AC vs. DC - Page 1
Forum section: electronics / beginners

author: Analog Kid
author_rank: Super Contributor
timestamp: December 06, 2024, 05:43:15 am
content: This is a question about nomenclature, really, not electrical/electronic theory, although I'm sure this discussion will wander far into that field. Nomenclature as in "what do you call _______?".OK, so looking at these waveforms:we can say for sure that 1 and 2 are AC, correct? Because they alternate between positive and negative.But what about 3 and 4? 3 is half-wave rectified AC, which I've seen referred to as "pulsating DC". But are these two really DC, just because they never go negative?What would you call those two waveforms (leaving aside sine vs. square for the moment)?Have at it.

author: IanB
author_rank: Super Contributor
timestamp: December 06, 2024, 05:50:17 am
content: In precise language:"Pure" AC is a sine wave, single frequency, centered around zero volts."Pure" DC is a flat line voltage, with no time variation.Everything else is a weighted sum of AC components and possibly a DC component (offset).In imprecise or casual language, one might call certain things "DC" or "AC" just for convenience.

author: Nominal Animal
author_rank: Super Contributor
timestamp: December 06, 2024, 06:57:08 am
content: In imprecise but casual/intuitive, usually correctly understood language:Sinusoidal ACRectangular ACPositive sinusoidal regular pulses/pulse trainPositive rectangular regular pulses/pulse trainWhen I'm writing technical documentation or a scientific article, I strive for the precise language.When I'm writing user documentation or a blog post, I prefer precise language with imprecise intuitive descriptions in parentheses or footnotes.When I discuss things here or elsewhere, either online or in real life, I use the imprecise language.The point in all these is to get the underlying description, the message, across with minimal error. I simply adjust the language to suit the context, to have the best chance at that. I still fail occasionally, but it seems to me that this works better than the other alternatives I've tried. In particular, always striving for precise language tends to lead to language/standard/term-lawyerism, which I find a waste of time; and imprecise language makes science and engineering weak and error-prone.

author: pcprogrammer
author_rank: Super Contributor
timestamp: December 06, 2024, 07:41:21 am
content: Purely on definition of language, AC (alternating current) can be used for all four of them. Why, because the current is changing in all four cases. Alternate does not have to be from positive to negative per se.(https://www.merriam-webster.com/dictionary/alternate)QuoteGoing from 0 current to full current is also alternating between two states.DC (direct current) can only be used for the waveforms 3 and 4 from electrical perspective. (DC stands for Direct Current, which is electrical current that flows in one direction.) In these cases the current is alternating (changing) but only flows in a single direction.The wiki articlehttps://en.wikipedia.org/wiki/Direct_currentshows a graph that support IanB his statement.Quote from: IanB on December 06, 2024, 05:50:17 amMy own usage is AC for mains voltage and everything derived from it through a transformer. DC for power supply voltages that have been rectified and or stabilized. Anything else I call signals.Pure DC based on IanB his definition is in my opinion almost non existent. Think of ripple caused by current draw and noise induced from the environment. Even when using batteries there will be some noise induced in the wires connecting the system. Without load a battery might be considered pure DC.

author: u666sa
author_rank: Frequent Contributor
timestamp: December 06, 2024, 08:31:17 am
content: Pulsations.

author: Zero999
author_rank: Super Contributor
timestamp: December 06, 2024, 08:43:11 am
content: Quote from: Analog Kid on December 06, 2024, 05:43:15 amAll of the waveforms contain AC.#1 & #2 have no DC offset.#3 & #4 contain both AC and DC components.

author: shabaz
author_rank: Frequent Contributor
timestamp: December 06, 2024, 09:38:28 am
content: I would say, sometimes it makes no sense to try to describe things solely in words. There's the universal language of a diagram. We all have pens and pencils, why not use the tools we have. Then, the text can be reduced or simplified, with hopefully less chance of misinterpretation.

