What the heck is negative voltages?

[tggzzz]

I guess I didn't know this because I skip any chapter or paragraph named the dreaded "theory".

In that case you will remain blind, and will remain as successful as a 3yo child randomly performing actions to see what happens.

Not quite true. I mostly work with digital and hardware stuff which, as I see it, I have done quite well. Until the time comes when I am designing a complex product or anything else that requires full knowledge of what's going on, I wont bother wasting my time reading theory's. For now I'm happy enough knowing how to put something to gather without know why it works.

... if it works, and under what circumstances it won't work.

It mostly does work (much to the disappointment of Dave  ) and if it don't work I usually manage to fix it. If not I know where to come .

do you think it is important to understand the theory of metastability, bridging terms and correct line termination, for example?

Only if I need to. I learn as I go or require.

And if you don't know the theory you cannot know that you "need to"! If forgotten, get the things that I mentioned wrong and the equipment will usually work on the bench, but will cause  intermittent failures in the field.

Software equivalents include two-phase transactions, priority inversion (infamously on Mars), cache coherence...

[raspberrypi]

Kind of off topic but if you had 80 AA batteries (120 volts without an awesome batterizer bateroo) in series, you could connect the + side of the battery bank to a meters red (positive lead for example),and connect the black lead to a grounding rod and see +120 volts? Or what if you connected up the the negative of the batt to the red lead you would see -120 volts? I thought that AC power could do that because ultimately the negative of the generator at the power station was tied into the plug(forget that it three phase keep it simple at = and +)? Now after reading this thread I don't understand the negative voltage.

[Galaxyrise]

Kind of off topic but if you had 80 AA batteries (120 volts without an awesome batterizer bateroo) in series, you could connect the + side of the battery bank to a meters red (positive lead for example),and connect the black lead to a grounding rod and see +120 volts? Or what if you connected up the the negative of the batt to the red lead you would see -120 volts? I thought that AC power could do that because ultimately the negative of the generator at the power station was tied into the plug(forget that it three phase keep it simple at = and +)? Now after reading this thread I don't understand the negative voltage.

You are correct that AC from the wall is tied to the grounding rod (in your breaker box), so you can measure a potential between the grounding rod and the mains "live".  But that's not universally true for anything "AC", just things which are "galvanically connected" to mains.

In your example with the meter positive on the battery pack positive and the meter negative on the grounding rod, your meter would not measure the battery. It would be the same as having the red lead waving about in the air.  This notion that battery voltages are independent is what lets you stack them up to get 120V in the first place.  If all battery negative terminals were at the same 0V potential, then putting the - terminal of one battery against the + terminal of another would short the second battery! 

Your voltmeter can be considered a fancy resistor.  If current doesn't flow through it, it doesn't measure anything.  (Measuring a circuit with a voltmeter does alter the current flow in that circuit slightly, which can be a real trap on sensitive enough circuits!)  So with the positive lead of the meter connected to the battery and the negative lead "isolated" from the battery, no current from the battery will flow through the meter--no voltage measured.

Your meter will measure a positive voltage if current flows into its positive lead and out its negative lead.  (conventional current)  Your meter will measure a negative voltage if current flows into its negative lead and out its positive lead.   Current always flows from a higher voltage to a lower one when a path is present.  The sign of the voltage on the meter is telling you which probe is on the higher voltage side. 

But let's return to your example of the meter positive on battery positive and meter negative on grounding rod. Can you predict what would happen if you then attached a second (identical) meter, this one with its positive on the battery negative and its negative on the grounding rod?

[Brumby]

..... because ultimately the negative of the generator at the power station was tied into the plug

First - that is not an accurate way to think about how a generator is connected.  This will only confuse you.

[Brumby]

... if you had 80 AA batteries (120 volts without an awesome batterizer bateroo) in series, you could connect the + side of the battery bank to a meters red (positive lead for example),and connect the black lead to a grounding rod and see +120 volts? Or what if you connected up the the negative of the batt to the red lead you would see -120 volts? I thought that AC power could do that because ultimately the negative of the generator at the power station was tied into the plug(forget that it three phase keep it simple at = and +)? Now after reading this thread I don't understand the negative voltage.

