AC vs. DC

[Analog Kid]

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.

[IanB]

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.

[Nominal Animal]

In imprecise but casual/intuitive, usually correctly understood language:

• Sinusoidal AC
• Rectangular AC
• Positive sinusoidal regular pulses/pulse train
• Positive rectangular regular pulses/pulse train

When 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.

[pcprogrammer]

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)

to change from one to another repeatedly

Going 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 article https://en.wikipedia.org/wiki/Direct_current shows a graph that support IanB his statement.

"Pure" AC is a sine wave, single frequency, centered around zero volts.

"Pure" DC is a flat line voltage, with no time variation.

My 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.

[u666sa]

Pulsations.

[Zero999]

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:

(Attachment Link)

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.

All of the waveforms contain AC.

#1 & #2 have no DC offset.

#3 & #4 contain both AC and DC components.

[shabaz]

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.

[SteveThackery]

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.

[Analog Kid]

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 would not be 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).

[themadhippy]

is no3 hells bells?

[golden_labels]

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.

[Sensorcat]

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.

Yes, 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.

[golden_labels]

Not sure, if it’s not too early. But you may take look at Paul Falstad’s Fourier 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.

[Xena E]

is no3 hells bells?

Yup.

Definitely AC/DC

🤘😛

[CatalinaWOW]

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. 

[Zero999]

Connecting a DC source in series with #2, you get #4.

[radiolistener]

Because they alternate between positive and negative.

Not because they alternate between positive and negative, but because their level is altering.

But what about 3 and 4?

these are the same AC, just shifted for some DC bias. In other word, this is a sum of AC+DC

[Analog Kid]

But what about 3 and 4?

these are the same AC, just shifted for some DC bias. In other word, this is a sum of AC+DC

No. 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.

[radiolistener]

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.

Both 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.

[Analog Kid]

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.

[Sorama]

If 1 thing is clear, then it's that it is not clear.

Strange that basics like these can have so many different interpretations.

[Aldo22]

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.

[SteveThackery]

DC is a straight line at a fixed level, while everything else is AC. 

That'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.

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.

Sophistry.

[Xena E]

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.

[SteveThackery]

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.