Negative voltage ? - Page 1
Forum section: electronics / beginners

author: wertyu
author_rank: Contributor
timestamp: February 20, 2019, 03:55:07 pm
content: What is it , i can understand most electrical principals but negative voltage i cannot , is it like a negative pressure in a water analogy ?any links to videos or article that can explain it simply for me ?ill be reading all the stuff on here over time ! first timei joined a forum in years i thought they had gone extinct .thanks

author: rstofer
author_rank: Super Contributor
timestamp: February 20, 2019, 04:07:39 pm
content: It has to do with the reference point and I get to pick that point.Consider 2 batteries in series oriented left to right with the + output to the right. Suppose you put your DMM black wire on the junction between the batteries. Now probe the terminal on the right end and you will get some positive voltage. Probe the terminal on the left and you will get some negative voltage -relative to the center point 'ground'.Now move the black probe all the way to the left. If you test the center point with the red probe, you get a positive voltage and if you test the terminal on the right end you get a more positive voltage.So, what happened to that battery? Did it change in any way? No, only the reference point changed and I get to choose that point.ETA: Expand the experiment by putting the black probe on the right end of the stack. Now probe the center point and you will get some negative voltage. Probe the far left and you will get a more negative voltage.Voltage measurements arealwaysrelative to something.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: February 20, 2019, 04:11:29 pm
content: Electricity is a flow of the mobile electrons in a metal. Metals tend to be conductors because they consist of a lattice of atoms which loosely share their outer electrons, therefore it is relatively easy to persuade the electrons to move from atom to adjacent atom.Since electrons have a -ve charge they will tend to move towards any point which is more positive than their present potential. (Positive charges arise from the protons in the nuclei of atoms)An atom with an equal number of electrons and protons is neutral. One which lacks an electron or two has a net positive charge, and will attract electrons until that deficit is made up. The reverse if it has too many.The principle is really much the same as a fluid in a pipe, in that it's thedifferencein pressure between the pipe ends which determines the rate of flow. In fact, in the early days voltage was called pressure.The convention that electrons are 'negative' and protons 'positive' is just that, a convention. The reverse naming could equally well be used.

author: ebastler
author_rank: Super Contributor
timestamp: February 20, 2019, 04:15:16 pm
content: Quote from: wertyu on February 20, 2019, 03:55:07 pmYes, that is actually the right analogy. For those simple circuits which can be explained via the water/plumbing analogy, a negative voltage corresponds to negative water pressure.As explained by rstofer, it's all relative:The proper "plumbing" analogy for a battery or other voltage source would be a pump with an input tube and an output tube, which maintains a constant pressuredifferencebetween the input and the output. Whether you call the input pressure "zero" and the output "positive", or call the output level "zero" and the input "negative", is a matter of definition.

author: wraper
author_rank: Unknown
timestamp: February 20, 2019, 04:37:00 pm
content: Quote from: ebastler on February 20, 2019, 04:15:16 pmActually it's not a proper analogy at all. Pressure cannot be negative, actual pressure does not go below zero, it's just pressure difference that produces force/flow in certain direction. On the other hand, both positively and negatively charged particles do exist and produce electric charge. And it's not necessarily electron flow that produces electric current. It can be positively charged particles as well.

author: Gyro
author_rank: Super Contributor
timestamp: February 20, 2019, 04:43:09 pm
content: Quote from: wertyu on February 20, 2019, 03:55:07 pmJust think of it as other things in the circuit being more positive. It's all relative.

author: ebastler
author_rank: Super Contributor
timestamp: February 20, 2019, 04:44:34 pm
content: Quote from: wraper on February 20, 2019, 04:37:00 pmSo -- which part of a circuit (which is in scope of the "water analogy" at all) can't be explained by a "negative pressure" model? Nothing wrong with a simple mechanical model where a pump is "sucking" at the pipe. Where does that simple model work for positive pressure, but collapse for "negative pressure" (negativ erelative to the environment)?Of course your more detailed explanation of positive and negative charges is "more correct". But it goes beyond the simple mechanical model, and is not needed in situations which can be explained by the simple model.And you may (or may not?) have noticed that I acknowledged the relative nature of pressure in the second paragraph of my post?