You have to have a circuit.  Connecting the black lead of your meter to a grounding rod will only work if some point of your stack of batteries is also connected to a grounding rod where there is an electrical path between the two grounding rods.

Now, to understand AC - pick a spot in the middle of your 80 batteries and connect that to your grounding rod.  Connect the black lead of your DMM to a grounding rod.

Starting at the middle point, move the red lead of your DMM to each battery terminal in sequence.  (Note: each of these steps is moving along by just one battery at a time.)

The starting measurement will be 0v, then 1.5v, 3.0v, 4.5v, etc... all the way up to 57.0v, 58.5v and finally 60.0v.  Now go back down the same sequence ... 58.5v, 57.0v all the way down to 3.0v, 1.5v, 0v - and then keep going.

The very next measurement will be -1.5v.  Then comes -3.0v, -4.5v, -6.0v, etc. all the way down to -60.0v.  Reverse the sequence again and we head up ... -58.5v, -57.0v, -55.5v, etc. to -3.0v, -1.5v, 0v (where we started from), 1.5v, 3.0v, 4.5v and so on.

This is AC.

If you plot all these measurements against time you will get a waveform.

If the time you take between each measurement is exactly the same, you will graph a triangular waveform.  If, however, you were to take the measurements at a varying rate (quickly at lower voltages and slower near the maximum (+ve or -ve) voltages) you can end up with a sine wave.

The sine wave is the natural result of a generator - since it is moving with a circular motion.


Now, in the real world of AC mains, it is usual to have these grounding rods - one at the generator/substation/pole transformer and one at your premises - but this is for safety reasons.  There is actually a wire directly connecting these two (electrically equivalent) points.  This is called the 'Neutral' and it's where the black lead of your DMM has been connected for my example.  The red lead represents the other wire ... the 'Active' or 'Live'.

[raspberrypi]

So why can you measure between hot and ground? I understand why you cant connect a battery to ground, it doesn't complete the circuit let alone get the chemistry going, that was just to make an example. Imagine you have a ground rod and you connect the hot to it and measure. Would you get a voltage? I know you get a voltage on the outlet ground because neutral is connected to it. But what about an isolated ground, like the one I use for my SDR. It works as a circuit for the antenna.

[Brumby]

So why can you measure between hot and ground?

This may sound a bit anal .... but I ask for good reason:

1. What do you define as "hot"
and
2. What do you define as "ground"

[abdullahseba]

And if you don't know the theory you cannot know that you "need to"! If forgotten, get the things that I mentioned wrong and the equipment will usually work on the bench, but will cause  intermittent failures in the field.

Software equivalents include two-phase transactions, priority inversion (infamously on Mars), cache coherence...

As I said I learn as I go or require. For the small amount of work I'm doing, I don't want/have the time to study theory. If I do, and end up up not doing electronics for a few months I will forget it.
The last time I had a reread of my first electronics book "electronics for dummies  " (which despite the name is not a bad book) was when I was 14.
If I need any quick theory such as an LC circuit I just read the section I need In "practical electronics for inventors" (one of the best electronics books in my opinion )
Obviously, this has its consensuses which is my this topic is here.
The only failure in the field was a public display with a row of buttons to choose what topic you want. The microcontroller sends serial strings to a c# program with a web bowser built in and when the program receives a number from the serial port it sends a keystroke (alt+x)  x been the number received to select a html accesskey. This is only a software bug and can be fixed easily.

[tggzzz]

And if you don't know the theory you cannot know that you "need to"! If forgotten, get the things that I mentioned wrong and the equipment will usually work on the bench, but will cause  intermittent failures in the field.

Software equivalents include two-phase transactions, priority inversion (infamously on Mars), cache coherence...

