Please correct me if I'm wrong about AC/DC understanding

[cowboybob]

"Alternating" being the operative, definitive adjectival gerund...

To put a finer point on it, the alternating voltage needs to be up and down in value, over a specified period of time, not merely a linear change like, say, the simple depletion or recharging of a battery.

Within the OP's graphs second example, it is a DC biased AC signal; the two elements (DC and AC) need to be considered apart from one another when discussing the waveform's (or "signal's") definition(s).

The bias is clearly DC (NOT alternating) while the voltage riding the bias is clearly AC (alternating). As we all know, stick a cap in the signal path and the DC disappears, leaving the AC.

Now, what of the case of a positive (or negative), slowly alternating DC bias (therefore an AC signal) upon which is riding a more (or less) rapidly varying AC component? Here again, the two AC components would merge (mix) such that the resulting signal would still be composed of a single AC signal riding a DC bias, more properly considered a biased modulation. The subsequent signal might well pass across zero, but the DC part could still be isolated from the AC part for definition purposes.

 

[MrAl]

Hi,

A couple points here...

First, when i try to explain a signal like the second one i try to give the most concise and descriptive explanation possible from the standpoint of electrical engineering, or at the very least the one that i think is the most concise and descriptive at the time of writing subject to change only by updated theory that i know of on the subject.

With that in mind, we can note that the first signal can be stated mathematically as:
V1=Vpk*sin(w*t)

where Vpk is the amplitude of the sine (and is positive) and and the second signal as:
V2=Vpk*sin(w*t)+V0 {Vpk<V0}

where V0 is the DC offset.

It should be clear that V1 is an AC voltage. For V2 with the constraint given {Vpk<V0} that means the entire waveform is above zero, but note that it can also be written as:
V2=V1+V0

and that's exactly how a signal like that in electrical engineering would be represented, and it should be clear that if it takes an AC signal added to a DC signal to make up the entire signal, then calling it simply "DC" is not a good enough description.

For a second example, note that if we call the second signal originally given "DC", then what do we call a perfectly straight horizontal line? We absolutely MUST call the straight line DC, but if we call them both DC then we have no way of knowing that the second signal originally given has an alternating 'component' to it as well as a pure DC component.
In circuit analysis this would be found from the Fourier components, where the first signal would have zero DC and the fundamental, while the second signal would have both the fundamental and a non zero DC value.

There will be contradictions to this way of thinking because the second waveform given is all above zero and therefore the current always flows in one direction, albeit it changes level over time. But when this happens there is almost always another term used to describe this action to show how it is different than pure DC. For the example of the full wave rectified waveform with no capacitance, the phrase usually used is "pulsating DC" to distinguish it from pure DC.

Also, when charging a battery with the second waveform, the battery still charges. The difference is that it does not charge at a single rate but the charge rate varies. It will be higher some times than other times. The average charge rate would be given by the mean value of the waveform which is the DC component. This makes it seem like it should be "DC", but again we could easily show a circuit that would not respond in the same way that it would respond with a pure DC signal.

So there is a difference and there should be some way to describe this difference. In electrical engineering it is described by the DC and AC components, and in more common language it is called "pulsating DC". Think of what would happen if we decided to no longer recognize ripple voltage.

A ramping signal still contains AC components even though we dont see anything going up AND down.

There will be times when this is more or less important. For example, for a battery discharging over time the DC voltage goes down slowly. It might be 12.000v at 9am, then 11.999 at 10am, then 11.998 at 11am, etc.. We usually refer to this as DC but a more exact analysis would show that it still has AC components.

So in short i think at the very least it should be called "pulsating DC" to distinguish it from pure steady DC.

 

[crutschow]

As Nigel noted, AC signals are not just pure sinewaves, they are any time-varying signal. Of course any such signal can be broken down into a series of sinewaves using Fourier analysis.

An signal that can pass through an arbitrarily large capacitor is an AC signal, even the slow voltage ramp of a discharging battery.

 

[KeepItSimpleStupid]

Wikipedia makes the distinctions of:

pulsating
direct
variable
alternating

I think AC/DC is basically a colloquial expression. Occasionally I here the term pulsating DC, usually when talking about battery chargers.

Clearly AC coupling is not strictly defined. It is really capacitive coupling, but who wants to use such big words. See: https://en.wikipedia.org/wiki/Coupling_(electronics)

The "world" or "discipline subset" we are in often defines the terms. e.g. In air conditioning a 7/8 fitting is the same as a 3/4 plumbing fitting.