author: wraper
author_rank: Unknown
timestamp: February 20, 2019, 04:56:10 pm
content: Quote from: ebastler on February 20, 2019, 04:44:34 pmThe question is about what is negative voltage. It's a real thing that does exist. While negative water pressure is abstract and does not exist in reality. Pump does not "suck", it can only push.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 20, 2019, 05:36:00 pm
content: A "Voltage" is Human Speak, but not technical language.When talking / writing / comparing about voltages there always have to be 2 points, and the voltage is the voltage difference between those points.You can not measure a voltage with only one probe of your multimeter. You need both probes (Usually Red & Black) so the Multimeter can measure the diffeence between the 2 probes.If you now take a 9V battery and you measure it's voltage with a multimeter, it will ether be +9V or -9V.If it measures +9V the Red wire is at a higher potential than the black wire.If it measures -9V the Red wire is at a lower potential than the black wire.The battery is still the same, It is just your Multimeter telling you that the voltage differential is the other way around.That's all there is. Really, It's that simple.Now if you take 2 9V batteries and put them in series, then you wil measure either +18V or -18V over the ends.You can also connect the black probe to the center of the 2 batteries.One battery wil now measure as +9V and the other as -9V.In the next step, you can use the 2 9V batteries as a power supply for an opamp circuit.As long as you use the center of the 2 batteries as the reference you can use the opamp for example for amplifying alternating voltages.You can do the same with 2 separate power supplies.Just like the batteries you can connect them in series for twice the voltage, or for a positive and negative voltage.The topology in the power supplies themseves can either be "positive" or "negative" It does not matter, it is all just a matter of placing a reference. You can buid a power supply with a LM7905 (which is called a "negative" voltage regulator) for a uC controller circuit and the uC will run perfectly fine as long as you make sure that the Vcc pin of the uC has 5V more than the "gnd" pin of the uC.Why do they make "positve" and "negative" voltage regulators then you might ask.Good question.With the (lab) power supplies mentioned earlier you can only put them in series in one point, but all the other parts may not be connected anywhere to each other, that includes the inputs.If you have a transfomer with a single secunaray winding, or with a center tapped secondary winding, and you want to build a power supply with standard voltage regulators for (for example) an opamp circuit then it's easyer to do with an LM7815 and an lm7915.

author: wertyu
author_rank: Contributor
timestamp: February 20, 2019, 06:10:17 pm
content: thanks all and doc P , i know what i need is in your post i just got to work it out for myself lol .its in op amps and power supplies ( theory not done much practical yet ) i have come across the negative voltages and could not figure it out but now you mention the reversing of leads and measuring a negative voltage that has got me thinking , ill reread post and work on it .Thanks all some great answers .

author: helius
author_rank: Super Contributor
timestamp: February 20, 2019, 06:18:20 pm
content: The reason that positive and negative voltage regulators are different parts is that they regulate the input voltage by shunting current to ground.In a LM7805, the input and output are positive with respect to ground; in a LM7905, the input and output are negative with respect to ground. The ground is a single pin that is on both the input and output sides of the regulator, which makes themnon-isolated. This is the critical reason they must come in positive and negative versions, unlike "lab power supplies", whichareisolated from their inputs.The better analogy to understand voltage is "repository of potential energy", similar to height for gravitational potential energy. When Galileo dropped his weights from the tower, they were forced downward because their height above the ground (above "zero") gave them potential energy that could be released. After they were lying on the ground, they could fall no farther because no potential energyabove the groundremained. Yet if there were a well, they could fall down into it because movingto a negative heightreleases yet more potential energy. Potential energy can't be seen or measured directly, so it naturally may be negative, like a book-keeping entry. The position chosen to be called "zero" is somewhat arbitrary.With electrical fields, too, the zero is arbitrary*, or better call it a selectedreference point. A charge at a region where the field is high feels a force towards regions where it is low: at the moment you connect a wire across the terminals of a 9V battery, the field is 9V higher at the (+) side compared to the (-) side. We select which point to call our "zero": we can measure with the reference point at the (+) side, in which case the (-) side is -9 volts; or we can measure with the reference point at the (-) side, in which case the (+) side is +9 volts; or we can measure with the reference point in the exact middle of the wire, in which case the (+) side is +4.5 volts and the (-) side is -4.5 volts. Only the voltage difference between two points has any effect on the movement of charges (current).* It is possible to measure the electric field absolutely using electrometers, but this type of measurement is specialized and not often required. There is an analogous "absolute zero height" in gravitation, which is far out in interstellar space: again, not often used.

author: rstofer
author_rank: Super Contributor
timestamp: February 20, 2019, 07:41:02 pm
content: Quote from: ebastler on February 20, 2019, 04:44:34 pmWell, it fails when you try to lift water more than about 30'.

author: Nerull
author_rank: Frequent Contributor
timestamp: February 20, 2019, 07:47:42 pm
content: Voltage is a measure of electrical potential. In an analog to gravitational potential, it matches well with 'height'. If I'm climbing up a cliff and stop halfway and decide I'm going to reference myself as 'zero height', than anything above me is positive height and anything below me is negative height. If you drop something from above me, it will fall and reach me. If you drop something from below me, it will not.Now, its a bit reversed in electricity because the most common charge carrier is the electron, which 'falls up' and goes from negative to positive, but the idea is the same. Electrons will flow across a potential difference, but what point you pick as 'zero' is arbitrary. Anything above it is then positive, and anything below it is negative. '0' is simply a chosen reference, like the origin in a CAD drawing. All other voltages are given relative to the chosen zero point.

