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Difference between ground and -ve voltage

Discussion in 'General Electronics Discussion' started by Vamsi Krishna. M, Mar 4, 2016.

  1. Vamsi Krishna. M

    Vamsi Krishna. M

    2
    0
    Feb 13, 2016
    In a battery or a DC Power supply,What does -ve terminal mean?a ground or a -ve voltage?If it is a -ve voltage then what is ground?or if it is a ground,how can I supply -ve voltage to something (like op-amp)?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,136
    1,844
    Nov 17, 2011
    -Ve is a voltage negative with respect to ground. Ground or 0V is typically the reference potential within a circuit.
    When you connect the "+" pole of a battery to gnd, then the "-" pole of the battery is negative with respec to ground. You can use this a -Ve.

    For more detailed information show us your circuit diagram (schematic).
     
  3. Colin Mitchell

    Colin Mitchell

    1,419
    314
    Aug 31, 2014
    THE POWER SUPPLY

    [​IMG]
    Sometimes you will see a circuit as shown in the first diagram with 12v or +12v on the top rail and 0v or a negative sign or the word "negative" on the bottom rail. In this case the word negative means earth or "chassis of a car" and we commonly refer to this as "negative earth" or "negative chassis."
    In the second diagram, the output from a power supply has a positive 12 volts and a negative 12v with the 0v rail in the middle. In this case the negative 12v rail is twelve volts BELOW the earth rail and that's why we call it the NEGATIVE RAIL.
    This means that when you hear "Negative Rail," you need to work out if it means the negative terminal of a battery (as in the first case - meaning 0v or earth) or if the voltage is below zero volts (as in the second case).
     
    Arouse1973 likes this.
  4. Anon_LG

    Anon_LG

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    Jun 24, 2014
    Measuring voltage is a relativity game. There will always be a point of zero volts, termed "common" or, more often, "ground".

    All voltages that you measure are stated in relation to this point.
    voltages.jpg

    So a positive and negative supply can be created from sources that would otherwise just be used as a positive voltage supply. It is all about the placement of the point that is decided as common.

    The diagram above will work with any batteries, ie, if you were to replace them both with 5 volt sources, you would get a 5 volt negative and positive supply.

    The symbol with three lines denotes the 0 volt position. Strictly speaking, it should be connected to an earth rod, but any point of 0 volts is nearly always shown as such with this symbol.

    I hope this helps,
     
  5. Minder

    Minder

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    Apr 24, 2015
    It is, but unfortunately incorrect.
    The Common or chassis connection International symbols should be used.
    It can lead to a lot of misunderstanding especially once you actually intend a point to be at Earth ground.
    In other cases where this symbol is used and someone actually interprets it to be earth and no galvanic isolation occurs in a power supply, it blows the P.S.!:(
    M.
     

    Attached Files:

    Harald Kapp likes this.
  6. Anon_LG

    Anon_LG

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    Jun 24, 2014
    It should be used for a true earth connection, but the fact is that you will find it commonly used as common in many, many circuit diagrams. Always be careful, but remember that battery based circuits very rarely need a ground connection, so you should be safe if the same is used.

    Minder has provided the correct symbol denotations. Also, remember that the digital/analogue ground may be marked with an A or D to represent ones relevant to Analogue or Digital respectively. These should be used if no connection to a ground rod or a large metal body or chassis is present.

    It is incorrect, but it has become informal convention among many.

    *Edit: A little off topic, but I thought I would mention that a chassis or earth connection can be important for a few reasons. A ground connection can prevent the build up of charges in circuits, the (usually) moist soil of earth dissipates any of these potentially destructive charges. A more novel use is the potential difference created between an aerial and a ground connection, with a rectifier and a capacitor, an LED can be made to briefly light if touched across the capacitor. An interesting demonstration of the power of electromagnetic radiation surrounding us constantly. Other uses include RF dissipation. Of course, one of the most noticeable uses is that of safety. The chassis of mains appliances can be earthed, therefore if the chassis becomes hot, due to a short, the dangerous live current has a less resistive path to ground. This has the obvious effect that the path to earth (usually <1Ω), diverts the current away from the person that comes into contact with the chassis.
     
    Last edited: Mar 4, 2016
  7. Minder

    Minder

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    Apr 24, 2015
    Unfortunately that does not make it right, you will also see circuits drawn correctly by those that know the difference and conform to the standard.
    One of the main perpetrators of this error in conformity is the widely acclaimed book 'Art Of Electronics' which use it throughout the book, of which they IMO should know better. and results in teaching of bad habits.
    M.
     
  8. Anon_LG

    Anon_LG

    453
    117
    Jun 24, 2014
    The long and short is. Use a little triangle, the three lines are wrong unless you are connecting it to the Earth itself.

    Vamsi may have got more than he asked for on this one.
     
    Minder likes this.
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