Interfacing 4-20ma sensors with PLC

Discussion in 'General Electronics Chat' started by maxwell_steuer, Jan 30, 2013.

  1. maxwell_steuer

    Thread Starter New Member

    Jan 30, 2013
    Hi All,

    I'm new to the forums so please let me know if I'm posting in the wrong place! I'm in a senior design project involving a PLC processing signals from some 4-20ma sensors. I know how the PLC operates generally speaking... A logical ON state is detected when say 12VDC is detected at its analog input.

    Does anyone know how 4-20ma sensors are interfaced with the PLC? Do we just hook a resistor in series with the sensor output and use the voltage dropped across that with the PLC?

    There is a minimum ON voltage that will trigger the PLC relays to detect an ON state on its analog input. Since the sensors emit a variable current, how can we measure that with the PLC? I would think we would choose what our threshold current would be to trigger the PLC... So, say we decide that 10ma should trigger the PLC input to ON. Since 12V at the PLC input is required for that, we could put a 1.2K resistor in series with the sensor output which would yield 12V across the resistor and we could wire that voltage in parallel with the PLC input. Is that how you do it or do PLCs have the ability to measure input current in addition to voltage?