author: ebastler
author_rank: Super Contributor
timestamp: February 20, 2019, 08:03:57 pm
content: Quote from: rstofer on February 20, 2019, 07:41:02 pmLook, I'm not trying to be difficult or stubborn here. I know that this "water model" for electrical circuits has limitations. But in fact, all models used in physics do. And nevertheless they are used, and are useful because -- as long as they are used within their limitations --(a) they allow you to predict reality, and(b) they allow you to visualize things intuitively.At some point, under some boundary conditions, in some parameter range a model "breaks down" and does no longer predict reality correctly. In that regime a new, more advanced model is needed. But still, physicists (and engineers) will happily use the simplistic model in the range where it works, since it is nice and simple.So why not use the water model, and even the colloquial term "negative pressure", in the regime where it works nicely to explain voltages, currents, and resistances? As I said in earlier posts, your explanations are "more correct", i.e. they describe a model that will work in a wider range of conditions. (But not under all conditions either.) But that does not make the simpler models wrong or useless within their domain.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 20, 2019, 08:34:48 pm
content: Quote from: ebastler on February 20, 2019, 08:03:57 pm... and when that happens, you should stop using the analogy.Vacuum is a pretty hard limit when trying to create a negative pressure and that is where this analogy breaks down and you shout STOP USING IT and switch to something else.To OP:Though the concepts are simple, they do need time to sink in.So indeed take your time and let it sink in.In electronics there are also a significant number of parts that have a mirror image.for example NPN and PNP transistors.I used to think for quite some time that NPN transistors were Normal, and the ohters were Perverted.Very similar with N-channel and P-channel MOSfets.Inductors and Capacitors are also very much alike if you change currents and voltages.If you put a voltage over an inductor, then current will start to flow.If you put a curent through a capacitor then voltage will start to rise.(Both are quite lneairly actually untill some limits are reached).If you ever get down to raw math and differential equeations youl also start seeing analogies between mechanical and electrical systems. They look different on the outside, but on the inside the math is the same.

author: rstofer
author_rank: Super Contributor
timestamp: February 20, 2019, 08:48:13 pm
content: Quote from: ebastler on February 20, 2019, 08:03:57 pmBecause I can demonstrate the arbitrary nature of the reference point (the purpose of this thread) with a couple of batteries. Something that can be done on a tabletop with 2 batteries and a DMM. There is simply no need to get into a discussion of electron flow, majority carriers, charge and a host of other details just to demonstrate something as trivial as 'negative' voltage. And we're describing electrical kinds of things using electrical kinds of things. I think it is terribly important to understand the arbitrary selection of the reference point. This understanding will be vital when the user gets to circuit analysis.All models have a domain. I don't prefer the water analogy but it has the feature that you can demonstrate it out on the lawn on a warm sunny day. What is overlooked is the discussion of the domain of the water analogy. If we double the current through a resistor, we expect the voltage drop across the resistor to double. That doesn't happen with water.I prefer to discuss Ohm's Law using batteries and meters.

author: IanB
author_rank: Super Contributor
timestamp: February 20, 2019, 08:55:03 pm
content: Why the water analogy works for negative voltages:Voltages are usually measured as a potential difference between two points. Voltages are not absolute single point measurements. You don't measure voltages with a single lead on a voltmeter, you need two leads and two measurement points.Similarly, consider water flowing through a pipe. You can measure the differential pressure of the flowing water between two pressure taps on that pipe (label them 1 and 2). You will read a differential pressure in pascals (or psi), for example. If you measure from 1 to 2 you may measure a positive pressure difference of say +5 Pa. If you measure from 2 to 1 you will see a reversed or negative pressure difference of −5 Pa.Analogies work as long as you keep the rules the same and maintain analogous circumstances. If voltages are potential differences, then by analogy you must treat pressures also as potential differences.

author: IanB
author_rank: Super Contributor
timestamp: February 20, 2019, 08:59:44 pm
content: Quote from: rstofer on February 20, 2019, 08:48:13 pmIt may do. If you make the "resistor" a porous block with many small passages in it you will find the pressure difference is proportional to the flow rate. (The system will be operating in the laminar flow regime.)

author: ebastler
author_rank: Super Contributor
timestamp: February 20, 2019, 09:26:32 pm
content: Quote from: rstofer on February 20, 2019, 08:48:13 pmSure does. Laminar flow of a non-compressible fluid through a tube follows Poiseuille's law, with flow rate = C * pressure difference. The constant C depends on the tube geometry and the viscosity of the fluid, and plays the role of resistance.So if you want to force twice the flow rate throug a given "resistor", you have to apply twice the pressure. It's a direct analogy to Ohm's law.

author: Jwillis
author_rank: Super Contributor
timestamp: February 20, 2019, 11:09:35 pm
content: I was bored .But the water analogy still holds water .As IanB explains it's all in potential whether positive or negative relative to a common steady state.

author: james_s
author_rank: Super Contributor
timestamp: February 21, 2019, 04:52:31 am
content: You could think of it like a staircase, hill, etc. If you're on the ground, the stairs go up (positive), if you're at the top the stairs go down(negative), if you're in the middle then you have both positive and negative, you can go either up or down. The stairs haven't changed, it's just a matter of where your reference point is.