As I said I learn as I go or require. For the small amount of work I'm doing, I don't want/have the time to study theory. If I do, and end up up not doing electronics for a few months I will forget it.
The last time I had a reread of my first electronics book "electronics for dummies  " (which despite the name is not a bad book) was when I was 14.
If I need any quick theory such as an LC circuit I just read the section I need In "practical electronics for inventors" (one of the best electronics books in my opinion )
Obviously, this has its consensuses which is my this topic is here.
The only failure in the field was a public display with a row of buttons to choose what topic you want. The microcontroller sends serial strings to a c# program with a web bowser built in and when the program receives a number from the serial port it sends a keystroke (alt+x)  x been the number received to select a html accesskey. This is only a software bug and can be fixed easily.

The last statement deserves another "Oh. Good. Grief". I hope you have a good lawyer.

As you only learn the things you know you need to know, one of the things you need to do is read learn and inwardly digest the first 15% of this. The bit before the "contents" panel is sufficient.

[abdullahseba]

And if you don't know the theory you cannot know that you "need to"! If forgotten, get the things that I mentioned wrong and the equipment will usually work on the bench, but will cause  intermittent failures in the field.

Software equivalents include two-phase transactions, priority inversion (infamously on Mars), cache coherence...

As I said I learn as I go or require. For the small amount of work I'm doing, I don't want/have the time to study theory. If I do, and end up up not doing electronics for a few months I will forget it.
The last time I had a reread of my first electronics book "electronics for dummies  " (which despite the name is not a bad book) was when I was 14.
If I need any quick theory such as an LC circuit I just read the section I need In "practical electronics for inventors" (one of the best electronics books in my opinion )
Obviously, this has its consensuses which is my this topic is here.
The only failure in the field was a public display with a row of buttons to choose what topic you want. The microcontroller sends serial strings to a c# program with a web bowser built in and when the program receives a number from the serial port it sends a keystroke (alt+x)  x been the number received to select a html accesskey. This is only a software bug and can be fixed easily.

The last statement deserves another "Oh. Good. Grief". I hope you have a good lawyer.

I'll get one if you pay for it.

this. The bit before the "contents" panel is sufficient.

That does not make sense in my case. I don't think I know all there is to know and I know my limits as they arise. But I know all I want to know for the present.

I do not fancy I know what I do not know

Very true, I don't.

[tggzzz]

I'd like to offer my apologies; I've just noted that the OP is, based on his profile page, young. That is a sufficient justification for some of his statements.

In particular, I'd like to commend him from asking basic questions, e.g. in this thread.

I would, however, caution him that theory is essential to understanding the practice. Yes, theory can be difficult, but that is not a reason to delay learning it. In particular, theory can indicate when you should not attempt something, because failure is guaranteed. Any engineer (as opposed to an amateur hacker) needs to be able to recognise where dragons lie.

As has been said before "if you think knowledge is expensive, you should see ignorance"!

[JiggyNinja]

Kind of off topic but if you had 80 AA batteries (120 volts without an awesome batterizer bateroo) in series, you could connect the + side of the battery bank to a meters red (positive lead for example),and connect the black lead to a grounding rod and see +120 volts? Or what if you connected up the the negative of the batt to the red lead you would see -120 volts? I thought that AC power could do that because ultimately the negative of the generator at the power station was tied into the plug(forget that it three phase keep it simple at = and +)? Now after reading this thread I don't understand the negative voltage.

Positive voltages push.

Negative voltages pull.

DC sources provide a constant, unidirectional force that doesn't change.

AC sources constantly flip between pushing out and pulling in.

This is probably the easiest way to thing of it without going into too much detail.

[Galaxyrise]

But what about an isolated ground, like the one I use for my SDR. It works as a circuit for the antenna.

I'm afraid I find that question a bit confusing.  Isolated ground as a circuit for the antenna? what?