author: SteveThackery
author_rank: Super Contributor
timestamp: December 06, 2024, 01:53:41 pm
content: To me, using "alternating" as a synonym for "varying" (@pcprogrammer) seems completely wrong.In the context of voltage or current, "alternating" definitely involves polarity reversal,in my opinion. I accept that the dictionary definitions refer to two states and don't mention polarity, but those are generic definitions. I think in the specific case of electricity, polarity reversal is fundamentally relevant to the concept of "alternating ", and that without polarity reversal it is merely "varying".However, that is just my opinion! And there's only one person on the planet who cares what my opinion is. 😄I agree with@shabaz: in reality it isn't very fruitful to get into a war over the definition of a particular word, because words are always imprecise. There is no "official" definition of the word "alternating" because nobody has the authority to define it. Thus we use common sense and other means, such as illustrations.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 06, 2024, 08:02:29 pm
content: Yes; since we're of necessity speaking non-technically here, with some looseness of the terminology, I'd suggest this:I'd say the consensus of most folks working in the field (electricity & electronics) is that the first two waveforms definitely could be considered AC, even if the square wave doesn't fit the absolutely technical requirement of a sine wave.The last two, which were really the ones I was asking about, are more open to interpretation. I'd hazard a guess™, though, that the consensus wouldnotbe to consider them DC. So you could really call them any number of other things: "signal" comes to mind, and is vague enough to cover just about anything (except pure DC).

author: themadhippy
author_rank: Super Contributor
timestamp: December 06, 2024, 08:05:34 pm
content: is no3 hells bells?

author: golden_labels
author_rank: Super Contributor
timestamp: December 06, 2024, 09:39:49 pm
content: I permitted myself to redraw #3 and #4.No doubt signal shape didn’t change at all. Right? But what about your perception?As with many things, it’s nothing more than a convention. And we choose the convention that is most fitting and convenient to use for the given situation.In the strict sense #1 is AC. None is DC. All four are varying.But if your circuit doesn’t care about higher harmonics, #2 may also be considered AC. And if it also doesn’t care about offset, so is #4. All four may be seen as a sum of many smaller purely sinusoid AC signals.We may also describe #3 and #4 as pulsating, if we use the offset from your original picture. Which is not opposite of AC. Just describing a different aspect.

author: Sensorcat
author_rank: Regular Contributor
timestamp: December 06, 2024, 10:30:35 pm
content: Quote from: shabaz on December 06, 2024, 09:38:28 amYes, and the diagram happily jumps over all natural language barriers. Probably worth noting in the context of this forum.Also, EE became so big, it has internal language barriers, possibly already observable in this thread. Depending on the main field in EE someone has, the preferred terminology might be different. For instance, in communication/computers/control, pure sine waves or pure DC are of little interest, because they do not hold/transmit/store information, and these fields are all about information. In contrast, power/drives is interested in them, as they at least sometimes represent the desired state of stationary operation.

author: golden_labels
author_rank: Super Contributor
timestamp: December 06, 2024, 11:01:17 pm
content: Not sure, if it’s not too early. But you may take look at Paul Falstad’sFourier Series Applet(GPLv2). Switch to magnitue/phase view (checkbox in the bottom right). You may also enable sound there (though not always work). If you select “Square” you get #2. If you now rectify it, you get #4. Can you see any difference? The charts below the time-domain signal are pure frequencies into which this signal may be separated. The first, leftmost slider is the offset. Between the original and rectified version the only difference you may see or hear is that magnitude falls by half. You may use the slider offset to move the signal around and also see/hear no difference.

author: Xena E
author_rank: Frequent Contributor
timestamp: December 06, 2024, 11:06:04 pm
content: Quote from: themadhippy on December 06, 2024, 08:05:34 pmYup.Definitely AC/DC🤘😛

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 07, 2024, 01:07:55 am
content: A few other ways to look at it.1. If it is safe to use a properly oriented polarized electrolytic capacitor it is DC. If not it is AC.2. If it is applied to one side of a transformer and a steady state (another ambiguous term) output from another winding results it is AC. If not it is DC.There have been a number of other definitions in previous posts. All of which are correct in some way. The real message is that each way of naming these conveys some nuance of the behavior of the waveform. When you see these terms in a post or article the author was trying to communicate something when using them. And may or may not have succeeded. A lesson for all of us in our writing.The two definitions I have given are examples of definitions which focus on very narrow behavioral results. The second would call all of the pictured waveforms AC, but would call both a steady state voltage and a summation of a time varying voltage with a voltage offset large enough to cause core saturation DC.

