confused!!!

[zachtheterrible]

I know that this is probably totally simple, but I have yet to understand the concept of ground, -, +, etc.

I keep on hearing reference to ground, and 0 potential. I have always thought that ground just means the negative side of a battery (if u have a 9 volt battery, ground = -9v, not 0v). The only other ground that I know of is tapping a transformer in the middle to achieve 0 potential. I am totally confused :?

thanx 4 explaining

 

[bmcculla]

All voltage is relative. There is no absolute 0V. So if you take any circuit you can just call any voltage 0V. You calculate all the other voltages as a difference from that 0V. So if you have an opamp with rails a +12V and 0V if you wanted you could relable the +12V rail as 0V and the lower rail as -12V without changing anything - the setup is exacly the same the only thing that has changed is you 0 reference voltage. The center tap of a transformer is convenient to call 0V because it makes it look like you have nice positive and negative supplies, but what you really have is 3 diffenernt voltages one high one low and one in the middle you can label them whatever you want as long as the relative voltages are the same.

This is one of the reasons that optoisolation is used: the 0V on one circuit is not nessesarily the same 0V on another circuit. You could have two circuits one has 0 and+5V the other has 100V and 105V both circuits work as 5V supplies (5V difference between the two voltages) but if you hook them together bad things happen.

 

[Optikon]

I know that this is probably totally simple, but I have yet to understand the concept of ground, -, +, etc.

I keep on hearing reference to ground, and 0 potential. I have always thought that ground just means the negative side of a battery (if u have a 9 volt battery, ground = -9v, not 0v). The only other ground that I know of is tapping a transformer in the middle to achieve 0 potential. I am totally confused :?

thanx 4 explaining

Zach, there is no such thing as absolute 0V.. all voltages are with respect to some other point. If I have a 9V battery I can call the - terminal ground or 9V_common or anything for that matter. If you have a 9V battery, ground = -9V only whith respect to the + terminal. it may not be so with respect to my wall outlet "ground" and may not be so with respect to my conductive wristwatch band.


Ground is usually the term given to a circuit node that currents return to for analysis purposes (this is easiest to see with the single 9V battery and calling the - terminal ground)

Lets call the -9V terminal ground go ahead make it = 0v. Now the positive terminal = +9V wrt -terminal (0V). If I measure the - terminal with respect to itself, I get 0V by my choice of what I call ground. Had I called the + terminal ground (0V) then when I measure the - terminal I would read -9V with respect to what I called ground (0V)

You can choose any node in your circuit to be the reference point from which all voltages will be measured with respect to. But some choices are easier to deal with than others. It would not make much sense to assign a 0V (ground) to a node that moves around depending on circuit conditions.

How would you label your circuit if you had a stack of 9V batteries connected in series? Which - terminal would you call ground(0V)? You could call any of them ground and all analysis proceeds sticking with that label. If the number of batteries is even, a common choice would be the center-most node for symmetry.

Some argue that the only true ground is earth-ground but this too is just a reference. If I build circuits in an airplane, the airplane frame makes more sense as ground than does the earth (which I cant even get to anyways)
Oh, and what would the martian designers have to say?

The best way to keep it straight is to remember that ALL voltages (All potentials) have no meaning unless they are with respect to some node which is either stated clearly on a schematic or implied.

 

[zachtheterrible]

if this is right, it means i totally get this . . . if its wrong, well . . .

A 9 volt battery does not have +9 and -9 volts all at once. You can only have -9 volts with respect to the positive side of the battery. and you can only have +9 volts with respect to the negative side of the battery.

Thank you soooo much for explaining. This is the simplest thing, but I've never understood it.

 

[Dean Huster]

And Zach, a lot of portable op amp projects put two 9v batteries in series, connecting as you normally would for a full 18v and ground the common connection. With respect to that ground, you have +9v from one battery and -9v from the other battery, 18v total.

Dean

 

[zachtheterrible]

ahhhhh, its so nice to finally get this :lol: