Need to make 4 - 20mA loop isolators

Discussion in 'The Projects Forum' started by bowlingo, Feb 17, 2012.

  1. bowlingo

    Thread Starter Member

    Jun 29, 2011
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    Hi all,

    I have a combined PH and CF transmitter of which can be found here

    http://www.omega.co.uk/ppt/pptsc.asp?ref=CDTX300

    I need to make 2 x 4 - 20mA loop isolators that I can connect to each output. The unit runs on a 24V power supply

    Someone has pointed me to some of the components

    http://docs-europe.electrocomponents.com/webdocs/0028/0900766b800289f3.pdf

    http://www.rapidonline.com/Electric...106/?sid=003c3b70-f289-488d-b4a3-bb276c21a821

    This will be the fist time I have made something like this and would like to do it on small prototype boards with chip housings etc.

    Can anyone provide me with the correct parts from farnell or RS and possibly an easy to understand diagram?

    Thanks
     
  2. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    Anyone any ideas?
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    I would assume that if two readings are transmitted via a two wire interface, then this is a digital output. You will need a digital processor such as a microcomputer to interrogate the data.
     
  4. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
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    This is the unit I have used on it in the past

    http://uk.rs-online.com/web/p/produ...552267573743D343636323231342677633D4E4F4E4526

    As you can see its disgustingly expensive £134.40 including VAT

    I am sure these can be made easily using basic components and a bit of soldering..trouble is what components from RS or Farnells, chip housing etc and a wiring diagram...I would like to use the existing 24V DC supply

    Thanks
     
    Last edited: Feb 17, 2012
  5. MrChips

    Moderator

    Oct 2, 2009
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    I can never figure why people say it must be easy to do and they come running to us for help.
     
  6. bowlingo

    Thread Starter Member

    Jun 29, 2011
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    That is a very good point..can anyone who knows how to do this help me please as quite rightly as mrchips has pointed out its not easy to do hence the reason I am asking
     
    Last edited: Feb 17, 2012
  7. crutschow

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    Mar 14, 2008
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  8. bowlingo

    Thread Starter Member

    Jun 29, 2011
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  9. crutschow

    Expert

    Mar 14, 2008
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    I believe that's the same device my first reference op note discusses.

    If you want true isolation that circuit won't do. You need an added isolator as shown in the op note. Otherwise you can just power it from a DC supply within the circuit's voltage limits of operation.
     
  10. bowlingo

    Thread Starter Member

    Jun 29, 2011
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    Thanks for your help...can you help me out with the exact components?....I really would like to make this and test it
     
  11. crutschow

    Expert

    Mar 14, 2008
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    If you want a cookbook design here's a circuit shown in Figure 19 of this ap note.

    Edit: The circuit parts are readily available and low cost. It uses an optocoupler for high isolation between input and output.
     
    Last edited: Feb 20, 2012
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