Help me understand the relationship between positive, negative, neutral and ground in AC and DC
This question stems from notations on schematics and seemingly conflicting information I'm seeing. I suspect that I'm seeing different vernacular for the same concepts--but I'm in a place where no one's ever told me that an "elevator" is a "lift". Then again, I may have the concept completely off, and need to be schooled so I don't blow up my workshop. :)
With DC: Batteries have +/- terminals. Most schematics I see show the circuit with voltage in and ground. I've heard that most schematics don't trace the return path to the negative terminal because that's understood and it doesn't need to clutter everything up. I've also heard that on a DC circuit, ground is SYNONYMOUS with the negative terminal. On schematics, I've seen V-in and ground, I've also seen V-in, ground, and a separate trace connecting to the negative terminal.
Then, we move to AC. There's a hot wire (positive), a neutral wire, and ground. I ~assume~ that in an AC circuit, positive correlates to positive, neutral to negative, and ground to ground. Transformers will correlate the +/- when changing DC.
What are the facts and what are the myths? How can I tell if I need to ground something to earth vs. "ground" to the negative terminal? When do I ground to the chassis of my device? Are there standard conventions in a schematic that would indicate GROUNDED ground vs. return-to-the-source ground? Or is that something you know from experience and doing an analysis of the circuit? Is it generally safe to assume I can connect ground to negative? Or are there cases in which that would be a Very Bad Thing, and how do I identify those cases?
Just trying to wrap my head around +/-/ground in AC vs. DC, and how that voltage is used...
Ground means whatever is attached to this symbol in the schematic:
Everything that touches this symbol in the schematic is actually connected to everything else that touches the symbol. Since so many things connect to it, this makes the schematic easier to read.
Usually the negative side of a battery is attached to that. But, there are many circuits that work differently. Some circuits need a negative voltage, so the positive side of a battery would be "ground". Some circuits need positive and negative voltages, in which case there could be two batteries, one with the negative side attached to ground, and the other with the positive side attached to ground.
This works because voltages are relative. Put three \$10k\Omega\$ resistors in series, and attach them to a battery. The difference in voltage from one side of the battery is 3V (because it's a 3V battery). The difference in voltage from one side of a resistor (any of the three) to the other side of the same resistor is 1V, because the battery's 3V is divided among 3 resistors of equal value.
Since voltages are relative, ground exists as a sort of assumed reference voltage. If we say an input is "5 volts", we mean "the difference between the input and ground is five volts".
In the context of AC, things aren't really different, except that tradition has done a good job of making the same term "ground" mean many things. It still could mean whatever is attached to that symbol, or it could mean that 3rd connector on the wall. More on that later.
As far as the circuit is concerned, live and neutral are no different. Pick either one, and the other oscillates between a higher and lower voltage, relatively. If all you have are those two wires for reference, they are indistinguishable.
The difference is more important when you consider safety. The things around you are at some particular electromotive potential (voltage). Current flows when there is a difference in potential. The neutral AC line should be about the same potential as most of the things around you, so in theory, if you touch it, and also Earth, you don't get shocked, because there is no difference in voltage. If you touch the live wire, you do get shocked, because there's a difference in potential.
However, I said neutral should be about the same potential as Earth, and since you are probably touching Earth, you. But, I wouldn't trust your life on it. There could be a faulty transformer on the pole near your house. There could be a lightning strike nearby. The house would be wired backwards. Or, as I mentioned the circuit will function even if the wires are reversed, it could be plugged in backwards. In the US, one of the prongs is a bit fatter to prevent this, but you never know. This is why there's the third connector, called ground or earth. This should go to a big copper rod near your house stuck in Earth, like this:
It doesn't otherwise connect to anything else. There are some times this is important for safety, and other times it's important for other reasons. Point is, it has nothing to do with the electrical power supplied to your home.
How can I tell if I need to ground something to earth vs. "ground" to the negative terminal? When do I ground to the chassis of my device?
If we are talking about a device that plugs into the wall, leave these questions to someone else. Each country has safety regulations, and these regulations exist for good reason. Buy a DC power supply that takes care of all that for you, and connect to its output, and nothing else. Don't connect to Earth through the 3rd pin on the wall or you may circumvent the safety features of your power supply.
If you are wondering if the "ground" symbol on your schematic should also be connected to box your project is in, well, it depends. Maybe you want to do that for RF shielding. Or maybe you don't, because you don't want some other device with a different idea of "ground" to touch it, which could result in noise in your circuit or melting something. In many circuits, it doesn't matter at all.
Awesome explanation. RELATIVE voltages makes a lot of sense. My circuit has a wall-wart power supply, so I've let someone else worry about that already. By the time I get to my device, it's the +9VDC my circuit requires. . I've got a +tip, -ring, and a chassis ground tab on the power jack. In this case, there's no mention of a chassis ground in the schematic, in fact V-in is a single trace from the tip. Right now, I've got all my power grounds running to the ring. But what do I do with the chassis ground? Ignore it? Connect it to the ring? Ground it to the chassis? Nothing's specified...
@dwwilson66 I'm not sure what you mean by a "chassis ground tab on the power jack." This probably merits another question, with a photo and links to the datasheet for the power supply you are using.
"As far as the circuit is concerned, live and neutral are no different. Pick either one, and the other oscillates between a higher and lower voltage, relatively. If all you have are those two wires for reference, they are indistinguishable." Let's say I have a large copper block with substantial capacitance, and attach it to neutral through an ammeter. I can only imagine no current will flow. But if I did the same to live, I think I would get a current in alternating directions 60 times per second as the block charges and discharges. This seems like a fundamental difference between them.
@markasoftware You are describing two topologically different circuits, because you've included the Earth as part of the circuit. Of course they are different: they're not the same circuit. Most circuits aren't modeled to include the Earth, because (except safety considerations) it's not relevant to the device's operation.
@markasoftware In your example currently only flows due to the capacitance between the metal block _and Earth_ (or whatever neutral is connected to). Where else do you suppose the charges come from?
If the neutral is about the sane potential as me and things around me how come we measure negative voltage in an ac cycle? It's a real mental block for me. Hopefully you get my meaning.
These symbols are used in the US (at least).
Isolation diagrams are block diagrams, which show the isolation features, such as: transformers, opto-couplers, mains connections, safety capacitors.
Example of a well-detailed isolation diagram
Example of a crude isolation diagram
This helps. I always wondered if the two different ground symbols were the same concept or not. I'm glad to see differentiation between circuit ground and earth ground. Would the signal ground be equivalent to a chassis ground? ow what?
@Trevor I don't think that I have mixed them up (although if you provide a reference I'll reconsider).
with reference to alternating current. As I understand it for half the cycle, the live is positive to neutral and for half the cycle the neutral is positive to live. So, surely you will get the same shock by touching neutral and earth as you would with live and earth. In other words non rectified ac does not have a polarity and can be connected either way.