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Just a general electronics question: What is negative voltage, like -5 Volt?

From my basic knowledge, power is generated by electrons wandering from the minus to the plus side of the power source (assuming DC power here). Is negative voltage when electrons wander from + to -?

Why do some devices even need it, what is so special about it?

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23  
You will do better to think about electronics as a kind of applied math, and forget about the electrons entirely. –  markrages Feb 17 '11 at 3:34
3  
Yeah, don't think about electrons. Engineers use conventional current instead ("just a convention"), since charge can flow in both directions at the same time. amasci.com/amateur/elecdir.html And power is not "generated" by electrons. Energy is carried in electric and magnetic fields. amasci.com/elect/poynt/poynt.html –  endolith Feb 17 '11 at 15:13
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xkcd.com/567 –  Michael Kjörling Apr 23 '13 at 9:54

8 Answers 8

up vote 47 down vote accepted

Someone may have better words to explain this then me, but the big thing you have to remember is voltage is a potential difference. In most cases the "difference" part is a difference between some potential and ground potential. When someone says -5v they are saying that you are below ground.

You also need to keep in mind that voltage is relative. So like I mentioned before most people reference to "ground", but what is ground? You can say ground is earth ground, but what about the case when you have a battery powered device that has no contact to ground. In this situation we have to treat some arbitrary point as "ground". Usually the - terminal on the battery is what we consider from this reference.

Now consider the case that you have 2 batteries in series. If both were 5 volts then you would say you would have 10 volts total.

But the assumption that you get 0/+10 is based off of "ground" as being the negative terminal on the battery that isn't touching the other battery and then 10V as being the location of the positive terminal that isn't touching the other battery. In this situation we can make the decision that we want to make the connection between the 2 batteries be our "ground" reference. This would then result in +5v on one end and -5v on the other end.

Here is what I was trying to explain:

+10v   +++   +5v
       | |
       | | < Battery
       | |
+5v    ---   0v
       +++
       | |
       | | < Another Battery
       | |
0v     ---   -5v
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3  
Thanks. Just wondering though: Why would devices "choose" to require -5 Volt instead of +5 Volt? If I have a device that's powered by a "normal" Power Supply, why wouldn't the manufacturer just require +5 Volt? (Some LC Screens requite negative voltage) –  Michael Stum Feb 17 '11 at 5:17
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It's specific to the device. Read up on how LCD's work at a technical level and I bet it'll explain it. –  davr Feb 17 '11 at 5:43
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@michael, LCD screen contrast provides the reference for each of the cells. It may be rated for common 3.3V or 5V for easy integration, but the cells may need more of a drop in order to read them (molecules need to be twisted). Often 5V LCDs can use 0V contrast, even without an integrated (-ve) charge pump. –  tyblu Feb 17 '11 at 5:51
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+1 for the ASCII art. –  Robert Feb 17 '11 at 16:34
    
Very helpful description! I never really fully grasped this before, but this helps a lot. –  William Feb 25 '11 at 14:59

Imagine you're measuring the height of a car. You could take a tape measure and measure the distance from the ground to the roof of the car. "The roof of this car is 4 feet above the ground."

You could also stand on the roof of the car and dangle the same tape measure down to the ground. "The ground is 4 feet below the roof of this car."

Voltage works the same way. The negative sign is just a convention, in the same way that the car has the same height, regardless of which way you measure it. Flip your multimeter leads, and the negative sign will disappear.

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Ground is just a convention, too. You can pick any single point in a circuit and label it ground, but we choose specific points out of convention. –  endolith Feb 17 '11 at 15:16
    
@michaelStum, @pingswept, I always like to remind people that there is potential energy from height if you take physics. (mgh) is potential energy, if I pick the atmosphere as my reference we all suddenly have negative potential energy. –  Kortuk Feb 17 '11 at 23:11
    
Good points, the both of you. –  pingswept Feb 17 '11 at 23:30
    
Gravitational potential energy is referenced negative by setting it to zero at infinity. Electrical potential energy is negative between two charges of opposite sign and positive between charges of the same sign. Say there's an electrical potential drop from 5V to 3V. A positive test charge q would add -2q to its potential energy (becoming less positive), while a negative test charge -q would accelerate in the other direction (+2V, from 3V to 5V), but also adding -2q to its potential energy (becoming more negative). In both cases potential energy decreases. –  eryksun Feb 27 '11 at 13:06
    
@eryksun: While it's true that potential energy is measured by taking zero at infinity, if one thinks in terms of height (a more familiar concept, and the original one above), one could recognize that while one could define absolute altitude (distance to the Earth's center of mass), it is in most contexts more helpful to define elevation relative to something that's a little closer [e.g. the "natural" height of the terrain]. If e.g. a house is built in a flat area, one may describe the floor of the basement as having an elevation of -2 meters relative to the surrounding terrain. –  supercat Apr 17 '13 at 15:57

Voltage is a difference in potential. If I connect terminal A of a device to potential of 30 volts and terminal B to a potential 20 volts. The potential from A to B is 10 volts, but the potential from B to A is - 10 volts.

Think of it a tall building To go from floor 30 to floor 20 you go down 10 floors.

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oh why not ..

To say you have a voltage of +5V at point A means that point A is 5 volts more positive than your chosen ground.

To say you have a voltage of -5V at point B means that 'ground' is 5 volts more positive than point B.

The sign just tells you the polarity of the voltage, with respect to the ground node.

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Some OP-Amps, for example the veteran 741, require their supply as two voltages, one positive and the other negative with respect to the ground or zero level of the signal input and output.In this context it is natural to speak of the supplies being +15v and -15v (these are the values commonly quoted for the 741)

An alternating voltage, such as the a.c mains supply, swings positive and negative with respect to the neutral line, which is very close to the earth potential, so "neutral" is regarded as at zero voltage.

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Voltage between two physical points \$A\$ and \$B\$ is defined as

\$ V_{AB} = - \displaystyle \int\limits_{A}^{B} \vec{E}.d\vec{\ell}, \$

where \$\vec{E}\$ is the electric field on the path of the integration.

\$V_{AB}\$ becomes negative or positive (or just zero) according to this integration. For example, if you swap the points \$A\$ and \$B\$, the sign changes.

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Well, just to chuck in my two cents, let's say you have an ungrounded device. With +10 volts, you're expecting the current to come of the battery, through the widget, and then... where? It's only 10 volts, so arcing through the air to ground isn't really possible, but it could go through the case into the hand of the user, or the charge could simply stay on the far end of the widget. So now you have +10 volts on one side and +8 volts or something on the other, relative to ground. The wiget only sees a 2 volt difference though.

With a a +5V and a -5V the current is both pushed into the widget and pulled out of the widget.

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I like your approach, but if you only connect one side of a circuit to 10V and leave the other floating(both relative ground) you will find that the entire device should measure 10V, as you have just charged it to 10V, if it only went to 8V on the fare side you have a 2V drop and current is flowing. This would only happen if your measurement device was loading it. –  Kortuk Feb 17 '11 at 23:10

voltage is just a relative unit... consider a point as 5v and another point as 6v then voltage is 6v-5v=1v now take it in reverse manner 5v-6v=-1v... sign is just conventional...

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