Bench power supply ground plug is positive!?

Discussion in 'General Electronics Chat' started by McPit, Jul 27, 2015.

  1. McPit

    Thread Starter New Member

    Aug 17, 2014
    11
    0
    Hi there!
    If I set my bench variable power supply to say 5V DC and connect my voltmeter between the negative and positive terminals I read 5V DC. Perfect.

    Now my understanding was always that Ground = DC negative (not positive).
    Yet if I connect my voltmeter between the ground and positive terminals of my power supply I get 0V.. and if I connect it between the negative and ground terminals I get 5V.. :confused:

    Can someone please explain this to me?

    Thanks a lot!
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    I assume you have three terminals on your power supply; red, blk and green.
    There are three useful connections for your power supply:

    1. leave the green terminal unconnected, You how have a "floating" 5V supply which is isolated from the ground prong on the line cord, with 5V between the red and the blk terminals. The green terminal is connected to earth ground through the third prong on the line cord. Only leakage shows between either red to green or blk to green.

    2. connect the green terminal to the blk, You now have +5V supply on red because blk is connected to earth through the third prong on the line cord.

    3. connect the green terminal to the red. You now have -5V supply on blk because red is connected to earth through the third prong on the line cord.

    The readings you made are bogus because you are using a very high input impedance meter.
     
  3. McPit

    Thread Starter New Member

    Aug 17, 2014
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    0


    Mm.. I don't understand...
    Let me maybe try and summarize my test more visually:

    PSU 5VDC
    + GND -
    |.........|
    |.........|
    +........-
    Voltmeter: 5VDC. Perfect.



    PSU 5VDC
    + GND -
    |.....|
    |.....|
    +....-
    Voltmeter: 0VDC. Why? Shouldn't it read 5V?



    PSU 5VDC
    + GND -
    ......|....|
    ......|....|
    .....+....-
    ......Voltmeter: 5VDC. Why? Shouldn't it read 0V?
     
  4. Ramussons

    Active Member

    May 3, 2013
    557
    92
    Ground is just a reference point. What if it is a -5 Volts power supply?

    Imagine a 9 Volt battery. If you connect the "-" terminal to ground, you will get 9 volts between "ground" and + terminal.

    What happens if the + terminal, instead of the - terminal, is connected to "ground" ? What will you get between "ground" and the - terminal?
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    You are getting confused by some internal leakage inside the power supply, or you have a non-standard power supply.

    Can you provide a schematic or model number of the the supply? Did you build it? Is it a converted PC supply?

    What is the input resistance of the Multimeter you are using to measure the DC voltages?

    With the supply off, have you measured the DC resistance between Red(+5V terminal) and Green (Earth ground) ? Between Blk(0V terminal) and Green (Earth ground)?
     
    Last edited: Jul 27, 2015
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  6. McPit

    Thread Starter New Member

    Aug 17, 2014
    11
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    Ok great suggestion, and my ohmmeter shows open between negative and gnd, and close between positive and gnd.. I guess that explains it all..
    Now why is that? Is it a plain simple manufacturing error!?

    This is the exact psu in question:
    http://pages.ebay.com/link/?nav=item.view&alt=web&id=111711812295&globalID=EBAY-AU
    (Yes chinese cheapo needless to say.. but otherwise good enough for my basic needs.)

    Schematic here:
    http://www.eevblog.com/forum/review...bench-psu/?action=dlattach;attach=22502;image
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,798
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    Is that the right schematic? There's no obvious + or - 5V output. To add to the confusion, the negative pin of C39 (bottom left corner of pic) is marked 'GND', whereas it should be at about -18V (courtesy of zener D25) relative to the circuit common indicated by the triangle symbol made up of parallel lines.
     
  8. McPit

    Thread Starter New Member

    Aug 17, 2014
    11
    0
    Model 305D, it's supposed to be..

    It is a variable power supply, I only mentionned 5V as an example, if that's why you're asking(?).
     
    Last edited: Jul 27, 2015
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Okay.
     
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