What the heck is negative voltages?

[abdullahseba]

Hi
I'm sure this has been asked many times before but I cloud not find an answer to satisfy me.
What is the difference between positive, negative and common?
Example:

I always thought that if is do this I will simply double the voltage. 
So say the power supply's are 6V each, if I connect a bulb across the V+ and common or V- and common I would have 6V going through the bulb right?
And if I connect the bulb across V+ and V- I would get 12V?
So what is all this positive and negative voltage?
Its especially confusing in Op-Amps. Why would I amplify a voltage to a negative?


Thanks in advance

[StuUK]

Simply the voltage relative to a particular point.. i.e. the 0v/Ground... I'm sure someone on here will give a more thorough explanation (I hope)...

[Photon939]

I'll give this a shot -

Yes you are correct in that if you have two 6v psus, the total voltage from v- to v+ is still 12v.

As StuUK mentioned it all has to do with your point of reference. You may only want to deal with voltages from Gnd to +5v, but devices like opamps cannot output voltages right at the limits of their power supply (0v to 6v in this case) so you supply the opamp with a larger voltage range (12v) but split it in half. Your output circuit may only use a 0v to 5v or so range but the opamp is running on 12v so it can have full operating margin when outputting at 0v (as it has -6v on its -v power rail).

This is also often used when you are dealing with AC signals. Having a voltage tap at 1/2 your supply voltage is useful for things like controlling voltage offsets.

[Red Squirrel]

Yeah the common is just something you define yourself.  You could make the "- v" the common if you want, then you'd have +6 and +12 taps (assuming a 6v psus).

Heck in telecom the positive is ground.  You have a 48v battery  bank and positive is earth ground so it's actually a -48v supply.

You could have some batteries and PSUs in series and centre tap and define that as the common ground. For ICs and other chips the ground would connect to that.  Ex: op amps.

[abdullahseba]

Heck in telecom the positive is ground.  You have a 48v battery  bank and positive is earth ground so it's actually a -48v supply.
 

oddly enough its an op-amp in DTMF decoder for a telephone im working on.
http://www.microsemi.com/products/telephony/dtmf-transceivers/mt8880c
I think I give up I still don't understand why you would amplify some thing in to negative voltage.
So does AC swap polarity at say 50Hz or does go from its positive to negative voltage at 50Hz.
What I don't understand is how anything below 0V have any power?

[filssavi]

As you said there is ALMOST no difference between a 6 volt dual supply and a 12V single ended one in the end if you measure between the V+ and V- of the opamp you will still have 12V so why bother with all this c**p?

To understand why this is done, you will have to study the internal schematic of the classic opamp, you will be able in particular to spot 3 main blocks, a first gain stage(Differential pair current source loaded), then you have another gain stage (current source loaded common emitter/source) and an output power stage (class AB usually),  now if you take the input stage for example, as you get close to both rails (V+ or V-) a numer of transistor will want to go out of their usual operating mode (for example FET transistors will tend exit saturation and go linear) which is REALLY, REALLY BAD NEWS, in fact the moment this happens the gain amplifier will jump off the cliff and any further input variation will be lost at the output.

this happens because the mosfet at the bottom of the differential pair(the current source) , or the one at the top (the active load), depending on the input stage flavor (2 NMOS or 2 PMOS used as gain elements) will need a Vds drain source voltage to operate (about .15 to .2 volts for FET's for example).

now if you have a single 0 to 12V supply your input voltage will be from 0.15V to 11.75V (the numbers are just an example depending on the input stage topology and design spec they will vary wildly) so to amplify very low voltages you will need V- rail that goes below 0V, as simple as that

the same thing is valid also for the output stage (maybe more so than for the input one)

now you could use an asymmetric power supply (let's say +12V -1/2V) if you just need to get the input and output down to ground but once you add the DC/DC converter (or transformer tap) to get the negative supply you might as well go all in and have equal V+ and V- so you don't have asymmetric common mode voltage limits and so on


what you seem to have misunderstood is that voltage is not absolute, by saying 0V you are tipically saying that there is no potential difference between the point you are measuring and the pole stuck in the ground close to your substation transformer so by saying 6V you are saying that there is a potential difference with a magnitude (absolute value if you want) of 6V and that the point at your end (the V- terminal) is the one close to you has more negative charge than the one at the transformer-pole-in-the-ground point ( I hope I've been clear enough) to put it brutally the sign is there to tell you which way the electrons are going

[slicendice]

Watch this tutorial by Dave where he explains what an OPAMP is, what it does, and how to use it. In this tutorial you will see why we sometimes might need negative voltage as reference.



