newbie DC question

[Rob Snyder]

Greetings.

I've been studying electronics on my own for a little while now, and
there's one basic concept I can't seem to get my head around. I
apologize in advance for the really basic question, but I just can't
find the answer and it's keeping me up at night.

I thought I understood DC, at least at a primitive level. In circuits
with a battery, things make sense - there is a positive and a negative,
current flows from the negative through the circuit to the positive...
makes sense.

Where I get lost is when I'm looking at a circuit with an AC to DC
transformer. Typically, the output of this, after the rectifier, is a
positive DC source, a negative DC source, and ground.

What *is* this ground thing? Does current flow from the negative output
to ground? From ground to the positive? Both? Neither? What they heck am
I missing?

Most explanations I've read just say something to the effect of "ground
being a reference, with the current being more negative or more positive
than ground". Needless to say, I can't turn that into something I
understand.

I appreciate any guidance anyone has to offer.

Thanks!

Rob Snyder

 

[Bob]

"Rob Snyder" <rsnyder@reversedthebrokenglass.com> wrote in message
news:qJ3Ad.38117$ld2.15952316@twister.nyc.rr.com...

Greetings.

I've been studying electronics on my own for a little while now, and
there's one basic concept I can't seem to get my head around. I
apologize in advance for the really basic question, but I just can't
find the answer and it's keeping me up at night.

I thought I understood DC, at least at a primitive level. In circuits
with a battery, things make sense - there is a positive and a negative,
current flows from the negative through the circuit to the positive...
makes sense.

Where I get lost is when I'm looking at a circuit with an AC to DC
transformer. Typically, the output of this, after the rectifier, is a
positive DC source, a negative DC source, and ground.

What *is* this ground thing? Does current flow from the negative output
to ground? From ground to the positive? Both? Neither? What they heck am
I missing?

Most explanations I've read just say something to the effect of "ground
being a reference, with the current being more negative or more positive
than ground". Needless to say, I can't turn that into something I
understand.

I appreciate any guidance anyone has to offer.

Thanks!

Rob Snyder

Rob,

Great question.

The term "ground", in electronics, is often misused. Technically, if you
have a ground connection it means that some part of a circuit is connected
to earth (or the ground).

People often use the word "ground" when they really mean "common". A
circuit's common is merely any node in the circuit that has been given the
name "common" (or "ground") by some human. Typically a circuit's common is
another name for one its power supply connections, and it's often also
connected to a piece of equipment's chassis (metal). Sometimes this "common"
is also connected to earth (ground) -- usually via the third wire (green) of
a three wire electrical outlet.

As you probably know, if you have a battery and you connect a passive device
(e.g., a resistor) across the two terminals of that battery, the current
(conventional) will flow from the more positive lead of the battery to its
more negative lead. The current can be made to flow from negative to
positive, but this would require the inclusion of some type of
energy-storing device like a battery, capacitor, or inductor in the circuit.

It's very typical to have the output of a transformer hooked up (to diodes
and capacitors) such that you end up with three nodes -- a "common", a
positive supply (+), and a negative supply (-). This will act the same as if
you've connected to batteries in series (plus of one to the minus of the
other), with the middle node being what you're calling "common". No current
will flow into the earth (ground) with this hookup because it's not
connected to ground. If you connect a resistor between your (+) and common
then some current will flow. The same amount of current will flow if you
connect that resistor between common and (-). More current will flow if you
connect that resistor between (+) and (-).

I haven't discussed the (very real) capacitance that is always present
between any conductor and earth. This will allow currents of that circuit to
flow to earth, for time-varying signals of that circuit, even though there's
no apparent connection between your circuit and earth. For now, however,
don't worry about this.

Hope this helps.

Bob

 

[John Popelish]

Rob Snyder wrote:

Greetings.

I've been studying electronics on my own for a little while now, and
there's one basic concept I can't seem to get my head around. I
apologize in advance for the really basic question, but I just can't
find the answer and it's keeping me up at night.

I thought I understood DC, at least at a primitive level. In circuits
with a battery, things make sense - there is a positive and a negative,
current flows from the negative through the circuit to the positive...
makes sense.

Where I get lost is when I'm looking at a circuit with an AC to DC
transformer. Typically, the output of this, after the rectifier, is a
positive DC source, a negative DC source, and ground.

What *is* this ground thing?

It is a convenient reference point with which to measure many voltages
in the circuit. It may or may not also be connected to the Earth with
a grounding wire.

Does current flow from the negative output
to ground? From ground to the positive? Both? Neither?

Once you pick a convention (electrons travel around the circuit one
way, holes and 'conventional current' the other way) just be
consistent it its application. If you want to think of current as a
flow of electrons, then current will take all paths that connect a
more negative voltage to a more positive one. Ground, by definition,
is a zero volt reference point.

What they heck am I missing?

Most explanations I've read just say something to the effect of "ground
being a reference, with the current being more negative or more positive
than ground". Needless to say, I can't turn that into something I
understand.

