lab power supply, ground, negative voltage

Discussion in 'General Electronics Chat' started by PG1995, Apr 17, 2011.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Hi

    Please have a look on this picture:
    http://img696.imageshack.us/img696/8841/powersupplyg.jpg

    If I connect one lead to the ground terminal and the other one to -ve terminal, I will have 0V to -30V. Correct? But what does this really mean? What is the direction of electron current? Before you answer this, I would request you to go through the entire post.

    I have always thought -ve terminal is simply taken the ground because conventional current flows towards this terminal.

    In 'actuality' electrons (or electron current) flows from -v terminal toward +ve terminal. I could get 0V to 30V when I have leads connected to +ve and -ve terminals. But I could also get the same range of voltage, 0V to 30V, when I have one lead connected to the +ve and the other one to the ground. Is then the ground also a '-ve' terminal because there should be something which is pumping electrons into +ve terminal?

    I hope you can see my confusions and I do hope you would be able to help me out to clear them. Thanks a lot.

    Regards
    PG
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    [​IMG]

    you can host pictures here. Some members are reluctant to go offsite to unknown websites to view pictures, or to open file attachments with .zip, or .doc extensions.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You will probably have 0 volts, because the ground terminal usually is the metal shield of the device, and not connected to the actual supply (or through a quite high resistance).
    If the ground is actually connected to the supply, it will most likely be connected to the negative terminal, again giving you 0 volts.

    You will get the regulated 0 to 30V between the + and - terminals, that is for sure. (or 0 to -30V between the - and + terminals)
     
  4. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Not sure about that. Some days ago I had the lead coming out of +ve terminal connected to the lead coming from the ground which formed a junction. Then I attached the +ve terminal of the DMM to this junction, and the other terminal of the DMM to the -ve terminal of the power supply. The DMM gave me -ve readings. What do you make of it?
     
  5. mbxs3

    Active Member

    Oct 14, 2009
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    What to you read if you switch your DMM to read resistance and you read between the "-" jack and the ground jack on your supply?
     
  6. mbxs3

    Active Member

    Oct 14, 2009
    141
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    Also, is the power cord for this unit 2 prong or 3 prong? I would imagine it is 3 prong. If so, does the "E" lead of the power cord read continuity to the ground jack on the front of the power supply? If it does then there is a very high chance that the ground jack is connected to the chassis.
     
  7. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
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    Hi mbxs3

    Sorry. I couldn't understand your above post. I'm not an English speaker, perhaps that's the reason for this.

    It's 2 prong. I'm sure. Now what do you suggest?

    Please help me with this. Thanks.
     
  8. someonesdad

    Senior Member

    Jul 7, 2009
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    The usual convention is that the metal binding post is connected to the metal chassis of the power supply. It's also usual to have a three-prong power cord plug so that the chassis is grounded to the earth safety ground in the wall socket. Not having that three wire cord is unusual -- I wouldn't be surprised if someone cut the original off. I prefer having that ground connection precisely because it gives a reference potential. This allows you to connect either the + or - output of the power supply to the grounding terminal, which then lets you source a negative or positive voltage. Or, you can leave the terminals floating; it's your choice as a user.

    When there's room for it, I like to add a switch that allows me to connect the + or - terminal to ground or leave it floating. I've always wished manufacturers supplied a switch for this (a recessed slide switch would be the choice), but it's typically not done.
     
    PG1995 likes this.
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    There is some relationship between grounding and electron flow, in the sense that certain dominant electronic devices have a negative common electrode. In the past, electron tubes filled this role, but nowadays NPN transistors and N-Channel FETs are mainly used.

    Although the negative side of a power supply is most often grounded, this is not always done. Some systems like operational amplifiers have both positive and negative supplies, with a grounded common rail. Less commonly, a negative supply is used with a common positive return. This is found in telephone equipment (-48V supply) and some logic families (ECL).
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    I agree, but the OP could make a simple measurement and confirm, if he has the unit.

    In any case, you do not need a true ground for power supplies and op amps.

    Creating a Virtual Power Supply Ground
     
  11. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thanks a lot, everyone. I'm much grateful to you all for your guidance. I will ask some follow-on questions soon.

    Best wishes
    Jackson
     
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