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On a D.C. power supply.

Discussion in 'Power Electronics' started by jeffie3, Dec 30, 2019.

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  1. jeffie3

    jeffie3

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    May 21, 2015
    Is the negative terminal considered a ground?
     
  2. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Often yes, but it doesn't have to be.
    You could ground the Positive side, or ground the power supply mid-way between Pos and Neg to divide the voltage between the two with respect to ground. For example you could read 12v between Pos and Neg, so you've got -6v on Neg and +6 on Pos with respect to the common ground.
     
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  3. Minder

    Minder

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    Apr 24, 2015
    Earth ground or common ground? o_O
    M.
     

    Attached Files:

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  4. Nanren888

    Nanren888

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    Nov 8, 2015
    Theoretical question, or do you have something practical in mind?
    Some supplies and other equipment have a terminal actually connected to ground, though most bench supplies let you choose, by having the outputs "float".
    If you're planning to connect one or other of the outputs to a real ground, you may want to check your supply.
    .
    Philosophically, you can view just about anything convenient as ground and everything else is relative to that.
    .
    Many digital circuits nowadays are drawn with a notional ground and a positive supply. Analogue circuits are often either ground and positive or ground in the middle and positive and negative.
     
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  5. jeffie3

    jeffie3

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    May 21, 2015
    Part of the reason is I am learning. Also I have a CNC machine that has had a new control put in. A few things are confusing to me. It has 3 transformers in it. 72v, 12v, and 5v. Some things that are 12v have their negative side running to the 5 volt transformer. I think it was done so they didn't have to run more wire.
     
  6. Minder

    Minder

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    Apr 24, 2015
    What is the make of CNC controller?
    "12V negative side running to the 5v transformer" is that actually the DC common side?
    Generally all DC commons are taken to a star point Earth ground point, together with the service earth ground conductor.
    This is to prevent spurious tripping of L.S. inputs etc due to 'noise'.
    M.
     
  7. jeffie3

    jeffie3

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    May 21, 2015
    I am useing Linux cnc. I am using Mesa cards. 5i25 and 7i77. I have wanted to build this for a very long time. A friend of mine did most of the wiring. He is not available at this time to help. As I get deeper into this. I an understanding it better. The whole thing intimidated me so, I never dove into it all. But now that I am. I am feeling more comfortable. The people at linuxcnc have been very helpful. The reason I am asking here. I want to go beyond this machine and build. The question about the ground went unanswered.

    Thank all of you for the help.

    Jeff
     
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    No it did not go unanswered.
    There were explanations in each of the replies.
     
  9. Nanren888

    Nanren888

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    Nov 8, 2015
    Jeff, not sure what you are missing or wanting.
    .
    If I have an isolated 5 volt supply, I can treat the negative as ground, zero volts, if I like and the positive as +5 if that suits me.
    If I have an isolated 12 volt supply, I can do the same.
    If I now connect the negativbe of the 12 volt supply to the positive of the 5 volt, then I can't usually sensibly have two grounds with a fixed voltage between them, so I have to change my mind about where I consider earth/ground.
    .
    As above, it is common to connect the earths/grounds form multiple parts of circuits or wiring to one point so there are no loops in the earth paths for current. Finite to-ground resistances means that eart currents frm one circuit will tends to induce voiltages across any shared resitance and therefore into the other circuits.
    .
    So, if I have three supplies at different voltages, each isolated with respect to each other, it would be common to bring one side of each (say the negative) to a common point and call this the ciruit earth/ground. Often this point would be connected to true ground,
    THis would mean the ground return path form any circuit could end up in the same place, that common point for all three supplies.
    .
    Sometimes what seems a simple circuit is not so simple without details as experienced people have seen many variants of similar and need to work out exactly what you have. If you'd like to explain in more detail about the connections you have and what qustions you have, people may be able to answer in a way that best suits your situation.
     
  10. Minder

    Minder

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    Apr 24, 2015
    Obviously you did not read the answers!
    Also all this can be confirmed on a site such as the CNCzone forum.
    M.
     
  11. jeffie3

    jeffie3

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    May 21, 2015
    It seems I have upset a few. I never meant to do that. Yes on the linuxcnc board part of my question was answered. I took the advice and ran new wires. I am very thankful for the help. The part I was asking here. In some ways wasn't about the machine. I just used the machine build as an example. It was more about understanding why. I see many things thats causes me to ask deeper question. Bench top power supplies sometimes have 3 terminals. Negative, Common or ground, and positive. Computer power supplies have both 12v, and 5v positive and negative.connections. There are British cars that the positive that is considered the ground. I thought asking "is the negative considered ground" would be better to ask over here and keep cnc machine specific questions to the linuxcnc board. I guess I should have said part of the question went unanswered. So if I offended or give the wrong impression. I do apologize. I'm just trying to get a deeper understanding of electronics.
     
  12. Minder

    Minder

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    There are basically two schools of thought over earth grounding DC power supply common's etc, one, to keep every section isolated and the other to bond all DC P.S. commons to a common earth ground start point, usually a chassis mounted plate of some kind.
    I subscribe to this latter method and after several years in the CNC retro-fit business, has served me well.
    Of course, some kind of individual assessment of each system has to be made to avoid conflict and damage, but in 99.9% cases, it is usually possible
    ,M. .
     
    Last edited: Jan 2, 2020
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  13. jeffie3

    jeffie3

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    May 21, 2015
    Is there any condition to keeping every section isolated?
     
  14. Minder

    Minder

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    One such example is where the secondary of a transformer is connected to earth, and if the DC side after rectification would be grounded then it would cause a short circuit, just one of a few examples.
    Most are fairly rare now.
    Remember your P.C. PS common is also connected to earth ground via the MOBO securing screws.
    M.
     
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  15. BobK

    BobK

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    Jan 5, 2010
    The answer to your exact question is no. Whether or not the - terminal is considered ground depends on how you use the power supply.

    Suppose you have 2 12V power supplies and you want to power a circuit that needs + and - 12V.

    You will connect the + of one power supply to ground, and the - of the other supply to ground,

    Caveat: Some power supplies might have one of the terminals, likely the - terminal connected to the ground of your house wiring. If you had two of these, you could not connect then as above or you would create a short.

    Bob
     
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  16. Minder

    Minder

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    The vast majority of SMPS supplies are isolated from the AC service and earth ground. (other than the P.C. ver).
    M.
     
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  17. jeffie3

    jeffie3

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    May 21, 2015
    As you all are answering this, I am reading and coming up with more questions. Thank all of you so much. When I plug a SMPS to a 110v ac the negative is isolated from the 110v ac?
     
  18. jeffie3

    jeffie3

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    May 21, 2015
    So in a computer power supply. Where they have have connectors that are positive and connectors that are negative. How are they grounded?
     
  19. Minder

    Minder

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    Apr 24, 2015
    The typical SMPS, whether Wall-Wart or larger 24vdc etc supply, there is generally galvanic isolation between the AC supply and the DC output.
    In the case of the PC SMPS supply, the DC common is connected to the service Earth ground conductor via the chassis.
    The service GND is connected to the PC case, and the MOBO is connected to same through the board mounting screws.
    M..
     
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  20. BobK

    BobK

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    Jan 5, 2010
    A computer supply will have the ground connected to the earth ground in its 3-prong plug. It then has voltages both positive and negative with respect to that ground.

    Bob
     
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