Hope this one helps you understand the importance of negative voltages.

[Ice-Tea]

This may help you... Conside a battery. A common way of thinking about an AA cell is to say the "-" terminal is at "0V" and the "+" at +1V5. This is, however, entirely arbitrary (albeit logically and intuitively pleasing). Saying one terminal is at 3698,5V and the other at 3697 is equally correct. As is saying one (+) is at 0V and the other at -1.5V. What is defined is that one theminal is at a 1.5V higher potential than the other, that is all. As long as you don't connect either terminal to another reference, that is all you can say about them...

As for your question why you would negative voltages at all: one of natures building blocks, the sine wave, oscillates around a reference, going above and below it, negative and positive if you will...

[Galaxyrise]

What I don't understand is how anything below 0V have any power?

Mathematically, the signs cancel so you still end up with positive power.

Try thinking of it in terms of current flow.  Apply a voltage to the resistor, and current flows.  Reverse the voltage, and current flows the other way.  The same power gets dissipated. This direction gets encoded as positive vs negative in both voltage and current. 

Or perhaps a different analogy: height.  We tend to think of height as a purely positive quantity, but there are negative heights as well.    If I stand a 20cm ruler on top of a 75cm high table, you can say that ruler is 20cm tall--making the tabletop your 0 point.  Then when I hang another 20cm ruler down from the table, what's the height of bottom of that ruler?  It's 20cm below the table, or -20cm.  You could re-define 0 to be the bottom of that ruler, or you could re-define 0 to be the floor, or the ground, or the center of the earth... the height you call 0 is arbitrary but the bottom < table < top will remain consistent.  Regardless of where 0 is, current will always flow from higher voltage to lower voltage.

However, the location of 0 is important once you consider amplification.  Suppose I have something that doubles heights.  That is implicitly doubling with respect to 0.  So in the first example, the ruler on top would grow to be 40cm above the table and the ruler on the bottom would grow to be 40cm below the table because the tabletop was 0.  But if 0 was the floor, then the bottom of the ruler would go from 55cm off the floor to 110 cm off the floor and the top would go from 75 to 150.  Its length would still be doubled to 40cm, but it would now be 35cm above the top of the table!  When amplifying audio, you usually want the first behavior, where the center of the audio stays at the same place but the peaks and troughs get further away from the center--amplifying both "positive" and "negative" voltages.

By and large, Amplifiers can't produce a voltage lower than their V- supply.  This is not their 0. Their 0 depends on how the IN- and IN+ are set up.  Thus, what is considered 0 will be different in different parts of the circuit. There might be an amplifier that thinks of the table as 0 and another one that thinks of the floor as 0.  But they will all agree the table is higher than the floor.

[suicidaleggroll]

Voltages are all relative.  A "negative" voltage is no different from a positive voltage, you're just looking at it from the other perspective.  Like a person standing on the roof of a house and a person standing on the ground looking at each other.  The person on the ground says "wow, that person is really high", the one on the roof says "wow, that person is really low".  Both are right, it's just a matter of perspective.

What you call "common" or "0V" in your circuit is completely, 100% arbitrary.  You could just as easily pick the "negative" or "positive" rail as your "0V", it makes no difference.  The "0V" point in your circuit is wherever you decide to label it; the op-amps, passives, other components, and electrons couldn't give a flying crap either way.

Why would anyone label their circuit in a way that causes some points to be at a "negative" potential?  Sometimes it makes the math easier.

[edy]

Here's another analogy. Imagine you are on a huge mountain, there is no bottom and no top.... Just an infinitely large sloped side. You don't know what elevation you are either. You are just "there".

