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Analog signal 4-20mA

Discussion in 'General Electronics Discussion' started by Sangari, Oct 16, 2013.

  1. Sangari

    Sangari

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    Oct 16, 2013
    Why 4-20mA is used in Analog Signals? Why not 0-20mA? or else? please explain
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Consider what happens if a wire breaks...
     
  3. Sangari

    Sangari

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    Oct 16, 2013
    No signal...
     
  4. BobK

    BobK

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    Jan 5, 2010
    And how do you distinguish no signal from 0mA?

    Bob
     
  5. Sangari

    Sangari

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    Oct 16, 2013
    No signal ; no current
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    So what is different between 4-20mA and 0-20mA?
     
  7. Sangari

    Sangari

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    Oct 16, 2013
    Ya i got it but i want to know what are the advantages of 4-20mA analog current loop over 0-20mA analog current loop? Reason behind 4-20mA for industrial analog communication!:confused:
     
  8. shrtrnd

    shrtrnd

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    Jan 15, 2010
    Sounds like you didn't 'get it'.
    I worked industrial electronics constant current for most of my life.
    We started in the 1970's with 10 - 50ma, but everybody eventually went with the 4-20ma type.
    You cannot use (as *steve* & BobK tried to help you figure-out) a zero ma signal as
    your zero value point for the current signal, because if the circuit failed, you would also
    read zero mA, and you would not know that the circuit failed, you would just think that
    your circuit was alright, and you were actually reading a 0ma signal.
    The circuits you're wondering about, use the 4ma value as the signal 'zero', because
    you know the circuit is working when you read 4ma. If the circuit ever does actually
    read 0mA, you KNOW the circuit is not working.
    Does that explain it?
     
  9. duke37

    duke37

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    Jan 9, 2011
    Also, I thought the 4mA was used to supply the transducer with power.
     
  10. eman12

    eman12

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    Aug 24, 2013
    Good explanation about the reason.

    I myself think that zero "mA" is nonsense;)? So it can be the one of the main easons why they do not use zero mA for this job!
     
  11. gorgon

    gorgon

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    Jun 6, 2011
    The reason for the 4-20mA system is the supply of the transducer. It is normal that they also need an voltage overhead of around 10V to function properly.

    0-20mA is normally used when the transducer has it's own Power supply.

    Many transducers use either <4mA(2-3) or >20mA (21-25) as a fault indication in addition to the normal range.
     
  12. Sangari

    Sangari

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    Oct 16, 2013
    Yes clear explanation!

    Moreover i got the following reply from one of my colleague that "most of the TTL circuits operate on voltage 5V or lesser than this , beyond which they need to handled with power transistor which is not economic as well handling heat dissipation

    What value add do you point in increasing the loop resistance, in general practice, we try to keep the loop resistance as low as possible to reduce voltage drop and signal attenuations
     
  13. gorgon

    gorgon

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    Jun 6, 2011
    Why you mix TTL into this picture, I don't know. (0)4-20mA is an analog signal in nature.

    The max value of the loop resistance is purely dependent on the overall voltage overhead in your current loop. For a 4-20mA loop the tranducer will normally need around 10V of the loop voltage to function. If your loop is supplied with 24V you'll have around 14V, minus a safety margin, left to use in the loop resistance. This resistance will be a combination of wire resistance and measurement resistance. If your max current is 20mA, the max loop resistance will be less than 700 ohm. In the real world you'll always need a safety margin, so the practical value should be less than that, maybe around 500 ohm. If the max current is 25mA, for error indication, the max loop resistance will be even lower. The measurement resistance is the sum of all inputs connected to the loop. There is no attenuation of the signal, as long as you keep a free overhead voltage available for the driver.
     
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