You have to separate in your mind voltages from currents. Nodes can
be have a more positive or negative voltage than ground. Pick any two
nodes, and one may be the more positive and the other the more
negative, or vice versa. This potential difference is what drives
current through the things connected between those nodes. Currents do
not have the same sort of positive and negative polarities that
voltages do. Current polarities just refer to whether an actual
current goes the assumed way (positive current) or the opposite way
(negative current) so that you can add them up correctly for things
like Kirchoff's law (the sum of all currents into a node is zero).

I appreciate any guidance anyone has to offer.

Thanks!

Rob Snyder

Have I helped or added to your confusion?
--
John Popelish

 

[Terry]

"Rob Snyder" <rsnyder@reversedthebrokenglass.com> wrote in message
news:qJ3Ad.38117$ld2.15952316@twister.nyc.rr.com...

Rob wrote in part:

What *is* this ground thing? Does current flow from the negative output
to ground? From ground to the positive? Both? Neither? What they heck am
I missing?

ROB: Just to see if I can add to your confusion by using some examples.

My car has a 12 volt battery. The negative terminal is connected to the
chassis of the vehicle.
So within the vehicle that is a 'common' point for all the negative
connections. I suppose in a way it is the vehicle 'ground' although clearly
it is not connected to the ground since the vehicle rolls on rubber tyres!

Many years ago my father had a car in which the positive end of the battery
was connected to the frame/chassis of the car so the positive potential was
'common' in that car!

In telephone exchanges they used very large 48 volt battery installations
charged by large rectifiers. In almost all cases the positive side of the
battery was connected to the ground of the building. This -48 volt power was
used to operate electromechanical telephone equipment. But there were often
other 130 volt power supplies inside the same building that had their
negative end connected to the same ground; this +130 volt power was mainly
used to operate tube type equipment. So that 'ground' was a common point
reference point for both supplies.

Occasionally you will get circuits where neither positive or negative is, or
needs to be grounded, these often referred to as 'floating'.

So the term 'ground' is best thought of as a common reference point, for a
particular piece of equipment or situation.

Just to confuse some more? Most telephone circuits require a pair of wires!
But there were and are very simple telephone and signalling systems which
use one wire as a connection and the ground as the other connection. Often
called "One wire and ground/earth systems".

Email if you'd like to discuss some more. Terry.

 

[Rob Snyder]

John smith wrote:

"Rob Snyder" <rsnyder@reversedthebrokenglass.com> wrote in message
news:qJ3Ad.38117$ld2.15952316@twister.nyc.rr.com...

Greetings.

What *is* this ground thing? Does current flow from the negative output to
ground? From ground to the positive? Both? Neither? What they heck am I
missing?

Thanks!

Rob Snyder


I remember when I was a kid...
Behind my grandfather's TV was a bottle of Coca Cola with dirt in it and a
piece of wire
from the TV in the dirt.
I asked my father WTF? (not in those words.)
It wasn't until I started reading about "toobs" and other stuff that I
realized...
In Spanish literal translation, Ground = Earth = dirt (tierra)
So, there was a bottle of dirt connected to the TV.

Rob, you are in the right place. You got some good questins and good
answers. Keep'em comming to try to
get these kids in the NG straight and narrow. Otherwise they turn into black
matter.

Wow... all of these responses were extremely helpful to me, and I'm
grateful for each person's time in responding. Thanks!

 

[Darmok]

All good responses you are getting here, one thing you are likely to see in
books or in formal classes that can initially be confusing is the convention
of current flow. Most texts even today define current to flow from positive
to negative as the standard "convention". This makes sense intuitively as
flowing from higher potential (or pressure, think water), to lower. But in
reality we know that current flow is the movment of charge (electrons) which
is the opposite, for the most part (I don't want to talk about hole current
right now).

But as already mentioned for the purposes of solving circuits it doesn't
matter what convention you choose as long as you stick with it and stay
consistent in the circuit analysis. I just wanted to bring it up in case you
decide to formally study electronics or engineering, then you WILL see this.

Best regards,

Darmok

"Rob Snyder" <rsnyder@reversedthebrokenglass.com> wrote in message
news:qJ3Ad.38117$ld2.15952316@twister.nyc.rr.com...

Greetings.

I've been studying electronics on my own for a little while now, and
there's one basic concept I can't seem to get my head around. I
apologize in advance for the really basic question, but I just can't
find the answer and it's keeping me up at night.

I thought I understood DC, at least at a primitive level. In circuits
with a battery, things make sense - there is a positive and a negative,
current flows from the negative through the circuit to the positive...
makes sense.

Where I get lost is when I'm looking at a circuit with an AC to DC
transformer. Typically, the output of this, after the rectifier, is a
positive DC source, a negative DC source, and ground.

What *is* this ground thing? Does current flow from the negative output
to ground? From ground to the positive? Both? Neither? What they heck am
I missing?

Most explanations I've read just say something to the effect of "ground
being a reference, with the current being more negative or more positive
than ground". Needless to say, I can't turn that into something I
understand.

I appreciate any guidance anyone has to offer.

Thanks!

Rob Snyder