AC voltages are considerably easier to "accidentally" create. If you were to put a metal sheet on a rubber mat, not connected to anything, and then measure the AC voltage from the metal plate to mains, you may find a voltage there.  It will be some mixture of capacitive coupling (the metal sheet being one plate of the capacitor, and house wiring or even earth as the other plate), induced voltage (mains creates an oscillating magnetic field, inducing a small current in nearby conductor loops), and acting like an antenna.  In all those cases, you're measuring a tiny current over a large resistance.  However, if you put a 1k resistor in parallel with the meter, you'd find the voltage basically disappears because there's still only a tiny current.

Or were you asking after how antennas work?

[Brumby]

But what about an isolated ground, like the one I use for my SDR. It works as a circuit for the antenna.

I'm afraid I find that question a bit confusing.  Isolated ground as a circuit for the antenna? what?

Indeed.  I, too, was wondering if, in regards to an antenna, he was referring to a ground plane.

That whole post from raspberrypi has some fuzzy bits that need to be clarified before any helpful answers can be offered.  This is why I put my questions to him (reply 31) ... but they are yet to be answered.

... and when (if?) they are, there are several more questions that might follow that will (hopefully) lead to understanding.

[raspberrypi]

So why can you measure between hot and ground?

This may sound a bit anal .... but I ask for good reason:

1. What do you define as "hot"
and
2. What do you define as "ground"

So to keep it simple forget my SDR comment.

Hot is the right side contact on a US 110VAC plug, its the one that has the voltage.
Neutral is the opposite contact that is connected to your house's ground and the other side of the power line to your house.
Ground is the round contact that connects to your houses grounding rod. Thats how I think of it. So how did early telegraph lines work that only used one wire? I always thought of a circuit as a loop. When you plug something in at your house you are ultimately connecting to both terminals on the power stations generator. So why can current flow when you only connect to one (hot side AC connector) and your houses ground? Thats not making a loop.

[timb]

So why can you measure between hot and ground?

This may sound a bit anal .... but I ask for good reason:

1. What do you define as "hot"
and
2. What do you define as "ground"

So to keep it simple forget my SDR comment.

Hot is the right side contact on a US 110VAC plug, its the one that has the voltage.
Neutral is the opposite contact that is connected to your house's ground and the other side of the power line to your house.
Ground is the round contact that connects to your houses grounding rod. Thats how I think of it. So how did early telegraph lines work that only used one wire? I always thought of a circuit as a loop. When you plug something in at your house you are ultimately connecting to both terminals on the power stations generator. So why can current flow when you only connect to one (hot side AC connector) and your houses ground? Thats not making a loop.

Your house ground and neutral are bonded at the circuit breaker box. The ground is also connected to a grounding round driven a certain distance into the soil.

The reason ground is literally grounded is for fault protection (think about the neutral wire breaking on the power pole or in your breaker box).

[Brumby]

So how did early telegraph lines work that only used one wire? I always thought of a circuit as a loop.

A circuit is a loop.  It is always a loop.  If there is current flowing, there will be a loop.  Sometimes it may not be obvious, but it HAS to be there.

The old telegraph worked by having one wire up in the air - isolated from the soil.  The soil becomes the other conductor...

When you plug something in at your house you are ultimately connecting to both terminals on the power stations generator. So why can current flow when you only connect to one (hot side AC connector) and your houses ground? Thats not making a loop.

The loop is there....

[timb]

So how did early telegraph lines work that only used one wire? I always thought of a circuit as a loop.

A circuit is a loop.  It is always a loop.  If there is current flowing, there will be a loop.  Sometimes it may not be obvious, but it HAS to be there.

The old telegraph worked by having one wire up in the air - isolated from the soil.  The soil becomes the other conductor...

When you plug something in at your house you are ultimately connecting to both terminals on the power stations generator. So why can current flow when you only connect to one (hot side AC connector) and your houses ground? Thats not making a loop.

The loop is there....

Your second image isn't quite right though, because as I said in my last post, neutral and ground are bonded at the panel. If you had to rely on the actual ground as a return, there would be a significant voltage drop, because that ground could be quite a high impedance depending on several factors (distance from substation or transformer, ground rod depths, soil moisture, soil type and so on).

