4-20mA Output Signal Degradation

Discussion in 'General Electronics Chat' started by dwinter, Aug 19, 2012.

  1. dwinter

    Thread Starter New Member

    Aug 19, 2012
    2
    1
    I am sending a 4-20mA signal from an Allen Bradley 1769-OF4 card to a variable frequency drive(VFD).
    I set the card output to 20mA and verified a 20mA signal at the end of the signal wires, prior to termination at the VFD.
    However, when the signal wires are terminated to the VFD, the 20mA signal degrades to about 12mA (half scale).
    I sent a 20mA signal to the VFD from my process meter, with the signal wires disconnected, and all was well.
    I don't think current is affected by impedence. Allen Bradley states the signal should be good if the resistance is less than 600ohms.
    Does anyone know what would cause the signal degadation and/or what I can do to resolve the issue?
    I don't have any info on the VFD yet.

    Thanks,
     
  2. #12

    Expert

    Nov 30, 2010
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    6,810
    That's a problem.

    The basic principle of a 4-20 ma drive is that it cares not about resistance in the wires and the load, up to a point. That point is where the driver runs out of voltage. Suppose you have a closed amplifer loop with a 5 volt power supply. If it wants to send 20 ma through the loop, the wires and the load must be less than 250 ohms or there won't be enough voltage to drive 20 ma through it. Just figuring the specs you know, 600 ohms and 20 ma, I can guess the power supply is about 12 to 15 volts.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Measure the voltage at the VFD input. If the input is <600Ω then the signal voltage due to the current will be less than 12V (<7.2V for 12mA). If it's higher than that, then the input is >600Ω.
     
  4. SANJAYAJRI

    New Member

    Feb 20, 2012
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    0
    To resolve this issue , use the signal isolator as this may happens due to current harmonics effect generated by VFD.
     
  5. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,648
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    It seems that you and dwinter know the suggested signal isolator. Can you identify it?

    Or maybe you are suggesting to use a signal isolator?
     
  6. #12

    Expert

    Nov 30, 2010
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    Either this is crazy or I'm about to learn something.
    Of course, I learn something almost every day on this site.
     
  7. maanga

    New Member

    Aug 20, 2012
    12
    3
    Hi,
    Most of the VSD the 24 volts supply (-) is grounded. This can cause a problem, but PLC AO is isolated.
    If the 4-20 ma signal calibrator can feed, the PLC card should be able to feed.
    12 ma is 50% of the current.
    I suppose you did not scale the analog output properly.
    Can you verify the current output from card in to a multimeter? Use a series resistor of 100 to 250 ohms.
    egards,
    Maanga
     
  8. dwinter

    Thread Starter New Member

    Aug 19, 2012
    2
    1
    I verified the output current with my DMM.
    I found the signal loss at a surge supressor. Removed the SS & all good.
    When I return to the site, I am going to get the spec on the surge suppressor and compare to the AB AO card.
    Thanks for the input.
     
    #12 likes this.
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
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    I've never heard harmonics being cited as cause for a signal isolator, but they are often used in situations like this. I suppose EMI could be a cause, but usually I see it in situations where connecting commons of separate circuits causes weird issues. Or when the input impedance of the drive (or other device) is too low.
     
    #12 likes this.
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