different types of ground

Discussion in 'General Electronics Chat' started by PG1995, Apr 27, 2012.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Hi

    There exist different kinds of grounds: chassis ground, earth ground, circuit ground, signal ground. My query (which consists of some related queries) for this post is about earth ground and circuit ground. I proceed with other queries once this one is settled.

    Circuit ground is essentially a reference point to measure voltages at different points around circuit. Earth ground is used to protect human life from electric hazard and for this purpose third wire (yellow/green) is used to take back the current to the ground.

    Do you think this power supply is dual polarity one? Don't you think to be completely sure one must refer to the manual/booklet which comes with the supply? I have once used this supply and its ground terminal is to be used as earth ground hence it cannot function as a dual-polarity supply. Would you refer to this supply as a multi-polarity one?

    Sometimes a term "floating ground" is used, what does it mean?

    Please help me with the queries above. Thank you.

    Regards
    PG
     
  2. Veracohr

    Well-Known Member

    Jan 3, 2011
    552
    76
    I think a "floating ground" is a common, like a circuit 'ground', that isn't connected to the earth.
     
  3. Stuntman

    Active Member

    Mar 28, 2011
    181
    47
    The key here is knowing that an isolated (make note) power supply "ground" can be at a different potential than earth ground.

    Why is this you ask?

    Let's pretend you want to build a 50V 10A power supply. All you have are two 25V 10A (isolated) supplies. Why not wire the two in series (connect + of PSU1 to the - of PSU 2)? Think AA batteries... If the supply (or AA batteries) were not isolated, connecting them in this way would not work. (instead, at least one supply would get shorted directly to ground).

    So, an isolated supply allows you to reference your voltage to something OTHER than earth ground. This has a variety of other uses besides being able to put multiple supplies in series, but you get the idea.

    The unit you show is a dual power supply, providing both + 15 and - 15V. I cannot tell, but the unit may be "isolated" meaning, it provides these voltages with respect to the ground you provide. Or, it may be tied internally to earth ground. I assume this supply is for some kind of special equipment that uses the connectors on the right? (ETA, I only noticed the "middle" picture when I loaded this post)

    As for a floating ground. When I use this term, it generally describes a grounded conductor with unsufficient current capacity for the circuit, therefore allowing the ground to develop a non-negligible voltage. Think of a voltage drop across a long string of wire going from the positive lead of a battery to a lightbulb. Now consider that you will can see the same effect if you were to run this long wire from the negative (ground) terminal of the battery to the light bulb.
     
    Last edited: Apr 30, 2012
    PG1995 likes this.
  4. MrChips

    Moderator

    Oct 2, 2009
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    Going by what I see in the photos:

    [​IMG]

    This PSU is a "floating suppy". The GND connection is CHASSIS GROUND.
    You can turn this into a +ve or -ve supply depending on how you connect the GND.





    [​IMG]
    The second PSU shown above is not floating. The COMMON is connected to CHASSIS GND.
    This is a bipolar PSU, supplying +15V and -15V as required in many op-amp circuits.





    [​IMG]

    The last PSU is not floating. This PSU provides many different voltages that are typically required in a desktop PC to supply many devices such as disk drives, SD memory interfaces etc. as well as the mother board.
     
    PG1995 likes this.
  5. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thank you, Veracohr, Stuntman, MrChips.

    @MrChips: I have found your post helpful but I need some help with the terminology.

    I take that by GND and COMMON you refer to circuit ground. You say that this supply is a floating supply. What does it mean? How do I turn it into a negative supply? Although this supply also has three output terminals (+), (-) and (GND) just like the other supply, you still say it's a dual-polarity supply. What thing in the photos leads you to this conclusion? Please help me. Thanks.

    chassis ground
    A connection made to the metal chassis on which the components of a circuit are mounted, to serve as a common return path to the power source.
    [McGraw Hill Science & Technology Dictionary]

    Regards
    PG
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    [​IMG]

    A general purpose bench PSU as shown above provides you with all possible options for configuration.

    The metal banana jack with the EARTH GND symbol is an indication that this terminal is connected to the GND pin on the power cable. You can check this with an ohm-meter.

    You can use this power supply in four ways:

    1) Connect the +ve and -ve terminals to power your circuit. In this case your circuit is "floating".

    2) As in (1) but connect the -ve terminal to the GND terminal as you have shown in RED.
    All voltage readings on your circuit will be +ve and referenced to EARTH GND.

    3) If you need a -ve voltage in your circuit, connect the +ve terminal to EARTH GND.

    4) If you need a voltage that is greater than what a single PSU can supply, you can stack two or more supplies in series just as you would with single AAA or AA batteries, for example, connecting +ve from one PSU to -ve to the next PSU. Make sure you do not connect the GND terminal.

    If you need a bipolar supply of +15v and -15v to power an op-amp circuit you can use two PSU connected as in (2), (3) and (4). Connect the +ve terminal of one PSU to the -ve terminal of the second PSU. Connect this junction to GND. You can connect to GND at one or both PSU.
     
  7. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thank you for the reply, MrChips.

    So, what if earth is not part of the wiring? Actually where I live only a number of places use earth ground. For safe purposes circuit breakers are used. I believe in the absence of earth ground, this supply will always be floating. What do you say? Please let me know. Thank you.

    Regards
    PG
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,442
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    [​IMG]

    This is a bipolar or split power supply delivering +15v and -15v as indicated on the front panel. The other give-away, besides the labeling, is the colour used for the three banana jacks, RED for +ve, BLACK for -ve and GREEN for GND. These choices of colours are common but not universal. On automotive wiring it is common to use BLACK for +12V and WHITE for -ve or GND.

    The GREEN COMMON terminal may or may not be CHASSIS GND or EARTH GND.
    To verify this, use an ohm-meter and measure the resistance from the GREEN terminal to something metallic (such as a fastener) on the case, assuming the case is also metallic. The resistance should be less than 0.1Ω for proper CHASSIS GND.

    Similarly, measure the resistance from the GREEN terminal to the GND plug on the three-pin power cable. Again, the resistance should be less than 0.1Ω for a proper EARTH GND.

    There is one other consideration when using a bipolar or split PSU in op-amp circuits. If the +ve and -ve supplies are independent then the two voltages may not both be at 15V amplitude if the loads are different. This can introduce offset voltages in the op-amp circuit. A proper op-amp power supply should use tracking supplies where the amplitude of both +ve and -ve supplies track each other regardless of load differences on the two supplies. The specifications and/or circuit diagram of this PSU should reveal if this is a tracking dual supply.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Does the PSU use a 3-prong plug? Is your wall outlet a 3-prong socket?

    For low voltage applications such as most electronic circuits, it is ok not to connect to the EARTH safety GND.

    Most consumer electronics are powered by wall-warts or AC adapters and these do not have GND connections.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    There is one other consideration when choosing power supplies.

    The two PSUs shown above are most likely linear supplies. These are preferred when powering analog circuits such as sensitive op-amp applications.

    The third PSU with the multiple voltages is a switching supply. The switching noise in the output may adversely affect the performance of sensitive analog circuits.
     
  11. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    It used 2-prong plug and in general earth ground is not used here. Almost all devices use 2-prog plugs. So, don't you think that PSU will always be floating and it's ground terminal is useless? Please let me know. Thank you.

    Regards
    PG
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Yes. All your circuits will be "floating". It would be interesting to know in what country you are living.
     
  13. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thank you for the reply.

    No, it's not country wide things. Only some areas have their ground wire missing and there are reasons for it. Once again many thanks for the help.

    Best regards
    PG
     
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