What is negative voltage?
I am reading up on logic circuits, families and levels because it's fun. I have no formal education in physics, computing or electronics.
For power supplies, sometimes one of the supply rails is referred to as ground (abbreviated "GND") – positive and negative voltages are relative to the ground. In digital electronics, negative voltages are seldom present, and the ground nearly always is the lowest voltage level. In analog electronics (e.g. an audio power amplifier) the ground can be a voltage level between the most positive and most negative voltage level.
I know from previous reading, that electricity - at least when it comes to direct current, but perhaps even when it comes to AC? - has a way in ("line"?) and a way out ("neutral" or "ground"? - disregarding for a second the fact that ground also carries current in case of a ground fault).
Again, from previous reading, I know that we work computers by either supplying them voltage or not (or in some circuits a higher voltage and a lower voltage). In any case, it's a choice between one or the other, since that is what we are trying to represent: boolean true or false.
So, what is this "negative voltage"? Is this a figure of speech or can voltage actually have a negative value? The part from the article that I quoted above states in relativistic terms, that "the ground can be a voltage level between the most positive and most negative voltage level" (italic text by me), which makes me assume "yes". But if voltage is electromotive force, how can it be negative? I amusingly imagine a force "sucking" the current backwards. 🤭
Explain it to me as if I was five. 👶
https://en.wikipedia.org/wiki/IC_power-supply_pinOpen linkView original on piefed.blahaj.zone
23 replies
Negative voltage means you have the probes swapped.
Voltage is always a difference between two points. It is not absolute - it depends on where you measure from.
When we measure voltage, we always choose a reference point (often called ground or 0 V).
If a point has more electric potential than the reference → voltage is positive
If a point has less electric potential than the reference → voltage is negative
So, a voltage is “negative” only because of the chosen reference point.
Example
Imagine a battery:
If you measure like this:
Same battery, different reference → different sign!
Sic. This explanation is clear as day. Thanks!!!
This explanation is good indeed.
Sometimes you design a circuit where the main chip runs on +3.3V reference to the chips ground at 0V, but somewhere in the proces you end up with an extension board that wants 5V below the ground. For easy of use we just call this -5V instead of renaming everything to 0, 5, and 8,5V. Renaming also has the drawback that I like to measure everything reffering to the ground of the main chip. So I personally just don't rename and keep my reference there.
Others have already answered, but one thing I think helps a lot in understanding electricity is to think of it as water. Water running through a hose behaves exactly the same as electricity running through a wire.
The amount of water running through the hose is similar to the amount of electrons running through the wire. We call this current, and measure it in Amperes. Whenever you read amperes think about the flow of material through the medium. Can this value be negative? Sure, it means the current is flowing in the opposite direction.
If one end of the hose is higher than the other the water will flow from the taller part to the lowest one. Measuring how tall one part of the system is compared to another tells you a what potential difference there is there. We call this Voltage and measure it in Volts. Whenever you read volts think about the potential of movement, if there is a 5 Volt difference between two points in your circuit, connecting a wire between those points will produce a flow, just like how if one reservoir is higher than another connecting a hose between them will move water around. Can this value be negative? Sure, it means the other side has more potential energy.
If the inside of the hose has a rougher surface water will have more difficulty flowing through there than if it's smooth. We call this resistance and measure it in Ohms. Whenever you read ohms think about how difficult it is for the current to flow through. Conversely Mhos (OHM spelled backwards) is how easily current flows through the material. Can either of these values be negative? Not usually, but things can behave as if they had a negative resistance, e.g. an amplifier, which in our water analogy is a device that uses a small water flow to control the doors for a larger door, if you have 1 drop per second it let's put 1L per second on the other side, so it can be seen as something that increased the water flow, therefore negative resistance.
