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Question about when to have a 'common ground'

Discussion in 'Power Supplies' started by aplant92, Sep 28, 2018.

  1. aplant92

    aplant92 Full Time Elf

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    Hi all,

    I'm planning on putting in more than one power supply in my show this year, and want to make sure I understand the need for a common ground between those power supplies.

    If each power supply is powering different, disconnected elements, do they need a common ground between the other power supplies?

    They other thing I'm not sure about is whether or not they need to come back to the controller or not - my controller gives power to each output, as well as data. Does this mean any injection that happens along one of those outputs needs to be grounded to the same power supply that is powering the controller?

    Any help is appreciated!
     
  2. AAH

    AAH I love blinky lights :) Community Project Designer Generous Elf

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    The common ground is required pretty much only when you have multiple supplies supplying power to the same string. This would be if it's too far to economically keep providing power injection from the original power supply or if you've run out of power availability on the 1st and need some more oomph from a 2nd.
     
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    aplant92

    aplant92 Full Time Elf

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    Great thanks for the clarification!

    So this doesn’t come to play with the controller providing power to the first set of strings connected to an output? Or do I need to make sure the 5v from the output doesn’t connect to a different power supply injecting later on to the same string?
     
  4. AAH

    AAH I love blinky lights :) Community Project Designer Generous Elf

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    I've got a bit confuzzled with your wording which may just be a result of my nanna nap this arvo.
    If you're powering strings via a controller directly there is no need to tie any grounds together. If you're then providing power injection 100 pixels (or whatever) downstream then it gets more complicated. Ideally the power injection of both gnd and 5V (or 12V) should come from the same fused output that the previous pixels come from. In reality it's often the case that people will provide power injection from a 2nd fused source coming from the same power supply. This doesn't provide the same protection as the 1st method and while there is no issue with fuse blowing it's all good. If either of the fuses blow then the other is likely to blow as well as it would be providing all the power.
    The better option and 1 that probably isn't used as much as it should be is when power injection is required the 5V or 12V line is broken at that point and the downstream power is supplied from that point on from the 2nd power source and in this case the gnd connections become tied together where the new power is tapped in. This option allows fusing to be correctly sized for the amount of pixels being protected and can actually allow voltage changes from 5V to 12V or vice versa along the same length of pixels. If I was any good at drawing pictures I'd illustrate the different good and bad methods.
     
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    aplant92

    aplant92 Full Time Elf

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    No worries, I think I understand what you’re saying!

    I thought about this a bit after my last comment and think the easiest option is just not provide power from the controller, and inject from a PSU on the very first string - this way I don’t need to worry about two different power supplies “clashing”.

    Does that sound right?
     
  6. scamper

    scamper Senior Elf

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    that is the way I do it.
    as far as common grounds go, the main reason you do it is because the ground is just a reference point, so the 5v is only 5v in reference to the ground. so if you have different power supplies, one of the grounds may actually be "floating"in other words in reference to the first power supply may actually have a positive voltage on it, so by tying the grounds together you are making sure they are the same potential.
     
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