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Thread: Different common between ddc and transformer

  1. #1
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    Different common between ddc and transformer

    For any DDC for instance, Eagle controller (Honeywell) all points analog and digital have the same common (ground) so my question is what problems happened if the ground of controller is not the same as ground of transformer?

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  2. #2
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    Different common between ddc and transformer

    Quote Originally Posted by Mohamed Lasheen View Post
    For any DDC for instance, Eagle controller (Honeywell) all points analog and digital have the same common (ground) so my question is what problems happened if the ground of controller is not the same as ground of transformer?

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    Typically you bond the two low voltage transformer commons together. So they have the same common reference. If you don’t it will cause issues.

    This is a good question. If someone can explain it more thoroughly than I feel free.


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  3. #3
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    You would very likely have a floating neutral on the DDC devices. I've seen it cause everything from temperatures being off to actuators that don't fully open or close. I've also seen it do nothing at all. It's a best practice to use the same ground reference for all the devices and code in most cases. You absolutely should be bonding the secondary of all transformers to ground.

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  4. #4
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    Quote Originally Posted by MPeterson21 View Post
    You would very likely have a floating neutral on the DDC devices. I've seen it cause everything from temperatures being off to actuators that don't fully open or close. I've also seen it do nothing at all. It's a best practice to use the same ground reference for all the devices and code in most cases. You absolutely should be bonding the secondary of all transformers to ground.

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    Yes, that's good but ddc has already built in common what exactly will affect on this common if we talk about temperature sensor, for instance

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  5. #5
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    This is basic electrical practices. If you don't have the same ground reference, you have improper voltages. If you don't bond your transformers, you're asking for things to not work, and fail.

    The answer to your question is, it will work correctly if you do this. It very likely won't work correctly if you don't. Look at the diagram, it shows the "external ground" being bonded to the secondary. Follow the diagram.

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    I hate to burst your bubbles.
    If you look closely at the wiring diagram you should notice the dot on the ground wire. This indicates term 9 and term 74 are at the same potential.
    In case of malfunction-Reconnect brain BEFORE starting mouth.

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    Quote Originally Posted by AutoPilot View Post
    I hate to burst your bubbles.
    If you look closely at the wiring diagram you should notice the dot on the ground wire. This indicates term 9 and term 74 are at the same potential.
    Yep, that's why I said follow the diagram. I also almost guarantee that this isn't done on the PCB and has to be physically wired together.

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  8. #8
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    Quote Originally Posted by AutoPilot View Post
    I hate to burst your bubbles.
    If you look closely at the wiring diagram you should notice the dot on the ground wire. This indicates term 9 and term 74 are at the same potential.
    I know the wiring very well and I follow it but I supposed if I have 1 ddc and 3 modules if all commons concerning to modules are not referenced what's happened, for instance for analog points

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  9. #9
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    Quote Originally Posted by MPeterson21 View Post
    This is basic electrical practices. If you don't have the same ground reference, you have improper voltages. If you don't bond your transformers, you're asking for things to not work, and fail.

    The answer to your question is, it will work correctly if you do this. It very likely won't work correctly if you don't. Look at the diagram, it shows the "external ground" being bonded to the secondary. Follow the diagram.

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    First of all I know what you mean but I supposed that common for ddc isn't connected to transformer common what makes it read improper voltage however there is common reference for all points in DDC like attached photo

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  10. #10
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    Quote Originally Posted by Mohamed Lasheen View Post
    First of all I know what you mean but I supposed that common for ddc isn't connected to transformer common what makes it read improper voltage however there is common reference for all points in DDC like attached photo

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    Well the transformer is in the bottom left and it does come up to the module common and ground terminals. So there's that.

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  11. #11
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    It looks like they are calling the "common" a ground bu it doesn't show the common being bonded. Does that happen in the board? It also shows the gnd/common being daisy chained between boards. That is not something I would do.

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