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Question about grounding

Discussion in 'General Electronics Discussion' started by Chengjun Li, Dec 14, 2014.

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  1. Chengjun Li

    Chengjun Li

    84
    7
    Oct 21, 2014
    Hi, I have a question about some materials I've read in terms of grounding. Below is the material.

    "
    III
    An Illustration of a Typical Power Supply Grounding Error

    As previously pointed out, the ground symbol, in many cases, has been taken as a generic symbol in electronic circuit diagrams to represent the current return path, even though no physical earth ground is used. This can cause some confusion to the novice engineering student when using instruments having an earth ground terminal. As an example, Figure 3 shows the front panel of a typical power supply. The supply is represented as a variable voltage battery. Note that three terminals are shown: a positive, a negative, and a ground terminal. The ground terminal of the supply is tied to the case of the instrument, which in turn is wired to a true earth ground such as a water pipe.

    Let's look at the load connection in Figure 3. Using the positive terminal of the battery and the ground terminal does not complete a current return path to the energy source (battery), so no current will flow from the source, i.e.

    Iload = 0.

    [​IMG]

    Figure 3 A Common Power Supply Grounding Error

    The positive and negative terminals must be used to have a return path exist. Use of the ground terminal will be discussed in a following section.

    Many circuits require both positive and negative voltages. A power supply must be used to provide each polarity. The supply for positive voltage will have the negative terminal as a return, and the negative supply will have the positive terminal as the return. These two terminals are connected together, forming a common return path for load current. Figure 4 shows

    the proper connections for these supplies to provide positive and negative voltages.[​IMG]

    Figure 4 Power Supply Configuration for Dual Polarity Voltages


    Although it may be shown as a ground in the circuit diagram, the connection between the negative and positive terminals of the supplies results in a common, or floating, return. If students feel that they must faithfully adhere to the circuit diagram, the floating common may be connected to the earth ground terminal of the supply. Generally, it will neither help nor hinder circuit performance."

    The first question is why the author says the floating common can be connected to earth ground while it won't influence the circuit performance?If the common is connected to earth ground, then it is no longer a floating source,right? Another question is from the material, it seems this kind of power supply is a floating one. I happened to find such an power supply, I set the voltage level the same with a battery and connected it to a circuit, but the performance is different from the battery, Why? Maybe it is not a floating power supply as the author says.
     

    Attached Files:

  2. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    A battery will not behave the same way, simply because of how it works...
    It's a chemically driven power source, and when you connect a load to a battery, the current draw causes a dip in the output voltage due to 'internal resistance'.
    This is true for power supplies as well, but to a lesser extant. They are commonly built with feedback to prevent the voltage from dipping, even under load. The exception here would be unregulated supplies, or using an excessive load.

    Now as far as the floating question, it would really depends on your circuit. Some power supplies are not 'floating' and have an output tied to ground, this will cause some issues. So as long as it is a 'floating' supply, and your circuit is also floating it will make no difference in the circuit's operation. You said it yourself that a return path must be present. So if there was only one 'ground' connection, do you think it would cause a current flow? Likewise, if there were multiple 'ground' connections and they were all electrically common there would be no flow.
    Only if two 'ground' connection were at different potentials would this be an issue.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Right. But before you connected the common to earth, it was floating, and that's what the author is getting at. The output needs to be floating before you connect either side of it to earth, otherwise you may have a problem.
    In what way is the "performance different" from a battery? Power supplies and batteries do behave differently. Neither of them is a perfect voltage source.
     
    Arouse1973 likes this.
  4. Chengjun Li

    Chengjun Li

    84
    7
    Oct 21, 2014
    For the attached circuit,the two resistors have same resistance value. I connect the +/- polar of a battery to the A, B point correspondingly. Then I measure the voltage of A to ground and B to ground, I find that the potential of A and B will always be 2+ half the battery voltage and 2- half the battery voltage. But when I use a power supply, I couldn't observe such phenomenon.
     

    Attached Files:

  5. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Please provide a complete diagram.
    How are you powering the opamp?
    If you are connecting A and B directly across a battery, are there any additional connections made to said battery? If not, this input is floating, and the value of resistor A or B will not make a difference.
     
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