Now you want to move a wheel with your water, for the water this wheel is seen like a hose that's harder to move through, so it offers some resistance. If you don't have enough water flowing it won't budge. You can move the wheel ba raising the other side of the hose, this increasing the speed the water flows and giving it more energy, or you can do the same by putting more water in the hose thus increasing the current. Therefore we need a unit to measure how easily the water at a given point can move a wheel. We call this Potence and measure it in Watts or VA (Volt-Ampere). Whenever you see Watt think about the amount of energy the el electricity has at that point.
Lamps have a measure in Watts, because they're like a wheel that the water will move, and they're letting you know how much water energy you need to throw at it for it to move, you can use high amperage low voltage, or high voltage low amperage to get to this result. Because both the Voltage on your house, and the resistance of the lamp are fixed values, you can calculate the resulting amperage and wattage.
I think I went a bit off topic, but I hope this helps clarify some of those concepts.
Great explanation. For anyone reading this, if this helped your understanding, and want more explanation of electrical concepts like this, check out the book There Are No Electrons: Electronics for Earthlings
Its voltage that just feels tired in itself, just wants to be alone and not push any charge.
If you think of a positive voltage as pushing then it is not anymore inaccurate to think of a negative voltage as pulling. At least if you think in terms of DC, which is what power supplies will typically give you.
Voltage is all relative. The sign just shows the direction of flow. If you plug a volt meter in backwards it will show a negative voltage. That means the current will flow from the negative to the positive terminals.
If you have three terminals, let's say A, B, and C. If you measure 12V between A and B. Then 24V between A and C. Then you will also be able to measure 12V between B and C. Or -12v between B and A. Or -24 between C and A. The direction matters.
You could label any of these as ground it makes no real difference to the system. Only how you talk about it. Without a ground label then you would not say C is at 24V as it has nothing to compare to. If you pick A as ground then it is said to be 24V, but if you pick B as ground then it is said to be 12V. If you pick C as ground then it is 0V. So when we say a point is at a voltage then that is implied that it is that voltage relative some other point we decided was ground.
Things get a lot more complicated with AC as electrons move back and forth in the wire they don't have the same direction as they do in DC.
That's awesome! Thanks for the explanation! So, with my (maximum) 12V computer chassi fans, what are we comparing to? What is "ground" here? I slapped the fans onto my server rack, bought a DC adapter with variable voltage and wired them up so that I can manually select the voltage and thus the speed of the fans. Is this also a situation in which the various voltages/speeds are 3V, 4.5V, 5V etc higher than something that the person who built the electric grid in my house decided to call "ground"?
Sorry for the blurry pic.
First, the DC voltage of these fans is a separate voltage system from the AC/mains of the house.
Whether the 12v comes from a transformer or a switching power supply, the DC and AC circuits are effectively separated by that device.
The ground reference of the 12v system exists only within that system (and is provided by the transformer/power supply).
Consider the tranformer/power supply to be a battery with a positive and negative terminal. Inside, a chemical reaction will take place that moves electrons from one set of plates to another, leaving "holes" in one set, and extra electrons on the other.
This creates an imbalance - an electrical potential - and entropy says everything should try to balance. So those extra electrons provide the voltage, and once a circuit is made, will immediately try to balance the potential by moving through the circuit to the other terminal.
Using a transformer/power supply provides those electrons (and a sink - somewhere for those electrons to return to) from AC/mains voltage.
This is a pretty simple way to view it, there are more details that an EE would take me to task for, but it's a good starting point.
It means you have the equation backwards.
You take some point of your circuit, call it "ground" and define the voltage there as 0.
If something has a higher voltage than it, than the voltage is positive. If the voltage is lower, than it's negative.
Also, you are mixing energy distribution names with your voltage references. Your ground wire has the same potential as the actual ground (the stuff made of dirt where your house is built). We usually define it as the ground in circuits that are connected to it, but you'll be surprised by how unusual it is to connect circuits to it.
The charge moves from higher to lower voltages. In a wire, that means that electrons move from lower to higher voltages. The absolute value of a voltage is meaningless, you will only ever use differences between them.