 

[MrAl]

Hi again,

KISS:
I think it is good that you brought the idea of a subset of a branch of knowledge into the discussion, and i'd like to continue that trend by mentioning the idea of a nomenclature subset which would be similar.
In one group we might find ourselves in the general consensus might be to call the second wave just DC without getting too much more technical about it. In another this would be totally and completely unacceptable.
"12v DC Wall Wart" comes to mind, where we definitely have ripple but we choose to ignore it in order to simplify the discussion. It's only when we go to replace a broken wall wart that we may need to find out if it needs to be pure DC (as a regulated model would be) or a regular old rectifier and filter cap type would work ok.

 

[KeepItSimpleStupid]

Yep, and we never say we need a 12 V, -100 W power supply or talk about a -8 MW nuclear plant. Try analyzing circuits without the sign.

So we have a -12 W power supply and a 12 W load; 12 W in = 12 W out or 0=-12+12

The + sign means power is dissipated and not generated.

Another that comes to mind is: "The red wire is positive". Not always true.

 

[crutschow]

..................................
Another that comes to mind is: "The red wire is positive". Not always true.

And I still have to think twice to remember that the black, not the white, is the hot wire in my house wiring.

 

[Nigel Goodwin]

Another that comes to mind is: "The red wire is positive". Not always true.

Don't EVER rely on that with car wiring - where VERY commonly red is negative and black is positive

Particularly in the wiring for car radios and accessories.

 

[flash_os]

Note how he's talking about currents, not voltages.

Get 500V battery. Put a load on it so that current flows. Get AC from the grid. Connect some load too. Ground them both - AC neutral wire and battery "-" go to the ground.

Two separate and unconnected currents flow - one is pure DC, other is pure AC.

Stick voltmeter between battery "-" and hot AC wire - what you get is the voltage graph from your first picture.

Stick voltmeter between battery "+" and AC hot wire - you get the voltage graph from your second picture.

Of course, currents were absolutely the same for both measurements.

I hope that all of us agree that AC and DC differ current flows direction?!
If simplify our problem...

So let judge us by simple (dc) motor which always rotate according to current flows direction.
On above case... in which direction will motor rotate? In one direction (DC) or back and forth direction (AC)?

 

[Ian Rogers]

So let judge us by simple (dc) motor which always rotate according to current flows direction.
On above case... in which direction will motor rotate? In one direction (DC) or back and forth direction (AC)?

Bad example!!!

AC motors run in one direction... A DC motor on AC wouldn't be a good idea...

I don't care what has been said...... AC stands for Alternating current! End of! If you superimpose an AC current onto a DC voltage, as long as the voltage doesn't go negative, the AC becomes a varying DC at that point...

If I connect a linear pot to a 5v rail and twist the pot back and forth fast.... I HAVEN'T CREATED AC.....

 

[crutschow]

...............................
If I connect a linear pot to a 5v rail and twist the pot back and forth fast.... I HAVEN'T CREATED AC.....

And I think you have created AC with a DC bias. The signal flowing through an AC amplifier is still AC even if it's biased at some DC level by the internal DC bias levels in the amp, no?

 

[Ian Rogers]

And I think you have created AC with a DC bias. The signal flowing through an AC amplifier is still AC even if it's biased at some DC level by the internal DC bias levels in the amp, no?

This is the confusion..... You are talking about an Alternating voltage..... If the current in a DC circuit is omnidirectional then how can you even call it AC..... Nigel has the idea that if you reference the circuit from the center of the AC waveform you would then be correct....

Take a battery.... 6 volt.... then inject a 1v pk to pk around the 6v point then stop time at 6, 6.5 and 5,5v and observe the current....

I have said that almost everyone calls it AC and I agree, but!!!! Its the wrong phrase.... simply a signal injected onto DC is correct...

 

[fezder]

Don't EVER rely on that with car wiring - where VERY commonly red is negative and black is positive

Particularly in the wiring for car radios and accessories.

yeah, i'we heard some english (?) cars have case POSITIVE. Hehe, my father fears nowadays car batteries when i told current levels and other ''flashy'' stuff that can happen with car batteries. Me, i dont fear, i respect. Totally different. Same applies to all machines i use, (chainsaws etc, you get the picture) i don't stress them, give them some eating, and they don't bite .

