Laptop power supply grounding

Discussion in 'General Electronics Chat' started by jack33, Jan 1, 2011.

  1. jack33

    Thread Starter Member

    Dec 31, 2010
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    My original laptop power supply has a 3-prong AC plug (with a ground pin). I have a new power supply with the same power specs which has a 2-prong AC plug (no ground pin). The ground pin on the original plug is directly connected to the negative terminal of the DC power plug that plugs into the computer (tested with ohm meter). I have tried the ungrounded power supply with the laptop and it works fine as far as I can tell. All the desktop computer power supplies I've checked have the earth ground connected through to the common wires. An old laptop power supply I checked has an earth ground pin, but it is not connected through to the DC power plug. Apparently some laptops are supplied with ungrounded power supplies.

    Here are my questions:

    1. Is there any problem with operating my laptop with the ungrounded power supply?

    2. Is the common (or ground) inside the computer connected to earth ground or is it isolated internally?

    3. I built a bench power supply from a desktop computer power supply. The common (or ground terminal) on the power supply is connected to earth ground. Are there any potential problems with that? Most DC power supplies seem to have the DC ground isolated from earth ground.

    Thanks,
    Jack
     
  2. #12

    Expert

    Nov 30, 2010
    16,314
    6,818
    1) no
    2) Common inside the computer is connected to several things that look like ground, like the shells of connectors, but it is not connected to "earth" unless "earth" is connected to it through the power supply or some other device that is connected to "earth".
    3) No.

    You need to think about things like, "earth" actually means Planet Earth when you are talking about the mains supply. Common or ground are usually a convenient point from which to refer your voltage measurement. Neutral of the mains supply is connected to earth, but it has more implications. For instance, neutral is also called the current carrying ground for the mains voltage and often has a tiny AC voltage on it due to return current flowing through the resistance of the neutral wire, while "bond" is the third wire in a 3 hole receptical and that wire never intentionally carries current except during fault conditions. This is an incomplete explanation, but it will get you started on thinking about the various definitions of "earth", "ground", "common", and "neutral".
     
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  3. jack33

    Thread Starter Member

    Dec 31, 2010
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    0
    Thanks for the answers, #12. Is there a benefit in connecting the "bond" wire of the AC mains to the common of the DC output, as is done in the original laptop power supply and in desktop power supplies? The return path of the DC current from the load is thus connected to the bond wire and also to the common connections in the power supply. Since the bond wire is ultimately connected to the actual earth, I'm not sure what that looks like to the DC system.
     
  4. #12

    Expert

    Nov 30, 2010
    16,314
    6,818
    Don't go get a wire and add a "bond" to the DC circuit. The bond wire never intentionally carries current under normal operating conditions.
     
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