I will definitely keep in mind, that what we use are the differences in voltages (which even makes sense if one were to use the water pipes metaphor).
So, the only reason we have absolute values in our outlets is for compatibility (to make manufacturers lives easier?) and to have some "wiggle room"?
We don't use absolute values in out outlets because there are no absolute values for voltage.
The line in your outlet oscillates between positive and negative (sqrt(2)*V) relative to the neutral, where V is the nominal voltage of your place. And because this is symmetric, you can also say that the neutral oscillates on the same range relative to the line.
Once you add in outlets, there's a reference.
Assuming you're in a US home similar to mine here: Ac power typically comes into an outlet as two to three conductors. One is live, one is neutral, the other is ground.
The neutral is your 0v. In the house, that's often tied to a water line at your breaker, or a rod inserted into the earth, a ground rod. The ground is also tied to neutral in the breaker, and is there for protection. (GFCI outlets test for current on the protection line and if there is any, shuts off the outlet because that's a fault condition).
Tldr: 0v/ground in your home is referenced to actual ground, the earth, typically in the breaker box.
Voltage can be viewed very much like pressure in a water pipe. Positive voltage is equivalent to pushing water into a pipe, negative voltage is like sucking water out of a pipe.
I'm not an expert, but I think I know what the deal is.
First off, the voltage of a single line has no meaning except relative to the voltage of another line. (Just like, say, "in motion" or "at rest" have no meaning except relative to a frame of reference. "At rest" relative to, say, the earth or the sun or the galactic center or car interior or whatever.) If you step on one rail of an electric rail line, nothing happens. You don't even feel anything. If you step on the other, similarly nothing. If you bridge both, you're gonna have a bad time</southpark skier voice>.
"Ground" isn't zero volts in any objective sense. And Vcc isn't 5 volts (or 3.3 volts or whatever) in isolation.
And in fact, it's only convention that ground is arbitrarily labeled "zero" and Vcc is 5 volts rather than Vcc being labeled zero and ground being labeled -5 volts. (Ok, it's a little more than just convention. The latter would be awkward in practice. But it would be consistent, and you could do all your circuit design that way, just like if you labeled the white keys on a piano "Z", "Y", "X"... instead of "A", "B", "C"... You'd just have to think differently about it to make it work.)
So, I think what's going on here, and we'll pretend "Vcc" is 5 volts (relative to ground) just for the sake of this example, is that basically:
So, for instance, if the device has some components that operate at 5 volts and some that operate on 10 volts (I dunno, let's say the CPU requires 5 volts, but the... I dunno, optical drive motor?... requires 10 volts), you can get 5 volts by using Vcc as my source and ground as my sink, and you could get 10 volts by using Vcc as my source and Vee as the sink.
That all make sense?
And to one of your specific questions:
So, what qualifies as "negative" and what qualifies as "positive" is just convention. Just like what qualifies as the "north" end of a magnet and what's "south" is convention. (I guess "north" and "south" refer to which direction a magnet points when used as a compass, but then again which end of the Earth is "north" and which end is "south" is just linguistic convention as well.)
In the same way, what we label "positive" voltage and what we label "negative" is convention. However, we do have an established convention with voltage. (And with poles of a magnet.) Just because we "made up" what qualifies as "positive" or "negative" doesn't mean there's any disagreement on the topic. So unless you purposefully switch your language just to be contrary, you're not going to get funny looks or requests for clarification using the terms "positive" and "negative" for electrode voltages.
However, if line A has a lower voltage than B (and what's "lower" or "higher" is convention) we can say that the differential between B and A (again, order is important) is negative whereas the differential between A and B is positive.
The analogy between voltage and pressure only goes so far, but in this case I think it's helpful. If tank B has higher pressure than tank A, you can say the pressure differential between B and A is negative. Similarly, you could say that the air pressure differential between sea level and the peak of Mt. Everest is negative. (Though, with pressure, there is such a thing as "absolute zero". "Perfect vacuum" is technically a meaningful concept. I don't think there's any such thing as "absolute zero" for voltage. Only "differences" between voltages on different electrodes.)
