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. 👶
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, <southpark skier voice>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.