Why is there no GND on my DC Powersupply?

Discussion in 'General Electronics Chat' started by cds333, Nov 6, 2007.

  1. cds333

    Thread Starter Member

    Nov 5, 2007
    16
    0
    Hey there,

    I know this is a stupid question, but here goes:

    Why does my power supply have only a (+) and a (-), whereas the more expensive power supplies have a (+) , a (-) , and a GND?

    Isn't the (-) and the GND the same thing?:confused: I mean, in every schematic I look at, there is only the latter two.

    I know it has something to do with digital, but I haven't yet grasped the concept of +5V, 0V, and -5V. I am still learning the basics and I only know about the "binary" system of (+) and (-). How does a battery (or another DC power source) put out 3 different voltages?

    If anyone can help me understand this I will be most grateful. :D

    -cds
     
  2. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    In most power supplies the + and the - would be floating (isolated). A third Gnd connection is really not necessary. Some power supplies may have an additional output connected to protective earth and then you can use this if you need it. If the PS has a metal case then it would normally be grounded to PE.
     
  3. Dragon

    Active Member

    Sep 25, 2007
    42
    0
    A wise person once said, "a question that IS NOT asked is a stupid question."
    Relax, you just excluded it from that category! Infact it goes on to show your interest in clearing major concepts.

    + and - terminals are somewhat 'local'. Current takes the path from the positive to the negative. The GND is an external terminal, from which the rest of the terminals are isolated. It is directly connected to EARTH ground.

    Its difficult to explain everything here. I found an interesting link that summarizes the 'terminal' concpets with applications.
    http://www.ese.upenn.edu/rca/instruments/misctutorials/Ground/grd.html
     
  4. cds333

    Thread Starter Member

    Nov 5, 2007
    16
    0
    I thought current flows from (-) to (+), or is that voltage? I read somewhere that this is because electrons are (-) charged, and that "positive" voltage is actually the "holes" in the conductor moving through it.

    Anyway, thanks a lot for the link; I now know the difference between the chassis and the earth GND symbols. Up until now I always wondered why people used them interchangeably. I do have a GND terminal on my oscilloscope, which I gather is to eliminate the 60hz interference that may come from my standard 110 electricity, but I thought that a power supply manufacturer would, IDK, maybe put that on the back of the case as opposed to right on the front next to the other terminals.

    Most of my current uncertainty has sprung from my descent into the realm of digital electronics. Until recently I had always experimented with analog stuff- mostly RF and ham radio related circuits. I just the other day acquired my first PIC programmer, as I was getting tired of spending $50 per BASIC stamp.

    I am having trouble understanding the concepts of +5V, and 0V as a power source. In analog applications it was simple, you had +12, and -12 (or whatever); but now that I am seeing this 0V, I just don't see how you get +5V and 0V out of a standard DC powersource. I thought that once my previous question was answered, I would understand; however it seems to be unrelated:(

    I am using National Instruments Multisim (an "Electronics Workbench" virtual simulator) and when working with digital CMOS chips, I must use Vdd and Vss "nets" that are in essence a +5V and 0V source in place of my real-life power supply which puts out +5V and -5V. I just don't see how a powersource can not have a (-)counterpart:confused:

    If you (or anyone else) can direct me towards a simple explanation, I will be forever grateful. Also can anyone tell me the definitions for Vcc, Vdd, and Vss. I've tried to Google them but come up with nothing :(

    Thanks again,

    -cds
     
  5. Dragon

    Active Member

    Sep 25, 2007
    42
    0
    Current flows from -ve to +ve. Agreed! But we take 'conventional' current from +ve to -ve terminal. Makes no difference. Just a convention.
    Remeber, voltage never 'flows', its the current. The concept of holes in semi conductors is a different one. Dont mix it up.
    Voltage on the other hand just indicates the possiblity of flow of current.

    I dont understand why you mix +5V with 0V. The ground/neutral/common is percieved to be at 0V.

    Next, you are mixing up the concept of negative supplies with the terminals of the supply. On a power supply, the +ve and -ve terminals dont indicate whether you can have a negative voltage through it. If its a +5V supply, then it cannot supply -ve voltage. The -ve terminal visible on the supply is for providing the return path to the current.

    Negative supplies are made by inverting their +ve couterparts. The concept of 'inverters'. But no need of going into its detail.

    As for Vcc, it is used with analog circuits.
    Vdd and Vss are like +ve and -ve terminals for a digital circuit.
     
  6. Distort10n

    Active Member

    Dec 25, 2006
    429
    1
    The (-) terminal is common while the GND connection is connected to chassis ground. Chassis ground is literally tied to the earth through the third prong of the electrical socket (earth ground).

