DC Wave Questions

Total and utter horseshit.

"DC" is simply the first (or "offset" term in the Fourier expression of any repetitive waveform.

"AC" are all of the remaining components.

Changing the relative amplitude of the terms does NOT in any manner change which is the first term and which are the remaining terms.

DC, of course, cannot exist at all ever. Because it would have to be unvarying through infinite time.

Tutorials on my website.

Boy, you are *pedantic*!

Can't we just define DC as current that doesn't vary "much" for at last a "long" time. Granted that is ambiguous, but what else would we the argue about, weather?

2. >

It is not changing polarity. I would hesitate to call it alternating current. On the "dc sine wave" issue, I wouldn't even get into that debate. To me the terms involved are open to too many interpretations. As evidenced in this thread, I suppose.

--- Well, Al, correction (no matter how gently offered) is often met with varying degrees of resistance, particularly by those who have become convinced that their way should be temporarily accepted as the 'right' way, for their convenience, regardless of whether their way conforms to universally accepted standards.

Case in point, the OP, whose attitude seems to be (and I paraphrase) "You know what I mean, so why should I have to say it your way?"

As for me, I'm perfectly capable of conducting myself politely in the presence of polite company. I'm also perfectly capable of atrocious behavior and have no qualms about stooping to that level if, in my opinion, the situation warrants it.

Finally, I don't see how you came to the conclusion that it pains me to engage in the edification of imbeciles. Actually, it's quite gratifying to be able to take on the challenge of breaking through the barrier of ignorance and realize that you've shown someone how to use a tool they didn't even know they owned.

-- John Fields Professional Circuit Designer

--- Your point is flawed. Alternating Current, by definition, causes electrons to move in one direction for a time, and then to reverse direction for a time. The sinusoidally varying unipolar voltage under consideration _always_ forces electrons to move in one direction only.

Since the voltage varies, the current will also, but the _direction_ in which the electrons are travelling will never change.

That means that the signal is DC. A varying DC, but DC nonetheless.

-- John Fields Professional Circuit Designer

No.

Sum a 1 volt peak sinewave with a 0.6 volt dc term and you have a waveform whose polarity continuously changes but whose average value is continuous.

Looking at the Fourier terms makes this waveform perfectly clear. Calling it "AC" or "DC" does not.

"AC" or "DC" are gross and meaningless oversimplifications.

Hey, you moved *my* goal post! I said nothing about average values. If it wiggles, it's AC. The difference is that you are being so precise that you're saying if it wiggled since the dawn of time, it's AC. I'm just saying that if it was so long ago that I can't remember (which seems to be a pretty short time anymore), that's long enough. :-)

EXACTLY! And while you and I can make jokes about just how pedantic we should be with definition of terms, the fact is that anyone who actually thinks "AC" and "DC" are the determinative definitions based on word meanings, is going to be wrong.

That isn't true.

A non-sequitor.

If it varies, it's AC.

If there is such a think as "varying DC", connect a load to it... through a capacitor. Now, how do you describe the effect that load has on your "varying DC". The load see's *only* AC, even according to your definition. That AC came from somewhere, and it certainly was not generated by the capacitor.

That's because AC is *not* defined by any change in direction, but only by a rate of movement change.

--- No, you have a waveform with a polarity which changes _periodically_, making it an AC signal. Do the electrons traversing the circuit change direction? Yes. Do the electrons in a DC circuit ever change direction? No.

Ergo, because of the periodic polarity reversals what you're looking at is AC.

---

--- Why go there? Your description was adequate to indicate that polarity reversals occur, therefore making the signal voltage alternate between two different polarities, therefore making the current alternate between polarities as well. That's why it's called "Alternating Current".

---

--- Uh-huh...

-- John Fields Professional Circuit Designer

It's not unusual to speak of the AC and DC _components_ of a waveform that does not readily satisfy the simplification.

One also speaks of _DC offset_ of an otherwise AC signal.

FWIW, most waveforms can be created as the sum of sine waves. I wrote an interesting computer demo once that showed how a sine and it's harmonics could be added graphically to form a better and better approximation of a square wave, running through what looked like Butterworth etc. responses.

Who do you want operating on your gall bladder? Someone who knows what he is doing or someone who is making it up as he goes along?

