4-20mA current loop transmitter

Discussion in 'The Projects Forum' started by aaron_tonka, Sep 26, 2013.

  1. aaron_tonka

    Thread Starter New Member

    Sep 24, 2013
    2
    0
    I am trying to design a 4-20mA current loop for a project. I have a circuit that is nearly working however I cannot get the current below 5.5mA. I attached my circuit. I used an analog devices circuit as a reference which I also attached. The portion of the analog devices circuit I am using starts at R9.

    At 5V input I get a current of 20mA, but at 0V input I get a current of 5.61mA. I have tried changing all of the resistors, and I have tried a couple different op amps with no luck.

    Any help would be appreciated.

    Thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,120
    3,046
    What are you using for V1?

    Are you getting more-or-less linear current-vs-voltage but you can't get down to 4mA?
     
  3. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,678
    2,737
    The LM340 has a quiescent current greater than 5 ma, even at a low 10 mA load. Also, the LM358 is probably running around 2 ma. The total current consumption of the transmitter cannot exceed 4 ma at the max loop voltage, or the transmitter will not work properly.
     
  4. aaron_tonka

    Thread Starter New Member

    Sep 24, 2013
    2
    0
    Thanks. I changed the regulator to one with a very low quiescent current and it will now go below 4ma.
     
  5. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,678
    2,737
    You're welcome.
     
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