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

[flash_os]

AC when current goes back and forth, between plus and minus.
and DC when current goes to the same direction, in range plus or minus.
Is it correct?
**broken link removed**
**broken link removed**

 

[MrAl]

Hi,

A signal can contain both AC and DC. Your second plot shows AC + DC not just DC.

DC is a perfectly straight line across the screen, horizontally. Any movement up and down means there is also AC. If the movement is up and down and the average value over all time is 0, then it is pure AC like your first plot shows.

 

[Ian Rogers]

Not sure I entirely agree here.... The second plot is DC.. The plot looks like there was an AC signal within a DC signal BUT!! for a current to be AC the current has to alternate it's direction... Here it is a DC signal that is always a positive direction but just less or more voltage..... I think I see the question in a slightly different scope...

 

[Nigel Goodwin]

The second plot is DC.. The plot looks like there was an AC signal within a DC signal BUT!! for a current to be AC the current has to alternate it's direction.

That's where we disagree

I'm with MrAl it's an AC signal sat on a DC level - the only difference from what you seem to consider 'AC' is the point of reference.

 

[NorthGuy]

AC - Alternating Current. By looking at the voltage it's impossible to tell if current changes direction (or is alternating) or not.

 

[flash_os]

"A DC voltage is always positive (or always negative), but it may increase and decrease."
Source: https://electronicsclub.info/acdc.htm

 

[ronsimpson]

Here I thought; AC/DC is an Australian hard rock band, formed in November 1973 by brothers Malcolm and Angus Young.

 

[audioguru]

In a DC voltage that is modulated up and down in level then the modulation is alternating the level, it is not alternating the polarity. So the DC has AC level modulation on it.

 

[KeepItSimpleStupid]

We can get wierd about it. When I first learned about DC, where basically talking about a battery that supplies a relatively steady voltage.

Let's fast forward a bit and talk about two other items:

1. A DC powered radio
2. A car's electrical system
3. A flashlight using 2 D batteries.

A flashlight is truely DC, the others are "called DC", but in the limit they aren't. They are a nominal steady state value with ripple or variations and/or transients. The battery has a v(t), voltage with respect to time, because it discharges slowly.

So, for now AC and DC are rarely obtainable. The sources approach an AC and DC, but are not mathematically: DC v(t) = k; for all t and AC: v(t) = Vp*sin(ωt+θ)

Another thing to bring up is that scopes have AC and DC coupling. With AC coupling, you will see only the part that is varying with time. When Direct Coupled, you will see both.

I think in both cases, you can apply the term nominal and sort out the difference.

 

[flash_os]

Here the answer from John Hewes (electronicsclub.info)

If the current is always flowing in one direction it is DC. The difficulty comes when the current is varying but never changing direction - I like to describe that as 'varying DC' to distinguish it from the simple case of constant DC.

Where the variation is small relative to the steady part of the DC, for example the ripple present on the output of a power supply which has been rectified and smoothed, I think most people will be happy to describe it as a form of DC.

Where the variation is large (but the current never changes direction) I say it is varying DC but some people will prefer to describe it as AC superimposed on steady DC - that is perfectly valid, it is just another way of describing it, and it is certainly the best description if the variation is so large that it is greater than the steady DC, making the current change direction.

Context is important in determining the best description when you are not dealing with simple situations.

I hope that helps!

John Hewes
electronicsclub.info

 

[NorthGuy]

Here the answer from John Hewes (electronicsclub.info)

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.

 

[JimB]

If we wind back the clock one hundred and fifty years or more, the early electrical pioneers would say without doubt that the first case was AC and the second case was DC.

Back in the present day we would certainly say that the first case was AC, but in the second case the answer to is it AC or DC would depend who you asked.

If you ask the guy who is concerned with a bank of batteries, he would measure it with a meter and say it is DC, if he looked at it with an oscilloscope he would say there is a bit of hum on there, probably from the charger.
If we ask the guy working on the audio amplifier, he may say that there is a nice clean sine wave signal on the collector of that transistor, and also the DC voltage is correct.