    These things are hardly ever explained in class!
     
  7. jpitz31

    Active Member

    Oct 24, 2007
    37
    0
    This brings up a good question for me.

    GS3 indicated "In most power supplies the + and the - would be floating (isolated)."

    Here is the definition I am using for isolated:

    Two circuits that are completely electrically separated with respect to DC potentials, and almost always AC potentials. In power supplies, it is defined as the electrical separation of the input and output via the transformer.

    What then is floating and what is the relationship to isolated?

    Thanks

    Joe
     
  8. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    I usually use the term floating to mean disconnected. When you connect a battery to an ungrounded circuit, the complete circuit is floating with respect to ground. But all measurements are referenced to one terminal of the battery which I will refer to as common. The potential between common and ground is unknown. You could say that the whole circuit and battery are floating with respect to ground.

    Floating essentially means that there is no path to ground. Ground is earth ground and is defined as zero volts. Common is the return to the source.

    In some circuits, you will find a 6.8 uf capacitor between common and ground. The capacitor represents an AC connection to ground because AC signals see a finite impedance between common and ground. However, any DC potential will not find ground because the capacitor is like an open circuit to a constant DC voltage.
     
  9. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    There is no such thing as a stupid question. My suggestion is: If anyone uses derogatory terms in response to your question, ignore that person. You can also report that person to the staff of this forum.
     
  10. jpitz31

    Active Member

    Oct 24, 2007
    37
    0
    Thanks John,

    "Floating essentially means that there is no path to ground. Ground is earth ground and is defined as zero volts. Common is the return to the source."

    If Ground is defined as zero volts then would Common be defined as zero volts as well?

    Thanks

    Joe
     
  11. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    If you connect the (-) output (usually black) to your project ground, then the (+) output will provide a positive voltage to your project.

    But, if you connect the (+) output (usually red) to your project ground, then the (-) output will provide a negative voltage to your project.

    This is what is meant as a floating output, it has no reference to ground, the outputs are referenced to each other.

    The "GND" lug (usually green) on a power supply is connected to the power supply chassis and to the AC plug ground. This connection, typically, is not related to the red/black outputs. Therefore if you want a safety ground, you will also connect the green AC ground to your project ground.

    I use the term "ground" loosely, whereas "common" may be more appropriate.
     
  12. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    The usual term applied to circuits is "isolated."
    Lets consider a simple circuit: A battery connected to a bulb in a flashlight. The voltage across the battery is known, but the voltage between any terminal of the battery and ground is undefined. Therefore the whole circuit is floating with respect to ground. Most technical people would say that the circuit is isolated from ground.

    Is common at zero volts? If you are measuring between common and ground, the voltage is unpredictable because the resistance between common and ground is infinite. Common does not have to be at zero volts. In our little circuit, the bulb does not care what the voltages are on each wire with respect to ground, long as the bulb only sees it's rated voltage across the two wires connected to it. Another words, the bulb sees the difference in voltage on the two wires, not the voltages with respect to ground.
     
  13. cds333

    Thread Starter Member

    Nov 5, 2007
    16
    0
    Let's see if I'm beginning to understand:

    So even on the 3 terminal power supply ((+), (-), GND), I will only get positive voltages. The GND terminal is simply if I need to connect to earth GND, and the (-) terminal is like the return path, or "common" for the (+) terminal; Right?

    This whole concept of negative voltages is not very well covered in my textbooks. And when I hook up my multimeter or scope to my PS terminals, it says +5 and -5, depending on the polarity of the probe.

    So you're saying that when referring to my basic DC-benchtop-powersupply, I should think of the red terminal as +5V, and the black terminal as 0V; and to attain a true -5V, I would need a "dual polarity" or "negative" powersupply?

    Thanks again to Dragon and everybody else for taking the time to assist me with these basic concepts. It's so frustrating because everyone who writes books on electronics has already mastered the concepts, and therefore take for granted stuff like this when writing the books.

    -cds
     
  14. Dragon

    Active Member

    Sep 25, 2007
    42
    0
    Absoultely right!

    Right again!

    Right!!

    Yes. But only upto a certain extent. Authors at times have to skip minute details. You cannot blame them!:p

    Such things are learned through patience and experience. When people interact (like the way they do on forums), they are learning.:)

    AAC is different from other forums because people here believe in learning through discussions! My experience was similar to yours. I stumbled upon a basic problem, googled, and landed here. What followed was a continuous following of this forum!

    Also, do check the AAC book for interesting concepts which other books overlook at times.
     
