newbie DC question

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

That's what .basics is for :)

You got good answers here but let me nitpick since you're new.

Rob wrote in part:

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.

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!

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