 

[crutschow]

This is the confusion..... You are talking about an Alternating voltage..... If the current in a DC circuit is omnidirectional then how can you even call it AC..... Nigel has the idea that if you reference the circuit from the center of the AC waveform you would then be correct....

Take a battery.... 6 volt.... then inject a 1v pk to pk around the 6v point then stop time at 6, 6.5 and 5,5v and observe the current....

I have said that almost everyone calls it AC and I agree, but!!!! Its the wrong phrase.... simply a signal injected onto DC is correct...

And what do you call that "signal"?

Okay. You are arguing semantics. But, to me, the common meaning of a phase is the proper semantic. No one calls a signal AV instead of AC just because it's technically not current alternating back and forth. If you think you can convince anyone to do that, well good luck.

 

[MrAl]

Hi,

I dont think there is that much of a distinction between AC current and voltage. We call it AC and AC stands for "Alternating Current", but we also call voltages AC such as "120vac" or "230vac", and if we have an alternating current then we almost always have an alternating voltage, and if we have an alternating voltage then we also have an alternating current somewhere in the circuit. The only way we could have an alternating voltage without an alternating current is with an infinitely high impedance or some weird non linear impedance designed just for that purpose. Even linear impedance will draw an AC current from an AC source and produce an AC voltage from an AC current source.

As for the motor example, i think what they mean was if you use a DC motor and drive it with either DC or AC. In this kind of device if we drive it with a DC voltage it draws an DC current and it turns in one direction, and if we reverse the polarity it turns in the other direction. If we drive it with normal lnie frequency AC then it does not turn at all but might vibrate a little as the shaft tries to turn back and forth. If we drive it with much lower frequency AC then it might turn in one direction and then later sometime stop then turn in the other direction, etc. If we drive it with a DC voltage that goes up and down a little (say 10v to 12v then back to 10v then back to 12v etc.) then the motor turns in one direction only but it speeds up and slows down, speeds up and slows down, etc. So we see a few different reactions to the different excitations:
1. Turns in one direction at constant speed [DC]
2. Turns in one direction, then in the other direction [DC then -DC]
3. Turns back and forth so fast that it seems to be vibrating not turning much [fast AC]
4. Turns in one direction, then stops, then turns in the other direction, then repeats [slow AC]
5. Turns in one direction only but slows down and speeds up repeatedly [DC+AC]

Now i ask the question that i think everyone should try to answer, at least for themselves:
I apply unipolar DC to the motor, what does the motor shaft do, is it:
A. "#1 alone", or
B. "#5 alone", or
C. "either #1 or #5"
D. "not possible to tell"

Note that i have not specified if the DC is constant or not yet, but according to some definitions this isnt necessary.

 

[throbscottle]

So if I connect 2 fixed resistors across a 5v supply, so I get 2.5v, and connect a pot across the 5v rails, twist it fast, then measure the wiper relative to the 2.5v point - THEN I can call it AC, can't I? (sorry to be devils advocate here )

About motors - I've connected a brushed AC motor across a battery, and it turned. So, if I connect a brushed DC motor across an AC supply, will it not also turn (in one direction)?

But the really important point everyone has been missing since post #7 is, AC/DC are Australian? And all these years I thought they were from the US. Jeez....

 

[Ian Rogers]

Okay. You are arguing semantics. But, to me, the common meaning of a phase is the proper semantic. No one calls a signal AV instead of AC just because it's technically not current alternating back and forth. If you think you can convince anyone to do that, well good luck.

I agree with the semantics..... If the whole of electronic mankind wish it to be AC!!! Who am I to stand in their way.... It's just that it is a subject that newbies come across and really need all sides.... Nigel, You MrAl and I are all correct but the correctness has a tad of uncertainty

 

[Ian Rogers]

AC/DC where brilliant back in 1979.... I saw them at Leicester De Montfort Hall... Bon Scott was still alive and singing carrying Angus on his shoulders around the audience...

Wonderful times.... No Electronic debates atall!!!

 

[crutschow]

.................................
About motors - I've connected a brushed AC motor across a battery, and it turned. So, if I connect a brushed DC motor across an AC supply, will it not also turn (in one direction)?
..............................

If it's a universal type brushed motor with the field generated by the input power (not a permanent magnet type) then it will run on AC or DC and will turn in only one direction with either type of voltage (oops, current ).

 

[throbscottle]

Ah, I understand now