[KD0CAC John]

Look at failures to really learn - as an example , if the neutral were missing coming into the house , you not have enough circut / loop to do much work , some light bulbs would light up , but any motors [ like in your furnace would not run ] .
And if you measured the hot leg , depending on the grounding system - including the makeup of the earth / dirt , you would get a varying voltage of around 50 - 90 volts .
The ground is not part of the circuit , its a safety backup - in this case if you lost neutral hopefully your ground would be enough to trip a breaker / GFI , when needed , also to bring potential down .   

[Brumby]

Your second image isn't quite right though, because as I said in my last post, neutral and ground are bonded at the panel. If you had to rely on the actual ground as a return, there would be a significant voltage drop, because that ground could be quite a high impedance depending on several factors (distance from substation or transformer, ground rod depths, soil moisture, soil type and so on).

Better?

[timb]

Your second image isn't quite right though, because as I said in my last post, neutral and ground are bonded at the panel. If you had to rely on the actual ground as a return, there would be a significant voltage drop, because that ground could be quite a high impedance depending on several factors (distance from substation or transformer, ground rod depths, soil moisture, soil type and so on).

Better?

Close enough.

[Brumby]

I'll take that at 2am.

[Vtile]

So how did early telegraph lines work that only used one wire? I always thought of a circuit as a loop.

A circuit is a loop.  It is always a loop.  If there is current flowing, there will be a loop.  Sometimes it may not be obvious, but it HAS to be there.

-Snip-

When you plug something in at your house you are ultimately connecting to both terminals on the power stations generator. So why can current flow when you only connect to one (hot side AC connector) and your houses ground? Thats not making a loop.

The loop is there....

Your second image isn't quite right though, because as I said in my last post, neutral and ground are bonded at the panel. If you had to rely on the actual ground as a return, there would be a significant voltage drop, because that ground could be quite a high impedance depending on several factors (distance from substation or transformer, ground rod depths, soil moisture, soil type and so on).

In US (in my knowledge) and many other countries, yes the GND and Neutral is attached together "inside the house", while there is also systems that are indeed build like on that picture. I now can't get it to my head the name and specific on this to my eyes exotic arrangement and I'm too lazy to walk downstairs to get the IEC-standard from the self. Even the TN-C and TN-S while in fault state can allow the GND to Neutral etc. to rise in to tens of percentages of the line voltage, because of line(cable) resistances and high currents if not done properly.

PS. Terra-Terra the system were. I'm not particularly sure how it is wired inside the consumers property, since it is not in use here. https://en.wikipedia.org/wiki/Earthing_system
Just to note in a place like this that the TNC/TNS aren't the only ones in use.

[Cerebus]

Your house ground and neutral are bonded at the circuit breaker box. The ground is also connected to a grounding round driven a certain distance into the soil.

The reason ground is literally grounded is for fault protection (think about the neutral wire breaking on the power pole or in your breaker box).

A word of warning (which I suspect Tim already knows), that scheme (ground and neutral bonded at the building's incomer) is not universal.

It's true for domestic installations in the US (and many other places) but there are a number of supply schemes where it is not true. You can read about various schemes here including the IEC terms for them. The usual US domestic  scheme is a TN-C-S scheme where the protective conductor stops at the incomer and doesn't physically continue to the substation/pole transformer. The commonest UK one is TN-S, where separate  protective and neutral conductors run all the way to the substation and are grounded there.

Never assume that neutral is at ground potential and never assume that it will stay there (even if you've measured it at ground) unless you know the particulars of the supply in question. Failure to observe this rule can result in "Bang! You're dead!" type outcomes.

[Brumby]

A word of warning (which I suspect Tim already knows), that scheme (ground and neutral bonded at the building's incomer) is not universal.

Yes, please understand that any diagrams I have provided are only for the purposes of illustration.

There are several power distribution structures in use around the world.  If it isn't obvious enough, let me be very clear: DO NOT rely on any of these diagrams for accurate depiction of your situation.