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Has anyone got this universal analog output circuit to work?

Discussion in 'General Electronics Discussion' started by leeb_965, Feb 5, 2012.

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  1. leeb_965

    leeb_965

    34
    0
    Feb 5, 2012
    Hi I am new to this forum and I'm struggling to find a 4-20mA voltage to current converter. I was recommended this circuit:

    http://www.piclist.com/techref/microchip/aout.htm

    But I can't get it to work.

    Has anyone ever used this or can recommend a circuit for me please?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,993
    2,498
    Nov 17, 2011
    I'm not familiar with the circuit you reference, but it looks reasonable to me.
    Can you specify more detailed what you "can't get it to work"?
    Do you get any output at all?
    If yes: what is the expected output vs. the measured output?
    In which environment is the circuit expected to operate (what supply voltages, what load resistance/burden)?

    See also this thread: https://www.electronicspoint.com/0-5v-4-20ma-t243919.html

    Regards,
    Harald
     
  3. leeb_965

    leeb_965

    34
    0
    Feb 5, 2012
    Thing is I dont know the typical changes in resistance so I randomly altered the gauge resistances on the simulation. I dont really know much about this stuff and Im not really sure what to expect at the output. Should there be only a current at the output or will there be a voltage as well?

    I also found:

    https://www.electronicspoint.com/atta...ge-current.gif

    but Im not sure what the wires are for going through the center of the 1st op-amp.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,993
    2,498
    Nov 17, 2011
    O.K.
    first: randomly altering values seldom does any good. Try to understand the circuit first. And on basic thing: Except under very rare circumstances (like superconductors) whenever there is a current, there is also a voltage. A current at the output can onlky flow if you add apath (ressitor) between the two output pins. Without load, there is no way for a current to flow and the circuit will just output max. voltage.

    So which resistor did you change? In the schematic there is no resistor labeled gauge.

    Also which wires "going through the center of the 1st OpAmp" are you talking about? I can't see any wires going through the OpAmp. Are we talking about the same schematic?

    I recommedn you build the circuit on a breadboard, then you can experiment.

    Harald
     
  5. leeb_965

    leeb_965

    34
    0
    Feb 5, 2012
    Yeah I found another but Im wondering if the vertical wire through the middle of the 1st opamp is part of it. What kind of voltages am I expecting at the output. I put in a volt at the input and got out 4mA but about 11 volts also.
     

    Attached Files:

  6. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    It is not just a random vertical wire. It is the power to the IC containing the op-amps. It is a given to know that an op-amp needs power to work.


    edit: look at the schematic you posted a link to. It states "1/2 lm358". This is a double op-amp IC package, so they only show power connecting to one half of the LM358, the other one half of the IC is powered by from the same source. Checkout the datahseet if you need a clearer understanding of the pin-out.
     
    Last edited: Feb 6, 2012
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