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DC current draw question

Discussion in 'General Electronics Discussion' started by rotokid, Dec 15, 2014.

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  1. rotokid

    rotokid

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    Dec 15, 2014
    Hello Everyone,
    I just joined and I am happy to have found these forums.

    I am reconnecting and making some editions to a rather complex home security, entertainment, computer network system here at home. Everything is centrally located in the basement. The problem I have is that there are a total of 13 "wall wart" dc transformers for repeaters, hubs, etc. Of the 13 transformers 9 are 12v with varying amp/Ma ratings. The total amperage of all of the 12v units equals 7.6amps. Is it possible to use an 12v 8 amp power supply run to a screw down terminal strip then feed all of the 12v units and eliminate the pile of transformers? I would think all units such as these would have a way of regulating the amount of current they draw but I cannot afford to be wrong. I have been trying to find the answer with no success so I figure it is either an obvious answer I just don't know or a very obscure idea.

    Thank you in advance for your time and any help you can give on the matter.
    Sincerely,
    Rotokid
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Hi there and welcome to Electronics Point :)
    For 12V DC, almost certainly, yes. The only difference is that the power supply inputs of the devices would all be connected together, rather than all individually isolated from each other, as they would be if they were powered from separate wall warts. This would only be a problem if the devices internally connect their power supply inputs to other connectors in an unusual way.

    Many of these devices have data connectors that are not isolated from (have electrical continuity to) the internal circuitry, and therefore not isolated from the incoming power connector. USB ports are a good example of this. Ethernet ports are isolated, and aren't a concern, as long as you're not using "power over Ethernet" (POE").

    A USB port's 0V connection will normally be connected internally to the 0V (common, ground) rail of the internal circuitry, which will normally be connected to the 0V (negative) terminal of the incoming power connector. This also applies to desktop and laptop computers. In this case, connecting USB ports together doesn't cause a problem, because the 0V connections on all the USB ports are already commoned together.

    You can do a continuity check on each device, between the negative terminal of the incoming power supply connector and the 0V rail and shield of any non-isolated data connector. If you get continuity, there's no problem, as all of the 0V rails in the whole system will be connected together. (In analogue systems this can cause a problem called an "earth loop" or "ground loop" but this isn't an issue in digital systems.)

    If you don't get continuity, there may or may not be a problem. If that happens, and the data port is not specifically advertised as isolated, post the details of the unit here and someone will advise you.

    Yes, that's right. Each device only draws the amount of current that it needs.
     
  3. rotokid

    rotokid

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    Dec 15, 2014
    Thank you for your quick reply.
    Rotokid
     
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