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Please help me understand + and - power supplies

Discussion in 'Electronic Basics' started by Eric R Snow, May 7, 2004.

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  1. Eric R Snow

    Eric R Snow Guest

    I see in catalogs that some power supplies supply both positive and
    negative voltages. A battery has electrons coming out one end, going
    through the load, and returning back into the other end. It would seem
    that a DC power supply should do the same thing. Apparently that's not
    the case. Could someone please explain this?
    Thank You,
    Eric R Snow
     
  2. JeB

    JeB Guest

    picture two batteries in series with wires from each end AND where the
    two meet. If you call the + end +1.5v and the middle 0v then the bottom
    end will be -1.5v. You still have electron flow, the + or - depends on
    what your refernce is.
     
  3. There are single supplies (that have a + and - output terminals) and
    stacked paris of supplies that have 3 output terminals ( +, - and the
    common terminal between them that is the - terminal of the positive
    supply and the + terminal of the negative supply). These dual
    supplies are equivalent ot two batteries in series that have a wire
    attached to the node where the two batteries touch.
     
  4. Eric R Snow

    Eric R Snow Guest

    Ok. Now I understand how they are wired. But why do it this way? Why
    not just reverse the leads to the devices powered?Is it be because the
    powered devices are interacting with each other?
    Eric
     
  5. grahamk

    grahamk Guest



    Have a look at http://homepage.ntlworld.com/g.knott/elect281.htm
     
  6. This sort of stacked dual supply is very handy when dealing with
    direct couples (capable of amplifying all the way to
    DC) amplifiers that have to accept and deliver AC waveforms that swing
    positive and negative with respect to a common (ground) node. One
    supply powers the positive half cycles and one powers the negative
    half cycles.
     
  7. -------------------
    Don't think of power supplies that way!!

    You cannot HAVE a supply that electrons "come out of" without that
    SAME supply ALSO having a connection that electrons "go into".

    When you have a multiple supply, you have several supplies all
    referenced to the same connector as their common "zero volt"
    reference, which means that the voltage of a single supply is
    spoken of as +12V when its + is considered +12V and its negative
    is regarded as its "zero", relatively speaking. ALL voltage is
    strictly relative to its other end.

    Here's how to rig two batteries as a +/-9V supply, simply by
    connecting them so that they share a common "zero" volt GROUND
    which then means the others have to be +/-9V.

    http://www.armory.com/~rstevew/Public/Tutor/OpAmps/DualOpAmpPS.gif

    You can just as easily connect to power supplies that way.

    -Steve
     
  8. Fraze

    Fraze Guest

    I would guess that your confusion is based on the idea that DC could
    be viewed as either + or - depending which way you would hook up the
    leads of a DMM, or which side you determine is your reference side.
    I've heard this same thing from many many technicians, "There really
    isn't anything as negative voltage, it's just the way you look at it".
    Well, ok in certain instances this is true. But consider this:
    If you have a battery in a certain circuit and the negative side it
    connected to a common earth ground and the positive side connected to
    a resistor which is connected to earth ground, the electrons would
    flow from ground, through the resistor, to the positive lead of the
    battery. (I am horrible at drawing schematics, so I will not trouble
    you with my scribbling.) Since electron current flow states that an
    electron will flow to a more positive source, we can conclude that the
    battery terminal was more positive than ground (0 volts). The battery
    is a constant positive voltage source. Now if we had a way to cause
    electrons to flow through the resistor to ground from a terminal of a
    constant voltage source, ground was obviously more positive than the
    terminal of the source. Guess what, you must have had a negative
    voltage source. So there most definitely is such a thing as negative
    voltage.
    In linear devices, such as resistors, it really makes no difference
    which way the current flows. But as Mr. Popelish stated, there are
    devices such as op amps, discrete transistor amplifiers, etc. that
    need negative voltages to function correctly.
    So to sum up my ramblings, there is a difference between negative and
    positive voltage sources. The positive is X volts as referenced to
    ground and the negative is -X volts as referenced to ground.
    I hope this helps.

    Fraze
     
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