Galvanicaly isolated 4-20mA Trans

Discussion in 'General Electronics Chat' started by mfsd, Dec 14, 2012.

  1. mfsd

    Thread Starter New Member

    Jul 16, 2011
    11
    1
    Hello...In my design I have a 4-20mA output that needs to be galvanicaly isolated.

    Any recommendation on a transformer to use?

    I have been looking at audio transformers, that seem that might do the job, but I dont know if there is something specific for this application.

    Thanks andh regards

    mfsd
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    This 4-20mA output is DC so a standard transformer will not work.

    There are galvanic isolators on the market but expensive.

    Basically there are 2 ways of doing this:-

    • Converting the analog 4-20mA signal to some kind of binary code or pulse frequency. Isolating via an optical isolator (as a train of pulses) and then re-constructing the analog afterwards.

    • Using an optical isolator in an analog mode (usually with some kind of isolated feedback to improve accuracy).
    The first method is best (better accuracy, lower drift). The device will also need to include an isolated DC-DC converter to power the output electronics.
     
  3. mfsd

    Thread Starter New Member

    Jul 16, 2011
    11
    1
    JDT, Thanks for you reply!

    One question, perhaps it is stupid, but i am just a bit confused.
    I am designing a replacement of a system that claims to be galvanicaly isolated, but it doesnt have anything as mentioned above in the output. Can it be that the isolation mains->secondary in the main transformer acts as isolator? I also see that the circuit ground is not connected to chasis, neither to earth.

    See a diagram in the attached file.
    https://dl.dropbox.com/u/14521863/Photo 17-12-2012 11 52 13.jpg

    Can we really say it is galvanically isolated?
    Apart from EMC problems, can there be other drawbacks to having this floating GND?

    Thanks again and regards,
    mfsd
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    Galvanic isolation can be so much. As an example by using a proper transformer you will the have the mains on the primary input isolated from the secondary input. In theory if something goes wrong on other side. It will not affect the other side. Quite often you may want send information from equipment A to B. The most common way to this. Is to have a common ground. But for some reason this may not always be applicable. Then you must use some sort of galvanic isolation. From your drawing you are on the correct path. But you should be aware of that most optocouplers are quite nonlinear. They are meant for digital signals (on or off).
    I think you should tell us more about your project. In this game it is hard to give to much information
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If I follow you, you are intending the galvanic isolation between your device and the AC line that powers it, and you are intending to use a power transformer to both get your power and isolate from the line.

    Yep, that's the standard way to do it. No worries.
     
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  6. mfsd

    Thread Starter New Member

    Jul 16, 2011
    11
    1
    Thanks guys...

    Here is a little bit more of info about the set-up.

    See diagram:
    https://dl.dropbox.com/u/14521863/Photo 17-12-2012 16 11 59.jpg

    Building A:
    AC/DC -> 15VDC_1 powering the 4-20mA output circuit.

    ...

    Building B:
    AD/DC -> 15VDC_2 powering the 4-20mA INPUT circuit

    15VDC_1 and 15VDC_2 can be at different potentials.

    Rephrase the question: Is the isolation on the mains transformer enough?
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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  8. mfsd

    Thread Starter New Member

    Jul 16, 2011
    11
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    Thanks! would it be enough if just one side is isolated?

    /mfsd
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Yes, you need to separate the circuit at a single point.
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    From email
    Sorry please keep it in the thread, so that other can learn as well.

    The point of galvanic isolation is basically to keep power supplies and noise separate. The maing question is if the two devices you want to connect through the 4-20mA link are connected to something else.
    For example let´s say you have it set up like this: PC-converter-4 to 20mA line-converter-PC. Both PCs are powered from normal switchmode supply, so their data ground is connected straight to the local mains earth.

    If you don´t use galvanic isolation in the middle, then it is very likely that there will be potential difference between the two grounds on each side, so there will be large current flowing between the two PCs through the 4-20mA cable. You see that even if the converter is supplied from its own transformer and so seems to be isolated, when connected to anything else it will likely stop being isolated from mains ground.

    You should make a diagram of what you´re connecting together, so that we can discuss this further based on your specific needs.
     
  11. mfsd

    Thread Starter New Member

    Jul 16, 2011
    11
    1
    Thanks...

    A simple connection diagram is in one of the previous posts.
    My basic requirement is to provide a galvanicaly isolated 4-20mA output, and it is the only part of the system that I have control of. It is up to the user what they are connecting to it.

    In your example, If I do NOT connect the circuit GNDs (DC-side) to earth, and leave them floating, why would I have current through the 4-20mA cable?

    If we look at my second diagram (above), GND1 and GND2 are inter-connected between them, and not connected to EARTH or AC-GND. That means that the potential difference would be absorbed by the mains transformer. right?
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I somehow missed those diagrams. If the only input to your circuit is the opto-coupled ttl input as I understand it, then you should be ok with just the transformer.
     
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