Hot and ground definitions in simple 12V systems

Discussion in 'General Electronics Chat' started by Rob5, Jun 2, 2011.

  1. Rob5

    Thread Starter New Member

    Jun 2, 2011
    The common understanding is that in auto/RV simple 12V circuits the positive side is called hot and the negative side is called ground. I have been trying to understand this terminology and its origin.

    I understand that in 12V systems that are grounded to earth the side that is not grounded is hot. The same reasoning applies in 120V systems, that is, the wire that is not grounded to earth is hot.

    But in simple 12V systems that are not grounded to earth the same terminology seems to be used and the distinction appears arbitrary to me. As best as I can tell the negative side is called ground because it is typically run through the frame as sort of a giant buss while the positive side is run through only through wires. So in older positive ground cars the negative would be hot, right?

    Is this logic correct or is there something I am missing?

    This may be nitpicking but I am afraid I am missing something in my understanding of 12V systems.

    Thanks for your help.
    Last edited: Jun 2, 2011
  2. #12


    Nov 30, 2010
    You are right. It's arbitrary or "conventional". (Conventional means somebody had a convention and they decided to call it "ground".)

    Ground is a matter of convenience. It eliminates clutter from drawings. It helps you think in simpler terms. Use it for what it was meant for, a matter of convenience.

    ps, if this didn't clear it up for you, ask again.
    Last edited: Jun 2, 2011
  3. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    Just think, If you used the term 'common'. Doesn't that have a more 'intellectual' feel to it. Try it when your talking to engineers, and if they were using the term 'ground' (when not referring to earth), they'll quickly correct their mistake.
  4. #12


    Nov 30, 2010
    Good point. Common and ground are used interchangably so much that I have gone on for several hundred words explaining the many ways these words are used.
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    It's not quite arbitrary as far as amplifying devices are concerned. Amongst the most popular devices are the NPN bipolar transistor and the N-channel FET, both of which have negative common (ground-like) electrodes in the most frequently used configuration.

    Historically, electronic valves (tubes) also had the negative cathode or filament as the dominant common electrode. There was however an intermediate period when PNP germanium transistors were quite widely used. These used positive-ground circuits, and from reading articles written at that time, it appears that some of the old-timers found them rather confusing.

    Having said that, a lot of circuits use both positive and negative "hot" supplies with a separate common or ground line (e.g. op-amps). There are also a few types of system which really do use a positive ground, such as landline telephones.
  6. steveb

    Senior Member

    Jul 3, 2008
    Also, when talking to engineers, if they use the term 'hot', when not referring to Jenifer Lopez, or other similar interest, - make them correct their mistake. :p
  7. someonesdad

    Senior Member

    Jul 7, 2009
    Some of this devolves from the concept of connecting a single conductor of a circuit to the Earth, allowing the Earth to be used as a reference of potential. It was convenient and practical. That's another use of the term "ground" -- as a reference of potential, regardless of whether it's electrically connected to the Earth or not.