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2024, 08:30:50 am
content: Connecting a DC source in series with #2, you get #4.

author: radiolistener
author_rank: Super Contributor
timestamp: December 07, 2024, 08:48:12 am
content: Quote from: Analog Kid on December 06, 2024, 05:43:15 amNot because they alternate between positive and negative, but because their level is altering.Quote from: Analog Kid on December 06, 2024, 05:43:15 amthese are the same AC, just shifted for some DC bias. In other word, this is a sum of AC+DC

author: Analog Kid
author_rank: Super Contributor
timestamp: December 07, 2024, 09:57:43 am
content: Quote from: radiolistener on December 07, 2024, 08:48:12 amNo. Waveform 1 "shifted for some DC bias" looks like this, not what I drew above:What I drew for waveform 3 was half-wave rectified AC, which completely cuts off the bottom half of the waveform. Not the same thing at all.

author: radiolistener
author_rank: Super Contributor
timestamp: December 07, 2024, 10:16:38 am
content: Quote from: Analog Kid on December 07, 2024, 09:57:43 amBoth are AC, just with different waveforms. Adding a DC bias shifts the waveform up or down relative to the zero value. So, you can add a DC bias to the first sine waveform, making it always positive, but it is still AC.DC is a straight line at a fixed level, while everything else is AC.Actually, even DC can be considered as a special case of an AC sine wave with zero frequency, where its value corresponds to the sine wave's amplitude at a fixed phase. Essentially, it represents a sine wave frozen in time.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 07, 2024, 10:58:37 am
content: But you're still wrong when you claim that my waveform 3 is my waveform 1 shifted by a DC bias. It's not, which is obvious from just looking at the pictures.

author: Sorama
author_rank: Frequent Contributor
timestamp: December 07, 2024, 11:04:09 am
content: If 1 thing is clear, then it's that it is not clear.Strange that basics like these can have so many different interpretations.

author: Aldo22
author_rank: Super Contributor
timestamp: December 07, 2024, 11:05:47 am
content: Do you even have to define it like that?Isn't it more a question of how you look at it, or what you want to measure?Can't every “signal” be measured as AC and DC? Perhaps one of the two is (almost) zero.With my ZT-225 you can display AC and DC as well as AC+DC on the same “signal”.Why would you assign it to one or the other?Just a noob question.

author: SteveThackery
author_rank: Super Contributor
timestamp: December 07, 2024, 11:16:19 am
content: Quote from: radiolistener on December 07, 2024, 10:16:38 amThat's your preferred definition, but not mine. I don't believe you have any authority to declare your definition as the correct one.The ONLY authority in the English language is common usage, and - as we can see from this debate - that is far from settled.Quote from: radiolistener on December 07, 2024, 10:16:38 amSophistry.

author: Xena E
author_rank: Frequent Contributor
timestamp: December 07, 2024, 02:20:48 pm
content: Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction.

