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BX-04THM weirdness

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  • BX-04THM weirdness

    I've been chasing issues with thermocouple wiring on an 04THM card for the BRX line of PLCs. I'm having some sort of noise or voltage issue.

    Attached to the card I have Channel 1 - N T/C, Channel 2 - K T/C, Channel 3 0-1.25V current sensor (P/N ACT050-10-S from AD. I know it's a 0-10V output, I'm just using the bottom of the range), Channel 4 - just a 1 inch jumper. Thermocouples are connected with unshielded extension wire, and there is a VFD in the same control cabinet as the PLC. I do have ferrites on the thermocouple wires, but I can't see that they have any affect.

    Both thermocouples are ungrounded, as is the current sensor. When the furnace is cold, or operating below about 650 degrees F, all the sensors produce data. Once the furnace gets above 675-700, the K thermocouple starts giving a burnout signal (and the raw reading goes to -0.1 because I have high burnout selected ) I was testing with my volt meter and noticed that if I take attach the voltmeter to ground and either leg of the K thermocouple, while in DC Volts mode, the reading comes back for as long as I hold the meter on the screw, plus a short amount of time after. The reading also comes back when the meter is switched to AC Volts, but only for a second or so, then it goes back to burnout, even if the meter is on the screw.

    I happen to have an oscilloscope here, and I attached the probe to one of the thermocouple legs at the card end. It does the same thing. When in AC coupling mode, the T/C reads burnout (it might be good for a short time when I'm attaching the probe, I didn't check). If I switch the probe to DC coupling, the thermocouple starts reading again. I attached a couple of screenshots, one showing the thermocouple beginning to work on it's own when the furnace cools sufficiently, and the other during the time I was testing with the oscope. The oscope screenshot shows the scope coupled in AC mode at the beginning, being switched to DC mode, then being switched to AC mode again. The probe was attached the entire time.

    As a bonus, I took a couple short videos of the oscope screen. Change the .zip extension to .mp4 to play them. Again, the only thing I did was change the switch from AC coupling to DC coupling. The voltage bumps you see when in DC mode are the same voltage bumps I saw when the scope was attached in AC mode and the voltmeter was attached in DC mode at the same time. I'm also noticing a DC offset. You can see it on the screen; the trace is higher in DC mode (voltmeter also shows roughly 0.5 V DC, but it jumps around a little).

    I'm a bit lost as to what to do next. I do know that grounding the K thermocouple doesn't work. I've tried it, and I get a negative temperature reading. No burnout indicated, but the temperature shows way negative and doesn't move. I'm open to suggestions for testing, but I might not get a chance to play with it until next week.
    Attached Files


  • #2
    Please return the modules to ADC for replacement. New modules should fix your issue.
    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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    • #3
      Is there a date code break? I have several of these modules. Not sure if this was a design problem, or a one-off type situation. I havenít done extensive testing with the other modules yet.

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      • #4
        Last two digits of the serial number should be at least B3 for a BX-04THM. If it is A1, A2, B1 or B2 then you should return it for replacement.
        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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        • #5
          Should the BX-08THM modules before B3 be returned and replaced as well? I have several that are A1 in a panel that has not yet been put into service.

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          • #6
            Originally posted by RogerR View Post
            Should the BX-08THM modules before B3 be returned and replaced as well? I have several that are A1 in a panel that has not yet been put into service.
            IGNORE<Based on Do-more PE's reply from 6/14, Yes.>
            Last edited by franji1; 06-18-2019, 08:55 AM.
            There are 10 kinds of people in this world, those who know binary, and those who do not.

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            • #7
              Those have a different date code associated with the issue. I'll have to see what it is.
              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

              Comment



              • #8
                BX-08THM should be A3 for the latest revision.
                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

                Comment



                • #9
                  Hmm.... What IS the rev numbering scheme? I'm terribly confused.

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                  • #10
                    There are two different modules being discussed, a 4 channel and an 8 channel T/C module. Presumably each has its own revision tracking numbers. The 8 channel was launched later, and probably started out with electronics similar to the B rev of the 4 channel module.

                    I'm guessing.

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                    • #11
                      Originally posted by bfitz View Post
                      There are two different modules being discussed, a 4 channel and an 8 channel T/C module. Presumably each has its own revision tracking numbers. The 8 channel was launched later, and probably started out with electronics similar to the B rev of the 4 channel module.

                      I'm guessing.
                      That would be correct.
                      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

                      Comment



                      • #12
                        Thx. That might clear up one of my mud puddles

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