Voltage is taken as the difference in potential energy between 2 points on the mountain.  Where you are on the mountain doesn't matter in so far as the electrons.... they only know to roll "down the mountain", no matter where they start. They follow the potential energy gradient.

So if you stick a 2 probes from a voltmeter and measure zero potential difference between 2 points on the mountain, the electrons won't want to move. They have no EMF (electro-motive force) or 0 voltage between the 2 points to push them from one place to the other.

The fact that a battery has a difference between the 2 ends, or if you stack a bunch of batteries in series together, or connect power supplies together in series... that's just like looking at sections of your mountain where one slope continues on from where the other left off.  You can stack as many as you want and set your reference to wherever you want.

For example, take two 1.5V AA battery connected in series and you get say 3 V difference from the first to the last. You can also just as easily "tap" the middle part (where you connect your batteries together) and use that in your circuit somewhere and call it 0 V. Then if you look at your circuit, you would have +1.5 V difference to the end of the top battery, and a -1.5V difference to the end of the bottom battery.

On the other hand, you could take three 1.5 V batteries and stack them in series. You could define 0 V where you want and end up getting (from ends and connection points of each of the 3 batteries):

0 V, 1.5 V, 3 V and 4.5 V taps
-1.5 V, 0 V, 1.5 V and 3 V taps
-3 V, -1.5 V, 0 V and 1.5 V taps
-4.5 V, -3 V, -1.5 V and 0 V taps

What you decide to do is up to you. Just make sure if you start connecting bits of mains-wired components together you know if they are connected to ground or "floating" because you need to know if these components have a potential difference between them. They may not be isolated but have a common conductive path and if the potentials are different between the points you connect, it has nothing to do with how you defined them... it is REAL.



 

[ziplock9000]

The best way to think about voltages is that they are NOT absolute, they are relative. Thats why you have potential DIFFERENCE. So normally you consider GND to be zero and your 9v battery to be 9v above that at +9v. This is relative however as YOUR GND may not be the same as my GND. So with that in mind, if you add another battery to that in reverse and relative to your GND, and are pushing 9v BELOW that you get -9v. Hence +9v, GND and -9v. But if we have a common GND (say through mains) then your +9v WILL be the same as my +9v.

I used caps not to be a dick, but to help keep it in your mind like keywords.. It's worked for me anyway.

Hope that helps.

[Brumby]

What is the difference between positive, negative and common?

To put it as simply as possible .... it depends on where you put your multimeter probes.

The following diagram shows the same battery setup, repeated 3 times.  Each of these has been labelled with voltages.  All of them are correct in themselves (you cannot mix them).  The trick is - you pick one point as your reference point and call that zero volts.  Once you've done that, connect the black lead from your digital multimeter to that spot and leave it there.  Use the red lead for your voltage measurements.  (Note: This is safe with digital meters.  Analogue meters usually require special care.)

Example:

I always thought that if is do this I will simply double the voltage. 
So say the power supply's are 6V each, if I connect a bulb across the V+ and common or V- and common I would have 6V going through the bulb right?

Absolutely correct.

And if I connect the bulb across V+ and V- I would get 12V?

Again, absolutely correct.

So what is all this positive and negative voltage?

If I wanted to get pedantic, I would ask you - if you connect that bulb across the V+ and V-, have you put +12v across it or -12v across it?

The answer is, as I originally stated, it depends on where you put your multimeter probes.

So, if they are all the same thing, then you might ask - "Which is the best?"  The answer to that is "Whichever is the most convenient" - which leads us to an answer for this question:

Its especially confusing in Op-Amps. Why would I amplify a voltage to a negative?

There are several ways to try and explain this - but I will choose this one: Because the mathematics is easier.

If you take the case of a sine wave, mathematically, it cycles between + 2 and -2, so setting a reference point on a circuit where the signal follows the same movement makes it a lot easier.

For example, say we have an amplifier circuit that takes a sine wave input signal and produces an output that is twice the magnitude....

1. Using the middle example of mine, lets say the input signal varies from +7v and +11v  i.e. 4v peak to peak.
OK - let's multiply everything by 2!
This gives an output signal varying from +14v to +22v i.e. 8v peak to peak.  But that can't be right.  The high voltage of 22v is impossible.  The maximum is 18v.  If we were to put the signals on a scope, we would see that our maths is wrong.