author: SteveThackery
author_rank: Super Contributor
timestamp: December 07, 2024, 02:33:58 pm
content: Here's a little question to test your definition:A test point on an electrical circuit has a voltage which varies between 5V and 7V, and this variation is random. It contains no data or patterns. Over the long term the random variations are centred around 5V, so they contain no DC offset. Think of it as noise, if you will.At this test point, is the signal AC or DC?Well it isn't moving between two values, so the dictionary definition of "alternating" doesn't apply. No polarity reversals take place, so it doesn't meet my definition of alternating, either. Also, I'm certain that "moving around at random" is not part of any dictionary definition of "alternating".So: can something moving around at random be described as "alternating"? That word implies alternates: that is, two or more identifiable values. It does not seem to imply an infinite number of alternates, because then they wouldn't be alternates.I'm just trying to suggest that applying a one-word label to my example doesn't work. It is neither DC nor AC, and arguing the toss is pointless. It is something like: "6V DC with 2V p-p random noise added", or possibly "A voltage varying at random over all possible values between 5V and 7V". The description cannot be reduced any further.I deliberately said the signal contained a random element because I wanted to test the definition of "alternating ". Does alternating include "random"?Putting that aside.....If we simplify it by having a 2V p-p sine wave added to 6V DC, is the signal now AC or DC? I still argue that you cannot apply either label alone because it clearly contains aspects of both. I did mention earlier that instead of AC and DC we could legitimately refer to "the AC component" and "the DC component". This is much better because it embraces the the observed phenomenon more completely: the signal comprises two components summed: 6V DC and 2V p-p AC.To summarise: I suggest that a voltage varying sinusoidally between 5V and 7V should not be described as DC or AC. Rather, it should be described as having both a DC component (6V DC) and an alternating component (2Vp-p AC), and the actual signal is the sum of the two components.No more need to argue about what counts as DC and what counts as AC.

author: SteveThackery
author_rank: Super Contributor
timestamp: December 07, 2024, 02:35:29 pm
content: Quote from: Xena E on December 07, 2024, 02:20:48 pmYeah - polarity reversal is part of my definition of "alternating", too. However, not everyone agrees.....

author: Aldo22
author_rank: Super Contributor
timestamp: December 07, 2024, 03:36:04 pm
content: Quote from: SteveThackery on December 07, 2024, 02:33:58 pmYes, isn't that exactly what I was trying to say/ask/illustratehere?

author: Terry Bites
author_rank: Super Contributor
timestamp: December 07, 2024, 04:14:35 pm
content: Homework??

author: SteveThackery
author_rank: Super Contributor
timestamp: December 07, 2024, 05:10:44 pm
content: Quote from: Aldo22 on December 07, 2024, 03:36:04 pmYep. I'm adding my voice to those of us who think it's unhelpful, and possibly silly, to declare it to be AC or DC. It's a simplification too far. I particularly loved the contribution arguing that it isallAC, because DC is just AC with a frequency of 0Hz.

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2024, 07:11:40 pm
content: As is the case with many terms, it's dependant on the context.If it's a rectified waveform, from a transformer, then it makes sense to consider it to be a DC signal, with some ripple.If it's an audio signal, superimposed on a DC offset, say in an audio amplifier, then it's AC superimposed on a DC offset.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 07, 2024, 09:16:09 pm
content: Quote from: Terry Bites on December 07, 2024, 04:14:35 pmWas that somehow aimed at me, the OP here?Well, yes, thisishomework for me, in the sense that I'm always trying to properly educate myself in the ways of electronics.But no, it's not homework for any college course.Happy now?

author: golden_labels
author_rank: Super Contributor
timestamp: December 07, 2024, 11:35:03 pm
content: Quote from: Analog Kid on December 07, 2024, 09:16:09 pmDon’t get offended. I believe Terry Bites just needs their detector to be recalibrated.Please remember there are many people, who come for nothing more than to have their homework solved for them. I believe you’re not one of them, but it’s easy to misjudge.Quote from: Terry Bites on December 07, 2024, 04:14:35 pmAppreciated, but don’t you think it’s worth first checking OP’s behavior before jumping into conclusions? The line between spotting a troublemaker and being a jerk is thin. Analog Kid clearly does participate in this thread way beyond the original question.

author: radiolistener
author_rank: Super Contributor
timestamp: December 08, 2024, 01:08:55 am
content: Quote from: Analog Kid on December 07, 2024, 10:58:37 amWhere did you see that I claimed that "3 is 1 shifted by DC"?I never made such a claim.What I said is that 3 is an AC waveform with a DC bias shift.

