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Thread: V- versus ground

  1. #1
    Join Date
    Oct 2014
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    Default V- versus ground

    I have noticed several posts referring to V+ and V- when talking about power supplies and the correct way to power inject. I am curious about the terminology.

    Why is ground referred to as V-?
    Isn't V- a voltage of opposite polarity to V+?

    My PC power supply has both 12+ and 12- but it also has ground. I don't think I should be connecting V- with the V+ circuit unless I had a very specific use case.

    Don't we mostly use V+ and ground in our light hobby?
    Isn't a WS2811 pixel using V+ and ground?

    Someone fill me in on V+, V- and ground please.

  2. #2
    Join Date
    Aug 2008
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    Default Re: V- versus ground

    It's all about what it's referenced to. A "ground" is exactly that, a connection to the earth. Here in north america, on AC systems, the neutral (AC Return path) is bonded (connected with very little resistance) to ground. There is a lot of electrical engineering into why this is, but I won't go into it here. The "ground" connection is designed to prevent electrical shocks by providing a path for any stray current in a chassis (exposed metal) to a place that won't harm anything. This can be current from a short, mis-wire, or static discharge - something you don't want getting sent into other devices.

    When it comes to power supplies/DC side of things, we continue this system - except most of the time we omit the "ground" connection and people like to interchange that term with negative. Technically this is improper to do as they are not the same thing. The big thing is to know that voltage is always a reference to something else. Most circuits reference v+ to v-, like a battery. This is where we get the v+ is "positive" and v- is "negative", but you can compare the voltage a v+ to ground, it may be different than that to v-. take a look at high-end bench power supplies, you'll see a v- gnd and v+ for this reason (and other noise/isolation reasons). Things we deal with - most connect v- to ground, but there are instances where it's not, and it can be dangerous if you don't understand this.

    Take your pc power supply as an example. Because they have multiple voltages, they don't call it v+, they call it whatever the voltage is referenced to ground. so if you take +3.3v, it is 3.3v between that line and ground, so a voltmeter with the red lead on +3.3v and black on gnd you get 3.3v. same with +5v, and +12v. now -12v is different - if you keep black on gnd, and red on -12, you get -12v. the PC uses these different voltages to mix-and-match to get what it needs. need 7v? grab +5v and +12v. need -

    In our hobby, it generally means this:

    v+ = positive (5v, 12v etc)
    v- = negative or 0v. Normally tied to ground.
    Ground = a rod in the dirt used to dissipate unwanted current.

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