Our maths needs some "tweaking".

The problem is that the amplifier does not amplify signals relative to our 0v reference - it amplifies them relative to somewhere between the two power rails.  With no input signal, we can see this is around the +9v mark and our scope observations confirm this.

We can now re-do the maths using this "offset" of 9v ... and it looks like this:
 - Input signal varies between +7v  and +11V   (4v p-p)
 - Subtract the offset and the "input signal" varies from -2v and +2v  (still 4v p-p)
 - We multiply the signal by the amplification factor of 2
 - The "output signal" varies between -4v and +4v  (8v p-p)
 - We add back the offset and the output signal varies from 5v to 13v (8v p-p) (confirmed by scope observation)



2.
Now let's try it with the first example.
 - Input signal varies between -2v and +2v   (4v p-p)
 - We multiply the signal by the amplification factor of 2
 - The output signal varies between -4v and +4v  (8v p-p)

Much easier.

[Brumby]

What I don't understand is how anything below 0V have any power?

Your confusion seems to stem from the notion that negative voltage equates to "negative energy".  This is not the case.  (Let's leave out the quantum mechanics for this ... OK?)

Let me offer you the following illustration: A person is standing, looking at a swing that is moving back and forth ... something like this:


Let's also say that that person considers where they are standing as the zero point - and any motion to the right is measured as positive.  Now, let's say when the swing is at it's furthest left position, it is 1m away from the observer and when it is at it's furthest right it is 5m away.  The swing moves through 4m and carries a corresponding amount of energy.

Now let's put the observer on the other side of the swing.  They are still facing the same way and still want to keep the same reference criteria: where they are standing as the zero point - and any motion to the right is measured as positive:



Now, when the swing is at it's furthest left position, it is 5m away from the observer, but in a negative direction and when it is at it's furthest right it is 1m away and, again in a negative direction.  The observer will write down that the swing as moving between -5m and -1m.  While these numbers are negative, the swing still moves through 4m and still carries the same energy that it did before.  Nothing has changed about the swing at all - just the observer's point of reference.



As a final example, let's say the observer stands in the middle (but out of the way enough that they don't get hit by the swing) with the same reference criteria: That they are still facing the same way, the zero point is where they are standing and any motion to the right is measured as positive.

They now report that the swing is moving between -2m and +2m ... yet it is still the same swing ... with the same amount of energy.

[ludzinc]

Why would you want negative voltage (power)?

Consider a loud speaker.

At rest, with no voltage applied it's in it's resting state. 

Apply a positive voltage to the speaker and the cone moves out from it's rest position. 

Apply a negative voltage to the speaker and the cone moves in from it's rest position. 

Useful!

Also - DC motors will turn one way with positive supply, then the other with a negative supply. 

[abdullahseba]

Thanks for the replies.  All these the analogy's where very helpful . I guess I didn't know this because I skip any chapter or paragraph named the dreaded "theory".

[Brumby]

Theory may seem dry and unappealing when you've got a soldering iron in one hand and a multimeter in the other...

BUT

... making the effort to get a handle on the theory gives you more than practical ability - it gives you understanding and this is the passport to far more varied and exciting endeavours!

[bson]

It's called common because it's the common point for the two supplies.  Because it consists of a + terminal that can source and a - terminal that can sink it can be used as a 0V ground for both positive and negative voltages.  Another way to think of the term "common" which is, ahem common in EE - is as "shared".

Current flows from the 0V common to the negative voltage point through the circuit between them.
Or from the positive voltage to either 0V or to the negative voltage point.  Or all of the above.

[tggzzz]

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

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

Do you know what a negative number means? Wait until you come across imaginary numbers, which are integral to understanding basic electronic sufficiently that you can predict what they will do.

Understanding takes time and hard work; if you aren't prepared to do that, find something trivial to do. As I taught my daughter, "ignorance can be cured, stupidity can't be cured".