And, according to what you've said in other posts, if that were a

But your definition of AC is faulty, because in fact they are the same thing, and *both* of them contain an AC component and a DC component, even if the general direction of electrons is always the same.

Except, polarity reversals are not significant to the definition of AC.

He's right.

Didn't you just say in your immediately previous post:

---------------------------------------------------------------- ""DC" is simply the first (or "offset" term in the Fourier expression of any repetitive waveform.

"AC" are all of the remaining components."

---------------------------------------------------------------- Did you intend to offer these descriptions knowing that they were "gross and meaningless oversimplifications"? Why bother in that case?

Where *do* you get this requirement for changing polarity? We don't call it "Alternating Polarity", we call it "Alternating Current". If the current is being altered, it's AC. You keep talking about AP, and it isn't the same.

Look, if the current's not alternating its direction, it's not alternating current. To be clear, just because its amplitude is changing does not mean it is alternating. In particular, a rectified AC waveform it isn't changing direction.

There are certainly AC components in the waveform. But the sum of all the components, including the DC component, never changes direction, so the total signal is a DC signal. Here, "DC" does NOT mean "constant"; it means unidirectional.

So you are saying that DC varying from 5 to 15 as the op referenced is AC? If you put a DC source across a capacitor and vary the source up and down, sometimes electrons are flowing into the capacitor, and sometimes they are flowing out of it. Same with an inductor.

For the record, I don't want to take one side or another in the debate about AC vs DC in this thread. The waters are muddy enough already. I view the op's scenario as DC with an AC signal imposed on it.

This whole discussion of whether it is AC or DC is a trap and diversion from the original. It does not matter whether it is AC or DC that the components see. For example, a capacitor operates the same on DC as it does on AC. If there is a path for it to charge, and a source sufficient to charge it, it charges. If there is a path for it to discharge, and no source applied sufficient to keep it charged, it discharges. Same thing for an inductor below saturation.

The op asked about a sinusoidal varying DC, but gave no info about frequency. He then asks about impedance of the (unknown) RLC circuit. The answer has to be arrived at by a consideration of how each component reacts. To say (not that you said it) the cap won't pass DC is crap. Connect a 15 V, 500 ohm relay coil to ground, and the other side to a 470 uF cap. Connect the other side of the cap to +12. The relay energizes briefly, proving that the cap did pass DC. Try the same thing with a supply that starts at 5 volts and increases to 15 volts at a rate of 1 cycle per hour, and it does not energize. But the relay coil DOES charge. For the op to understand the load impedance, he has to understand what each component does in his circuit. I see no other way to answer his question, in the absence of specifics.

Ed

debate.

evidenced

You are the one with the requirements, assertions, and definitions, not me. Where are you coming up with them? If it's from the same place where

- zero current is not definable

- magnitude of current needs an outside reference

- voltage and current are for all practical purposes different expressions of the same thing

- alter is the same thing as alternate then I don't even want to know.

You need to come to realize there is no clear cut correct answer on this 'AC' vs 'DC' issue at this time. If there was one, there would be much more consensus between people on what the correct answer is. This big long thread would not have occurred. Now let's turn it around and look at it the other way. This big long thread did occur. We can plainly see that there is disagreement between groups on what exactly the precise meanings of AC and DC entail. Therefore there effectively is no single exact definition for "AC" or for "DC" that will allow us to resolve which is correct and which is not correct.

Picture my flashlight, battery powered. Generally this is considered a dc circuit. When I turn it on or off, there is 'change'. So is it in fact an AC flashlight? If the battery starts to die there is a change so is it in fact an AC battery? Etcetera. (These questions are rhetorical by the way). I know better than to try to pin a strict name on these things where there is not an (adequately) universal and strict definition.

On another note, how long are the days getting to be way up there? Do you get continuous sunshine?

Or, one might refer to this as 10VDC with an AC waveform superimposed. The AC waveform varies sinusoidally with 10 V peak-peak. One can then solve two circuits, the DC one with just R and 10VDC, and the AC one with R-L-C. Combining the currents from the two solutions should be equivalent to the original circuit.

I agree, that 'DC sine wave' is a misnomer. It makes it sounds like the speaker doesn't know AC from DC. In order to not sound foolish, it would be better to use one of the alternatives suggested.

daestrom