Interfacing active 4-20mA device to active 4-20mA logger?

Discussion in 'The Projects Forum' started by zoombody, Nov 16, 2010.

  1. zoombody

    Thread Starter Member

    Aug 14, 2009
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    My post title probably isn't the clearest. I have a water treatment controller with an actively driven (i.e. the controller is a current source) 4-20mA monitor output. I am trying to interface this output to a data recorder that has 4-20mA inputs that are also actively driven (it's designed to connect directly to sensors). I'm at a loss. Any suggestions? Thanks in advance.

    Edit: more detail: The controller outputs a voltage on its + terminal and varies it based on the return current to its - terminal. The recorder outputs a fixed voltage on its + terminal and records the current returned to its - terminal.
     
    Last edited: Nov 16, 2010
  2. mik3

    Senior Member

    Feb 4, 2008
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    Any datasheets available?
     
  3. zoombody

    Thread Starter Member

    Aug 14, 2009
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    I have only manuals, not datasheets, and both are sorely lacking in any detail relating to the 4-20mA signalling. I've tried to get more detail from both manufacturers but have had no luck.

    What questions were you hoping datasheets would answer?
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    I want to see the specifications of the 4-20mA output and the 4-20mA input to make sure you can connect them directly.
     
  5. zoombody

    Thread Starter Member

    Aug 14, 2009
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    You cannot connect them directly -- that's my whole issue -- but I had to figure it out the hard way. The controller outputs a voltage on its + terminal and varies it based on the return current to its - terminal. The recorder outputs a fixed voltage on its + terminal and records the current returned to its - terminal.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    By the sound of it, the data logging hardware is probably designed to connect to loop powered transducers. You therefore can't interface directly to a self powered or actively driven transducer.

    If you have analog voltage inputs on the data logger as well as 4-20mA current inputs you could perhaps convert the 4-20mA to (say) a 1-5V DC signal using a 250Ω resistor in series with the 4-20mA with the latter connected across (in parallel with) an analog voltage input terminal pair. You then have to modify the logging program to correctly interpret the input data.

    Does the data logger not have an input option which allows for self powered 4-20mA transducers?

    What is the data logger device type?

    If you are stuck with loop powered transducer inputs to the data logger then you probably have to find an alternative transducer type. Alternatively, use a different data logger. You can probably buy an interface device which converts the active transducer signal to a corresponding loop powered equivalent - or design and build something yourself. In the end what's the least expensive option?
     
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