That made perfect sense in the sense that I interpret it as yes, "negative voltage" - here - is a sort of figure of speech. We (or the article or the electrical engineering community or the universe) is not trying to say that there are negative voltages. The article is just illustrating the common voltage ranges for ICs.
Did I understand you right? 😅
Well, to say "the voltage differential between A and B is positive" is saying exactly the same thing as saying "the voltage differential between B and A is negative".
There is such a thing as voltage differential. And the voltage potential at one electrode can be higher than the other, or the voltage at one can be lower than the other. But given that "voltage" isn't really a thing except in relation to something else, there's kindof no such thing as "negative voltage" or "positive voltage". Just voltage relative to some other electrode.
And if the voltage of one line is higher than the voltage of a reference ("ground"), then the voltage is said to be "positive". If it's lower, it's said to be "negative".
I don't think "negative voltage" is any less a thing than "positive voltage", really, but neither one is really a thing except in relation to a reference. Which is to say you can't connect a single electrode to an instrument and measure the voltage of that single electrode. That would be meaningless to try. You can only measure the voltage of an electrode by connecting your instrument the electrode and a reference.
Hopefully that clears it up.
This really helped visualize it! Thanks! 😊
It is a figure of speech in the sense that voltages are almost always being discussed between two things, and the figure of speech part is that "ground" or "earth" is implied in most, but not all terrestrial cases. Especially in microchips, which like you said have very limited safe voltage ranges, it is very important to give them a stable and consistent reference to ground whenever possible to make sure static voltages don't develop on the other side that can damage them internally.
Technically, if you're referring to voltages on an absolute scale (not relative to each other) you'd typically be talking about "electron-volts" which is why powerful electro-magnetic radiation like x-rays, gamma rays, and other cosmic phenomenon are often measured in mega-electron-volts (MeV) for example. This is useful in physics, but just like we don't use "absolute temperature" in day-to-day life, it is essentially meaningless to us standing down here on the crust of our pitiful little planet made mostly of molten rock covered in conductive water.
So in day-to-day life, we talk about voltages relative to the closest thing we have to a fixed reference point, and the "zero point" we rely on which is mostly a decent reference to the voltage of Earth's own water table tends to be reasonably consistent and practical for our purposes. Static electricity is what happens when a particular object starts to lose its reference to ground, and large voltages (positive OR negative) can build up RELATIVE to ground, which then sparks when it finally equalizes back to our stable reference point. When large static charges build up in the atmosphere, they too eventually equalize with ground through lightning strikes. This is why "ground" is important to us. High voltages relative to ground can be very dangerous. Even small voltages relative to ground can be damaging to sensitive electronics. There is nothing special about "ground" voltage in particular, in fact it changes constantly with the weather, and solar output too! The important thing about it is that almost everything on this planet is connected to it, either through humidity, water, plants, the moistness of soil and even rock has enough conductivity to maintain a stable connection to Earth's average voltage. And of course we're going to use that as a reference and baseline because it's super helpful for us. We don't care how many electron-volts the Earth has. We just care that almost everything is connected to it in some meaningful way.
Holy shit that escalated fast! In a fascinating way, that is! Thanks! So, truly, there is a measurable, fluctuating voltage (measured in electron volts) in the ground, but for our everyday appliances we rather measure voltage in the difference between the immediate source's terminals, such as the two terminals of a battery, for instance?
That's my understanding, I think I've grasped at least the general gist of it, if not the specific nuances and details. Granted, I'm not a physicist or scientist of any sort. My dad was an electrician if that counts for anything, I dabble in electronics and I watch a stupid amount of Youtube, but those are pretty much my only qualifications, so feel free to consider this having a similar accuracy and truthiness level to that of generative AI, I won't be offended.