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0-20ma generator help please

  1. Jan 14, 2008 #1
    Hi does anyone know of a simple circuit that i can build to generate a ma signal from 0 - 20ma ? using a 9v pp3 or similar

    there does not need to be a digital indicator on the circuit as i will series the generator with a digital multimeter, i just need to be able to generate a ma signal that can be adjusted between 0 - 20ma.

    can you help?

    thanks
     
  2. jcsd
  3. Jan 14, 2008 #2

    berkeman

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    Is this for 20mA communication signals, or you just want to build an adjustable analog current source that you can set with a potentiometer? How accurate do you want it to be? What will the range of load impedance be?
     
  4. Jan 14, 2008 #3

    yes i just want to build an adjustable analog current source to help in commisioning actuators/valves that i use at work. they operate on a 4 - 20ma signal, ocasionally we use 0 -20ma actuators/valves

    the circuit impeadance is in the range of 350 ohms.

    and yes as you say this would ideally be adjustable via a potentiometer

    thanks
     
  5. Jan 14, 2008 #4
    regards accuracy it needs to be able to go as low a zero ma but can far exceed 20ma afterall i will be monitoring the current flow by putting a multimeter in series.
     
  6. Jan 14, 2008 #5

    berkeman

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    Oh, this is 4-20mA signalling for a sensor application. Here are a couple primers on 4-20mA signalling, and an app note from Maxim on how to use one of their ICs for generating the 4-20mA signalling. If you still have questions, please post a follow-up.

    http://www.processautomationcontrol.com/download/4-20mA-Current-Loop-Primer.pdf

    http://www.sensorland.com/HowPage028.html

    http://www.maxim-ic.com/appnotes.cfm/an_pk/1064


    Quiz Question -- why is 4-20mA signalling used, and not 0-20mA signalling?
     
  7. Jan 14, 2008 #6

    mgb_phys

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    Presumably so you can detect a cable break?
    Follow up question, why not using +/- 10mA instead. Or does 4-20 assume a chassis gnd like in car electronics?
     
  8. Jan 14, 2008 #7

    ranger

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    Interesting question. I think its in that range because we will always have at least 4mA (as our "zero") flowing in our circuit. We can use that very same (pair) of wires to power other electronic components. In this way be save on wire and the installation is easier.
     
  9. Jan 15, 2008 #8

    berkeman

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    Good answers. In skimming the links that I posted, I noticed that they mentioned the 4mA base current made open-circuit detection easy, as mgb_phys says. Not sure why they picked 4-20mA as the range, though.
     
  10. Jan 15, 2008 #9

    dlgoff

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    "Not sure why they picked 4-20mA as the range, though."

    Well I think it has to do with 250ohm being a standard input impedence for display and other modules giving a 1 to 5 volt input.
     
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