Please help me understand + and - power supplies

[Eric R Snow]

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

 

[JeB]

On Thu, 06 May 2004 16:16:54 -0700, Eric R Snow <etpm@whidbey.com>
wrote:

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?

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.

 

[Eric R Snow]

On Thu, 06 May 2004 20:07:56 -0400, John Popelish <jpopelish@rica.net>
wrote:

Eric R Snow wrote:

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

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.
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

 

[grahamk]

Eric R Snow wrote:

On Thu, 06 May 2004 20:07:56 -0400, John Popelish <jpopelish@rica.net
wrote:

Eric R Snow wrote:

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

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.
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

Have a look at http://homepage.ntlworld.com/g.knott/elect281.htm

 

[John Popelish]

Eric R Snow wrote:

On Thu, 06 May 2004 20:07:56 -0400, John Popelish <jpopelish@rica.net
wrote:

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.
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?

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.

--
John Popelish

 

[John Popelish]

Eric R Snow wrote:

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

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.

--
John Popelish

 

[R. Steve Walz]

Eric R Snow wrote:

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
-------------------

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
--
-Steve Walz rstevew@armory.com ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public

 

[Fraze]

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


Eric R Snow <etpm@whidbey.com> wrote in message news:<fihl9016vcgl6u5i3d90lqb14krv0qbvj0@4ax.com>...

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