Electronics > Beginners
Some noob questions
<< < (25/28) > >>
james_s:
Any time you have two or more identical symbols with the same name on a schematic it means those points are all connected together. If you look at the actual physical circuit, "ground" will typically just be a trace like any other on a board that ties all the indicated points together. It may also be connected to the metal housing, but it doesn't have to be. It might connect to earth ground but again it doesn't have to. All the schematic tells you is that those points connect to each other, and calling it "ground" is common for a return path used by a large number of nodes in a circuit. It is usually tied to the negative terminal of the battery but not always, until the 1950s many cars were positive ground, doesn't really make any difference which way the current is flowing as long as every part of the circuit is in agreement. Terms like "ground" are really just imaginary concepts, but you have to pick some point in a circuit to call ground because any voltage measurement has to be the potential between two nodes. If you say a circuit should have 3.5 Volts on a certain pin of a component, that's assumed to be 3.5V relative to the node called "ground", otherwise you'd have to ask "Relative to what?" for every measurement.
james_s:

--- Quote from: Mr D on August 17, 2018, 09:42:35 pm ---Many thanks so far guys, i'm starting to develop the smallest inkling of understanding.

But still some things not making sense.

Attached is a pic with two, independent circuits.

To my mind, the connection of the ground at different points in the circuits should make no difference whatsoever, as they're simply connections to a bus that itself isn't connected to anything, not even to the negative battery terminal.

However, what i observe is that the voltage potential is different in the circuits. How can the voltage potential be effected when the ground is going to a dead-end street (bus leading nowhere)?

--- End quote ---

The voltage potential is different because when you measure a voltage, you are measuring it relative to ground.  If you define a different point as ground, you will see a different voltage relative to it.

The two circuits are indeed identical from a functional stantpoint, the only difference is in describing quantities. If you are standing in the street level of a building that has a basement below, you could call the place where you are standing "ground" and say that the floor above you is positive 1, and the basement floor is negative 1. Now you could instead decide that it makes sense to call the basement floor ground, then the lobby is positive 1 and the floor above that is positive 2. The building configuration has not changed, only the perspective from which you describe it. In order to specify the floor level you have to agree on what your measurement is relative to and in most cases it's relative to the ground, ie what you're standing on when you're outside the building. It doesn't have to be though, you could call the top floor 0 and assign negative numbers to any lower floors but it would likely confuse somebody else if you asked them to meet you on the negative 7th floor because the general assumption is that floor 1 is the ground level floor.
Mr D:
So are you saying that in reality, in that last pic i posted, the two circuits are actually identical, and the reason the EveryCircuit app shows a different voltage potential in the two circuits is that it's imagining putting the black lead of it's DMM in a different place?

So wherever you choose as "ground", if you put your black DMM lead there, the DMM will call that point 0v and work out the rest accordingly?
james_s:
Yes, that's exactly it. Whatever we call "ground" is the 0V or reference point, to which all other measurements in the circuit are relative.

You can also take a voltage measurement between any two points you want, so you might say "the voltage across resistor R3 should be 0.8V" and that would be perfectly valid, but if you were to just say "the voltage at the top of R3 is 0.8V" that implies relative to the node called ground, not necessarily across R3. Note that my "R3" example is completely arbitrary and not in any way related to the specific circuit(s) you have posted.

When dealing with devices powered by the national electrical grid, ground also happens to be connected to earth for safety and practical reasons but with something powered by an isolated source like a battery or a transformer "ground" can be anything you want as long as you are consistent.
Mr D:
Great, many thanks to all who replied, i feel i've come a long way in one day as this morning it made no sense to me whatsoever and now i feel like i have a bit of a handle on it!

And now to bed!

And to be continued! ;)
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod