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.