small voltage drop across the PT100 to 0-5V suitable for the ADC.