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Analog Inputs reading voltage with nothing plugged in

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  • Analog Inputs reading voltage with nothing plugged in

    Hello All,

    I have (2) BRX08AD-2B (0-10v) input modules in my system. I have all the input channels going through the recommended 0.032amp S500 fuses. My issues comes when a fuse holder is disconnected or with one of the spare (unused) channels, I have voltage/a reading with nothing connected. my readings are accurate while connected to my CTs. My concern is that since I am reading motor Current, If one the channels get disconnected or the fuse blows, I will still show a significant reading. the disconnected channels are reading about 3 volts.

    Any Ideas, Suggestions, or is this the nature of using Analog voltage inputs?

    I typically use 4-20ma but my customer insisted on 0-10v sensors for ease of troubleshooting.

    Any Help greatly appreciated.

    Thanks.
    fdiaz1


  • #2
    I don't think is ok to post from the manuals. I recommend that you read thoroughly the "voltage module tips and troubleshooting" from your module manual. I will provide two short excerpts:
    "
    • The DC power supply that powers the module should be checked for the negative side to ground voltage being under 0.1 V for both AC and DC. If this voltage is floating, it can cause errors and/or damage to the circuit.
    "
    and another one:
    "
    • Jumper the positive and negative terminals together on unused voltage input channels.
    "

    there is a list in several points of what a faulty reading should be related to. I don't think i should discuss it here. maybe later, after more people has read it.

    please understand that trying my suggestion could lead to unpredictable hazards. if you choose to follow, I can not be held responsible.

    I would never run an analogue channel disconnected from the transmitter. admitedly this could accidentally happen at runtime, so I would have a 10kohm resistance connected between channel and com. in normal operation, the current loss through the 10kohm is max 1mA at 10V, too little to affect the operation. however, should a fuse blow or anything else to disconnect the transmitter from the channel, then since the channel is shorted to com, will always indicate 0VDC.
    in ladder logic, the 0VDC is a fault level which should be monitored and a positive level (0.1VDC) should be considered for a positive detection of zero (similar to 4mA for current). therefore, my channel should never read 0VDC; when it does it, ladder logic will raise an alarm indicating channel fault.
    Last edited by Alexandru; 01-04-2019, 12:06 AM.

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    • #3
      That seems high and makes me wonder if there is some kind of grounding or supply issue, but for testing I usually have them looped, so that may be more typical than I realize. I agree that a small load would be adequate to solve the issue. Not sure what the input impedance is on those modules, but I would expect it to be pretty high. It's possible that going above 10k would still be effective, and would reduce the load on the sensor further.

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      • #4
        As suggested, I would try loading them with a 10K resistor. Also try grounding your DC supply negative if not already. I don't usually ground it myself and rarely have any noise issues.

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        • #5
          If you are worried about floating signals, you should never use an analog voltage signal. Voltage based analog signals pick up noise from everywhere. Current based signals will never float and don't pick up nearly as much noise as they are a driven signal.
          If you have an urgent issue, please contact AutomationDirect's Technical Support team.

          AutomationDirect.com Technical Support: 1(800) 633-0405 or (770) 844-4200 Email Tech Support

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          • #6
            Thanks for the replies...

            I have done everything suggested on the manual (there isn't much for analog voltage inputs), The power Supply is a Class 2 and its properly grounded. I can't imagine its noise since my issue comes from the not-connected channels. I can remove the connector plug on the module and only leave the power plug and all my CHs will be showing a reading.

            I thought about adding a resister but:
            1. I don't have them on hand
            2. I worry about affecting the reading when connected to the analog sensor.

            any thoughts on #2?
            Thanks
            fdiaz1

            Comment



            • #7
              Originally posted by fdiaz1 View Post
              Thanks for the replies...

              I have done everything suggested on the manual (there isn't much for analog voltage inputs), The power Supply is a Class 2 and its properly grounded. I can't imagine its noise since my issue comes from the not-connected channels. I can remove the connector plug on the module and only leave the power plug and all my CHs will be showing a reading.

              I thought about adding a resister but:
              1. I don't have them on hand
              2. I worry about affecting the reading when connected to the analog sensor.

              any thoughts on #2?
              Thanks
              Start with a very high resistor value...1M, or even 10M. See what it does. Somewhere between air and a short to ground is a value that will correct the issue. Once you determine what that is, then you will have a better sense as to whether the load is too heavy and is likely to skew your readings. The suggestion of 10K was just a starting point.

              Or you can use current.

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              • #8
                The manual also says to jumper the non-connected channels.
                you need to tie to com any channel which is not connected to a transmitter. However, if not connected, then why worry? You wont use in ladder logic a channel which is not connected, yes?
                if you worry that a channel might become disconnected at random during normal operation, then you need to employ a monitoring or a redundancy that will detect the disconnect. The resistor patch work is an option, if you use monitoring for zero volts d c in your ladder logic. Which means you cant use zero for anything else in your program.

                the resistor method is called soft pull-up when used in serial communication. In this case will be called a soft lockdown.

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                • #9
                  The specifications for the BRX08AD-2B claim an input impedance of ">10M " it wouldn't take much to get a reading even with an "open" circuit. The catalog page says "country of origin: USA" so I'll assume the circut boards were properly cleaned and high impedace sections coated with a qulity conformal coating, but you know what the say about assuming

                  I too will recomend a shunt resistor, the original recomendation of 10K sounds good to me, I wouldn't be using a sensor that couldn't drive 10K in any industrial setting, but even a 1M would be a big change from >10M

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                  • #10
                    The lower the shunt resistance, the more noise immune the channel will become... I'd see if 10k knocked it out, and possibly head toward 1k. I assume just about any sensor can source 10mA.

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