author: radiolistener
author_rank: Super Contributor
timestamp: December 08, 2024, 01:16:41 am
content: Quote from: SteveThackery on December 07, 2024, 11:16:19 amThis is not sophistry, it’s simply mathematics. For example, consider the Fourier transform, which decomposes signals from the time domain into the frequency domain. In this context, a constant DC level is represented as a sine wave with zero frequency.When you take the Fourier transform of a signal, the DC component appears as a spike at zero frequency in the spectrum. This demonstrates that a constant value in the time domain corresponds mathematically to a sine wave with zero frequency in the frequency domain. It's a fundamental principle, not a rhetorical argument.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 08, 2024, 01:18:38 am
content: Quote from: radiolistener on December 08, 2024, 01:08:55 amI don't think even that is true. Can you show us how that's the case? (Draw a picture on the back of a napkin if you like.)An AC waveform with a DC bias shift would look like what I drew inreply #17 above, not like the half-wave rectified waveform.

author: Simmed
author_rank: Frequent Contributor
timestamp: December 08, 2024, 01:30:04 am
content: how about using math depiction ?V*sinωt ?

author: radiolistener
author_rank: Super Contributor
timestamp: December 08, 2024, 01:58:39 am
content: Quote from: Analog Kid on December 08, 2024, 01:18:38 amNo problem:1) AC sine - dotted blue line2) Rectified AC sine - red line3) DC bias + Rectified AC sine - solid blue line4) Rectified AC sine without DC bias - solid green line

author: SteveThackery
author_rank: Super Contributor
timestamp: December 08, 2024, 08:40:51 pm
content: Quote from: radiolistener on December 08, 2024, 01:16:41 amNo, I entirely agree with that. My point is that, in the context of finding suitable words to describe AC, DC and their combination, I don't think pointing out that DC is AC with zero frequency helps much. Mathematically it is totally correct, but I think it is useful to keep the concepts separate at this stage. Fourier transforms are for later.At least, that's my opinion. I wasn't expecting anyone to agree with me. 😄

author: EEVblog
author_rank: Unknown
timestamp: December 09, 2024, 12:00:36 am
content: The term DC on its own, without any other context, implies just what it says, a "straight" DC voltage without any variation.If it has variation and you don't specify that then the fault is on the laziness of the person using the term.So if you call a half or full wave rectifed AC signal "DC" just because the current doesn't reverse polarity, then expect to be taken to task for that poor description.Of course, engineering is a practical field, so in the practical real world "DC" almost means "DC with acceptably low ripple/noise", and no one will evert take you to task for that.e.g. There is no need to call a DC plugpack "DC with ripple and noise".Any competent engineer who uses it knows that a plugpack will have ripple and noise and other specs, and is going to have the weigh up that later. But they won't be upset that you said it's a "DC plugpack".

author: EEVblog
author_rank: Unknown
timestamp: December 09, 2024, 12:10:30 am
content: Quote from: SteveThackery on December 07, 2024, 11:16:19 amElectronics engineering is ultimately a practical field, so the only thing that matters is whether or not you adequately convey the required practical information to another engineer.To quote you:QuoteIf you say just AC or DC then you are guilty of poor engineering communication, and hence be prepared to be branded "wrong" and expect to get called out for it.Your boss and fellow engineers are going to be pretty pissed off if you said your module outputs "5V DC" and it jumps around like crazy randomly by several volts.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 09, 2024, 01:22:01 am
content: Let me try something else here, which may (or may not) shed some more light on the subject.Seems to me there are basically 3 main "flavors" of waveforms:(obviously the bottom-left quadrant, alternating but not varying, is nonexistent)There don't seem to be any accepted terms for these categories, except that alternating/varying is, in its pure form, AC, and not-alternating/not-varying is, again in its pure form, DC.But of course there are variants of each of these "flavors" which can be disputed.Maybe we need some more terms (or categories) for this?Perhaps for some well-known examples (half- or full-wave rectified sine wave, positive- or negative-going square-wave pulses, etc.)?Or do we only need, as some have suggested, to be precise in our description of any given waveform?