[Vtile]

The common is also sometimes called as a virtual ground. It is merely a point at which point the direction of current (and energy flow) changes. For light pulb it doesn't matter but one directional device like diode it do. It is used mostly in situations where someone needs to reach zero volts (since silicon devices have this forvard voltage around 0.7 so you need to go under zero volts with some parts of the device if you want to have true zero output) also this is the way to work with AC which is alternating current. It alternates in both sides of the ground (+ to - to + ...) reference point (with household appliances that is the soil of earth).

Braindump with layman terms.

@Brumby.. Add to that analog meter sentence word "usually" as there is ie. center zero meters, in automotive current meters as example with plus and minus scales, recharge/discharge respectively...

[abdullahseba]

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.

[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.

Since you are mostly work with digital hardware, do you think it is important to understand the theory of metastability, bridging terms and correct line termination, for example?

Even when I was under 10, I was striving to understand the theory that would enable me to predict that something would indeed work.

[neil t]

Hi
I'm sure this has been asked many times before but I cloud not find an answer to satisfy me.
What is the difference between positive, negative and common?
Example:

I always thought that if is do this I will simply double the voltage. 
So say the power supply's are 6V each, if I connect a bulb across the V+ and common or V- and common I would have 6V going through the bulb right?
And if I connect the bulb across V+ and V- I would get 12V?
So what is all this positive and negative voltage?
Its especially confusing in Op-Amps. Why would I amplify a voltage to a negative?


Thanks in advance

Simples -v is more negative the ground and the inverse or mirror of +v
+v is more positive than ground and the inverse or mirror of -v
kind of like a see saw pivoting around the centre.

[abdullahseba]

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.

[Brumby]

@Brumby.. Add to that analog meter sentence word "usually" as there is ie. center zero meters, in automotive current meters as example with plus and minus scales, recharge/discharge respectively...

You are absolutely right - and I did consider such devices when writing what I did.

However, a DMM satisfied the requirement of my discussion point so I decided to minimise any tangent thoughts.  To that end, I consigned any and all analog meter considerations, such as centre-zero movements, under the 'special care' reference.

Having said that, your single word suggestion is one that didn't cross my mind - but I like it, so I will edit that post to include it.

[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.

[james_s]

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 .

My grandmother's house had precisely this happen when I was a kid back in the early 80s. Lightning struck on or near the pole out in the field and vaporized the neutral lug in the meter base which was on another pole in the yard near the house. Some of her lights were very bright and others were very dim. The 240V loads worked fine but turning on any light caused the other lights to change brightness. This is of course because the two banks of 120V circuits were now in series, having no neutral/ground reference. At the time it was freaky but looking back I understand exactly why everything behaved as it did.

[james_s]

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.

With an AC generator (alternator) there is no "negative output", rather there are two outputs that have a rapidly reversing potential across them. On one half of the rotation (assuming a simple 2 pole alternator) one output will be positive and the other negative, then the potential will flip through the other half of the cycle. You can tie either one of these two outputs to the earth and call it "ground", consider that point to be 0V, and then from that perspective the remaining output will cycle above and below ground. It doesn't matter which of the two outputs you tie to ground, the result is the same. When you look at a circuit that is rated at 120V, that is the nominal voltage. That is an incandescent lamp connected to 120V nominal AC will glow with the same brightness it would connected to 120V DC. In reality the voltage is constantly changing, it peaks at around 170V peak to peak, so 85V above "ground" and 85V below.

[tggzzz]

The commonest UK one is TN-S, where separate  protective and neutral conductors run all the way to the substation and are grounded there.

Every house in my road, and many in my village have their supply taken from 4 wires on poles outside the house. Every house is connected to the lowest wire, and every house is connected to one of the three other wires. Yes, that's neutral at the bottom and the other three are the three phases. I'm not exactly in the sticks; I'm 4 miles from a major city centre!

I do not intend to have the lead water supply pipe replaced - although I do run the water for 1 minute before drawing any for drinking

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.

As you are aware...

Even if you "measure it at ground" to within, say 10mV, it can supply a lot of current at 10mV. That could cause "bang you have a heart attack or are blind" or "bang you are on fire" situations.

The actual potential of the neutral wire depends on the degree of imbalance in the currents in each of the three phases. That's why in my road every third house is connected to the same phase; hopefully no one house takes "too much/little" current.

