Negative voltage

[Digi421]

I really am a n00b - I've always been interested in electronics since I was a kid (so about 40 years ago), but I never had any knowlege. That hasn't changed much, but I've started to learn the basics. One issue I'm struggling to understand is negative voltage. What exactly does that mean? Is it simply "a point of view" (as in measuring one voltage compared to another, be that "ground" or something else, depending on where you are) or does negative voltage actually mean...like...that electrons move from + to -?

[MosherIV]

Hi

"What exactly does that mean? Is it simply "a point of view" (as in measuring one voltage compared to another, be that "ground" or something else, depending on where you are)"

You are along the right lines. Negative voltage is always with reference to 0V or zero volts (often refered to as 'ground' but that is strictly incorrect as Ground techincally means Earth).

One thing that is not mentioned is that voltages can float with regards to Earth/ground.
For example, most double insulated power supplies (say a wall usb psu) do not connect 0V to Earth so the 0V could actually be any voltage above or below Earth.

[Cubdriver]

It refers to the reference point.  In general, the negative side of the power supply goes to some common point (usually the chassis back in the day, and often some sort of common 'ground' plane on circuit boards today), resulting in the power supply rail(s) being positive relative to the ground/common point.  On occasion, there is a need for a voltage that is negative relative to this common point (the negative rail for dual rail op amp circuits or grid bias for vacuum tubes, for instance).  This 'negative' voltage is achieved by connecting the positive side of that particular supply to the common point, thus resulting in it's being negative if measured relative to the ground.

I hope that makes sense.

-Pat

ETA clarification of positive supply rail

[IanB]

I really am a n00b - I've always been interested in electronics since I was a kid (so about 40 years ago), but I never had any knowlege. That hasn't changed much, but I've started to learn the basics. One issue I'm struggling to understand is negative voltage. What exactly does that mean? Is it simply "a point of view" (as in measuring one voltage compared to another, be that "ground" or something else, depending on where you are) or does negative voltage actually mean...like...that electrons move from + to -?

Voltage is measured as a difference between two points. A difference can be positive or negative depending on which way round you measure it.

Take a standard 9 V battery and a multimeter set on the DC volts range. Put the red lead on the small terminal and the black lead on the large terminal. You will see a measurement of about 9 V on the display. Now put the red lead on the large terminal and the black lead on the small terminal. This time the display will show -9 V instead.

So is this a +9 V battery or a -9 V battery? It is neither and it is both.

To be clear about voltage differences, leave the black lead touching nothing and put the red lead on either terminal of the battery. Notice how the display shows nothing much either way? This is because no single point has a voltage. To measure a voltage you must make a complete closed loop with the meter in the loop. When you do this the sign of the voltage depends on which way round the loop you go.

[Brumby]

I've used this graphic somewhere else on this site ... but it illustrates the point:



All 3 represent the exact same battery connections - it's just where you place the meter probes that changes the numbers.

[edy]

The way I understand it is that it is dependent on how you label things... whatever you decide to call "0" or neutral or ground, is your reference point. And if you happen to measure something MORE NEGATIVE in potential, it is negative voltage. Like the battery examples show.

Imagine you are on a huge mountain somewhere, infinite in size. Anything below you is called NEGATIVE and any point above you is called POSITIVE. There is no top or bottom, the mountain is infinite. All that matters is the relationship between you and another point on the mountain. It is meaningless to ask you "what altitude are you on the mountain" because it is infinite. The only thing you can do is compare 2 points.

Same goes for voltages. If you call the "+" end of a single 9V battery "0" then the "-" end becomes -9V. If you call the "-" end of a single 9V battery a "0", then the "+" end becomes +9V. It is all basically what you decide to call "0". There is nothing magic about a negative voltage or positive voltage. You can even flip a voltage with your multimeter by flipping your probes around... all of a sudden the multimeter will show a negative number instead of the positive number.

If you are troubled by this, then simply call the most negative voltage in your circuit a "0" and then all the other voltages would be positive in relation to it. However, when you start working with opamps it is more convenient to think about "+" and "-" voltages. There are many instances when you want to think in terms of "+" and "-".

You may also see a central tap on a transformer.... so you have 3 wires coming out of it. If you call the middle one "0", then you have "-" and "+" voltages available between the other ends of the remaining 2 wires. That is often how you see power supply rails in systems.

[Brumby]

You may also see a central tap on a transformer.... so you have 3 wires coming out of it. If you call the middle one "0", then you have "-" and "+" voltages available between the other ends of the remaining 2 wires. That is often how you see power supply rails in systems.

Be careful about using that approach on transformers.  They operate on AC - which means any voltage will cycle from Positive to Negative and back many times a second.

[edy]

You may also see a central tap on a transformer.... so you have 3 wires coming out of it. If you call the middle one "0", then you have "-" and "+" voltages available between the other ends of the remaining 2 wires. That is often how you see power supply rails in systems.

Be careful about using that approach on transformers.  They operate on AC - which means any voltage will cycle from Positive to Negative and back many times a second.

True! Good point. There are a few more components needed for rectifying the output on each end and smoothing it using caps, like this:

[Mechatrommer]

negative voltage means. your measured point is lesser than your reference point. you reference point not necessarily 0V, it can be 12V, but when your measured point is 11V, that -1V. electron doesnt flow from + to -, electron is negatively charged so it must be coming from the -, to + until both side leveled (flat battery or 0V difference). whenever you find text or post relating reference point as 0V or GND, thats just illustrative or relative figure to simplify things, not the absolute figure, fwiw.

[Digi421]

Thank you very much for all the replies, it is now much clearer to me. This really is a very nice and helpful community!