author: shabaz
author_rank: Frequent Contributor
timestamp: December 09, 2024, 01:44:43 am
content: Quote from: Analog Kid on December 09, 2024, 01:22:01 amDC is not just a straight line at a fixed level for many people; DC isn't an acronym owned by anyone, and for millions of people who are in technical engineering vocations and are indeed engineers, DC means some sort of connection with a pair of wires where one is always "plus" with respect to the other one, and if the signal doesn't visibly change level much on the multimeter, then that's maybe a bonus but not always.Even for design engineers who may well use equipment to observe signals in extreme detail, DC can still include a signal that varies over a period that realistically might not even be "long".Part of successful comprehension for engineers relies on recognizing context and recognizing when they need to give or seek additional information and knowing how to do that.Part of successfully conveying information comes from recognizing your audience.And, if you recognize that your audience needs particular levels of accuracy, then there's still really no point in trying to be accurate with written words alone, because if the audience is deeply technical, they will simply expect you to have provided things like diagrams, or maybe even photos or maths formulae, or additional material such as specifications or schematics, to understand the full detail of the signal of interest, to the level that they want or need to know at.My Proxxon hobby tools come with a power supply with the DC symbol on it, and that's absolutely the correct description for their customers, some of whom will be very technical if they are using such tools. The output, of course, looks nothing like a flat line by any stretch (another example would be old-school battery chargers, I expect, from memory).The manufacturer was still right to label it simply with a "DC" symbol with no further description.I don't think every situation needs to have precise well-defined words (which might not even translate well to hundreds of languages), and will still not be as good as annotated diagrams and numbers/maths, specifications, etc.

author: Xena E
author_rank: Frequent Contributor
timestamp: December 09, 2024, 02:56:15 am
content: Quote from: EEVblog on December 09, 2024, 12:10:30 amThis.When using language to describe a generalisation such as this, the context has to be stated as well as just trying to qualify electricity as AC, DC, Zero frequency AC, slowly varying AC, or fairydust cotton candy farts, as one would when defining a voltage source in a simulation for instance.OTOH, youre not going to put "Zero frequency AC output adaptor" on the label of your wall wart case, because you will look like a complete twat, and Joe public won't understand what that means.This is another waste of time. This thread is just an example of the TS trying to start controversy for it's own sake by picking abstract subjects, claiming confusion and wanting clarification which is a trait that is now abundantly clear, (and hats off to him, he's very good at it, and playing you all like a fiddle).X

author: Analog Kid
author_rank: Super Contributor
timestamp: December 09, 2024, 03:29:01 am
content: Quote from: Xena E on December 09, 2024, 02:56:15 amOh yeah, you nailed it: I only start these threads because I'm a stupid troll who doesn't give a shit about what I'm writing about, only the reaction it will elicit. Not.Methinks that perhaps you're the twat here ...

author: Xena E
author_rank: Frequent Contributor
timestamp: December 09, 2024, 03:54:44 am
content: Truth hurts, dont it?

author: EEVblog
author_rank: Unknown
timestamp: December 09, 2024, 05:15:40 am
content: Quote from: Analog Kid on December 09, 2024, 01:22:01 amAgain, it's about what you want to and need to convey as an engineer, often taking into account who the recipient is, and the context.Again, as discussed, that might include putting the onus on the recipient to determine if the description is adequate.Same with all aspects of electronics engineering. Take a multimeter spec for example, it's usually sufficient to say it's a 0.05% accuracy multimeter. But some people need to know it's a "0.05% +2 digits @20degC +/- 5degC with 1ppm/year aging" meter.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 09, 2024, 05:38:20 am
content: OK, so I'll stick with the last part of what I wrote there, edited:QuoteWorks for me.