  15. jpitz31

    Active Member

    Oct 24, 2007
    37
    0
    Ok,

    I am getting some conflicting info here, just want to clear this up?

    nomurphy indicates that:

    Then Dragon indicated:

    "Next, you are mixing up the concept of negative supplies with the terminals of the supply. On a power supply, the +ve and -ve terminals dont indicate whether you can have a negative voltage through it. If its a +5V supply, then it cannot supply -ve voltage. The -ve terminal visible on the supply is for providing the return path to the current.

    Negative supplies are made by inverting their +ve couterparts."

    And indicated "Right" to the below statement:

    "So you're saying that when referring to my basic DC-benchtop-powersupply, I should think of the red terminal as +5V, and the black terminal as 0V; and to attain a true -5V, I would need a "dual polarity" or "negative" powersupply?"


    If I want a -voltage supply I have always flipped the (+) and (-) leads on the power supply.

    Could someone clear up this concept?

    Thanks

    Joe
     
  16. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    even zero volts actually never exists so having a -ve voltage is impossible at least i do not have a knowledge of one.
    if u consider above replies with -ve taken as lower potential and + as higher.
    all those statements speak for themselves.
    like connecting + to ground will bring ground to that high potential and now - is low wrt grnd which we may call as -ve (since we have a bad habit of calling ground as zero volts) and vice-versa. it should be noted that circuit can not be complete without both + and -ve connected . ground are sometimes connected to avoid completing circuits thru large conductors as the metal body to which can act as one. common terminal which is ref is also referred to as ground.
    an example: we connect an led in forward bias by connecting either of the terminals to grnd and other to the reqd terminal .the grnd can be connected to + or - while the other terminal to led.

    EDIT:about dual supply the common point is such that pd measured across it with respect to other terminals is same but with diff.
    polarity since this point is higher than - and lower than +ve. again i think nothing but a confusion due to a different reference.
     
  17. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    I don't think Dragon is being accurate when he answers "right" to all your questions/statements.

    The isolation of the +/- on your power supply allows for either a positive or negative supply depending upon which terminal is common to your project ground. As you've noticed, the DC polarity depends upon how you connect your scope or DMM; in other words, which side of the probe is "common" determines the polarity.

    In better dual supplies, it is typical for both supplies to be isolated from each other. Which means that if you want a split supply (let's say +/-5V), you will need to connect the positive terminal of one supply to the negative terminal of the other supply, and this common point to the project ground. The -5V will be available from the open (-) and the +5V will be available from the open (+).

    You would have to do the same thing if you wanted to connect two separate supplies, such you have, to make a a split supply. Such isolation also provides the ability to combine supplies in parallel for higher output current, or in series for higher output voltage.

    An example of using a dual type of supply, would be if you made an audio amplifier circuit using op amps that required both positive and minus supplies (Google TL082 and check the datasheet).

    Also examine a full-wave bridge design using a transformer with a center-tap on the secondary that is common, the FWB will create an equal +/- supply relative to the center-tap which is typically referred to as circuit ground. However, if you were to measure between this "ground" and the AC or wall ground (say 115VAC) supplying the transformer primary, you would measure about 45VAC (typically, as I recall) -- and, this is why a person can get shocked from instruments that are not properly grounded -- you're circuit "ground" (common) is not necessarily earth ground.
     
  18. Dragon

    Active Member

    Sep 25, 2007
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  19. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    Lets consider two +5 volt power supplies. First, we have to isolate the minus terminal from ground on both supplies. The first power supply plus terminal provides plus five volts. The minus terminal of the first power supply is connected to the plus terminal of the second power supply and is labeled common. The minus terminal of the second power supply provides -5 volts.

    +________________+5 volts
    PS1
    -__
    +__|----------------Common
    PS2
    -__________________-5 volts

    A +5 volt power supply can not provide -5 volts with respect to ground. But if we isolate the minus terminal from ground, then simply using a meter to measure the voltage at the minus terminal [red lead on minus terminal, black lead on plus terminal] will prove my point.
     
  20. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    I don't agree with the last part. To simplify, the "+" and "-" terminologies are relative. You can consider your "-" pole your ground if you want a positive voltage PSU and the "+" pole the ground if you want a negative voltage supply.

    The part about the ground and earth is not true. It really depends on the PSU. Ground is a relative concept if it is floating, and in most PSU's, the ground is not connected to earth, only the chassis is. It is a mistake in a PSU to connect the output ground to earth, since the ground (especially a bad one) may generate potential to fry a circuit, especially if the supposed earth ground is "live" and the circuit is in contact with other ground, eg. the floor. So, don't trust earth for grounding.

    Bottom line, it is a good idea to keep isolation from the plug.
     
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