I suggest that there is no simple answer, and there are several ways of describing the waveform in the second case, and all of them are correct in their own way.

JimB

 

[Ian Rogers]

That's where we disagree

I'm with MrAl it's an AC signal sat on a DC level - the only difference from what you seem to consider 'AC' is the point of reference.

I agree with that statement... But!! The reference in the plot is an implicit 0v as shown.... the "AC" wave is above the reference so there is only one current direction.... I do, however, realize that we have the saying "Remove the DC content"... But that would change the plot back to the first one.... AC and DC only refer to current not voltage...

 

[crutschow]

I agree with that statement... But!! The reference in the plot is an implicit 0v as shown.... the "AC" wave is above the reference so there is only one current direction.... I do, however, realize that we have the saying "Remove the DC content"... But that would change the plot back to the first one.... AC and DC only refer to current not voltage...

Literally AC and DC means Alternating Current and Direct Current but in normal use it refers to voltage as well as current. Thus when you say you have an AC source it typically refers to the alternating voltage (which generates alternating current if connected to a load).

And whether the AC voltage has a DC offset or not does not change the AC component of that signal (i.e. it doesn't actually have to go plus and minus around zero to be AC, it just has to vary in voltage). For example an unfiltered rectified sine-wave has an average DC value with an AC component (the part that would go through a capacitor).

 

[Nigel Goodwin]

I agree with that statement... But!! The reference in the plot is an implicit 0v as shown.... the "AC" wave is above the reference so there is only one current direction.....

Again Ian, everything has to be referenced to 'something' - yet referencing it differently doesn't change what it is - AC is AC regardless of what you use as a reference to measure it.

Regarding just the sinewave in either of the diagrams, if you use the positive side of the sinewave as reference then it's an entirely negative going sinewave, likewise if you use the negative side then it's purely positive going - in either diagram both are EXACTLY the same AC waveform.

Having a DC shift on an AC waveform relative to some arbitrary point doesn't mean it's not AC - if it wasn't AC it wouldn't pass through a capacitor and almost all analogue circuits would cease to function

 

[Ian Rogers]

Like I said.... I agree with your theory... Its just the wording "AC" it should just be "Variable Voltage" as AC by definition ( In this application ) is wrong... the current is not changing direction....

 

[Nigel Goodwin]

Like I said.... I agree with your theory... Its just the wording "AC" it should just be "Variable Voltage" as AC by definition ( In this application ) is wrong... the current is not changing direction....

It is changing direction if you use the correct reference point - the 'failing' is simply how you're measuring it.

 

[Ian Rogers]

I was only talking about the examples posted above..... I know EVERYONE calls it AC... so as an EVERYONE, I'll agree..... This is exactly why they renamed voltage rails to potential difference....

 

[willeng]

As a beginner I find this interesting & have a couple of questions.
Please knock me on the head where due!

Don't all AC waveforms consist of a "zero voltage line" that divides the waveform into two symmetrical halves or other wise it's not AC at all.
The AC alternator works in just that manner.

The above "DC offset Sine Wave" isn't actually an AC waveform is it as it never passes through a ZERO point.

Another question Nigel mentioned that if it wasn't AC it wouldn't pass through a capacitor?

This is not entirely true is it because you can have a ramped or time varying DC signal & it does pass through a capacitor?

Shouldn't it be said that only time varying signals will pass through a capacitor?

Probably have my foot in my mouth here but that's life.

Cheers

 

[Nigel Goodwin]

Another question Nigel mentioned that if it wasn't AC it wouldn't pass through a capacitor?

This is not entirely true is it because you can have a ramped or time varying DC signal & it does pass through a capacitor?

Shouldn't it be said that only time varying signals will pass through a capacitor?

Yes it could - but that's just another name for AC

AC signals are simply varying ones.