Only if perfectly balanced will it be at 0V (except during lightning strikes and gross distribution problems!). ISTR a couple of volts being not uncommon.

[Cerebus]

The commonest UK one is TN-S, where separate  protective and neutral conductors run all the way to the substation and are grounded there.

Every house in my road, and many in my village have their supply taken from 4 wires on poles outside the house. Every house is connected to the lowest wire, and every house is connected to one of the three other wires. Yes, that's neutral at the bottom and the other three are the three phases. I'm not exactly in the sticks; I'm 4 miles from a major city centre!

I do not intend to have the lead water supply pipe replaced - although I do run the water for 1 minute before drawing any for drinking

Out of curiosity, what is the grounding arrangement TN-C-S, or TT? (The latter in my mind standing for not Terre-Terre but Truly-Terrifying)?

For those not in the know geographically, overhead (low voltage*) supplies are pretty rare in the UK. Not unknown, as the above proves, but I'd guess that the proportion of the total premises that are delivered overhead to the customer is in the low single figures percent. (I just tried to find actual figures but failed.)

Correction, I found figures. These are just based on total length of low voltage supply lines above or below ground, and one would expect rural supplies (where one finds most overhead delivery) to be physically longer and so use more length of cable per premise served. It takes 10 metres of cable to get to my (urban) next door neighbour; if that was rural it would be possibly hundreds of metres. So, 14.6% of UK low voltage distribution cable is above ground (European average 42.4%) - which I think is consistent with my guess of single digit percent premises served.

Only if perfectly balanced will it [neutral] be at 0V (except during lightning strikes and gross distribution problems!). ISTR a couple of volts being not uncommon.

The worst place for floating (relative to local ground) neutrals are light industrial trading estates where the demand balancing across phases can be all over the place, especially when there's big-ish machinery. Heavy industry tends to get individual three phase supplies and some thought put into designing the loading of them. Light industry shares one supply between many premises and there is little or no planning goes into load balancing. Add a few load dumps, some crappy power factors and you've got an 'interesting' supply to deal with.


* In the electricity supply world the term low voltage gets used for everything below about 1kV, and the term extra low voltage for what most of us would think of as low voltage - 50V and down.

[IanB]

Here's an example of the 4 wires on poles. We have three phase 11 kV coming into the transformer, and three phase 415 V/240 V leaving.

https://goo.gl/maps/s9cLaNfVciG2

If we look closely at the transformer it appears to be grounded at the foot of its mounting pole, but there is no evident ground connection carried along to the houses. At a guess each house will have it's earth tied to the neutral wire where the supply enters the premises?

[tggzzz]

The commonest UK one is TN-S, where separate  protective and neutral conductors run all the way to the substation and are grounded there.

Every house in my road, and many in my village have their supply taken from 4 wires on poles outside the house. Every house is connected to the lowest wire, and every house is connected to one of the three other wires. Yes, that's neutral at the bottom and the other three are the three phases. I'm not exactly in the sticks; I'm 4 miles from a major city centre!

I do not intend to have the lead water supply pipe replaced - although I do run the water for 1 minute before drawing any for drinking

Out of curiosity, what is the grounding arrangement TN-C-S, or TT? (The latter in my mind standing for not Terre-Terre but Truly-Terrifying)?

There's nothing on the pole, as I said; see attachment from google earth. Curiously it appears that some of the distribution around here has 5 wires, with a thin one on the top. I don't know what that is, and can't make it out on the google earth pictures; conceivably it is some form of lightning conductor.

As far as I can see, I confirm that in/around the fuseboxes. From memory there's a strap on the lead cold water supply, but I'm not going to move appliances to check that! These are 1930s houses, with modern internal wiring.

[james_s]

In the USA power is normally distributed from the substation as 13,500/7200V 3 phase. That will be either overhead or underground and run along main roads. Suburban neighborhoods generally get just one phase feeding off the main line. Then there is a transformer either on the pole for overhead or in a box on the ground feeding anywhere from 1-10 homes nearby with a center-tapped 240V secondary. The center tap is bonded to earth at the transformer, and also to the neutral wire that runs to each house along with a pair of hots. I've never seen a low voltage feed that was more than a few hundred feet long.