DC Wave Questions

There is no contradiction, so what is wrong with understanding both statements?

Bad example. That does *not* require a direction reversal. All it requires is understanding that it is relative to the static state.

It does happen that the static state in that specific case is when a polarity reversal takes place, but in the general case it is not required. In other examples both sides of the switch might well be at some DC potential, that happens to be equal on both sides at the time the switch is made, even though there is no direction reversal.

Everything concerned with reactance is AC specific. Nothing concerned with reactance requires a polarity reversal. Reactance is the essence of the difference between DC and AC, not some notion of reversing polarity.

Which means it is worthless. Reactance *is* the significance.

All of which *does* have practical value.

That is an hilarious idea! If the magnitude is zero all the way around... we aren't talking about AC or DC... maybe about blown breakers or taking a coffee break, but not about current.

Oh? DC doesn't exist? What about "steady AC"? (That's two exactly equal signals 180 degrees out of phase, combined in that capacitor which can generate AC mentioned by John Fields, perhaps???)

Then why would we be concerned at all about this reversing direction, and give it a specific name and have a whole separate field of study for it? Sounds like we need to be concerned with varying magnitude, *not* with reversing direction. (Which is what I've been saying...)

A nice goal.

Except that alternating direction has no significance. Changing magnitude does. Why bother with alternating-direction at all, it is just an insignificant, though interesting, part of the more general case of changing magnitude. All of the same equations apply.

And if you claim that only alternating direction current is AC, then you have to have two sets of equations for DC, one for non-varying magnitude and one for varying magnitude.

That doesn't make a lick of sense.

(snip)

True mathematical sine waves extend from infinite negative time to infinite positive time. Practical sine waves last long enough for things to respond to their frequency. How long that is, depends o what is reacting to it. A frequency counter operating in period mode needs only a single cycle to make its measurement. An ear needs several cycles to several dozen cycles, depending on exactly what part of the audible spectrum being detected (this property of ears is part o the MP3 music encoding scheme). A quartz lattice filter may need thousands of cycles to of a pure frequency before it develops a nearly steady state output.

Something like that. Each frequency component in the signal has to last long enough for the time response of that frequency of the ear's sorting system to respond to it. If two frequencies fall within a single reception band, they are not heard as two tones, but as a beat addition and cancellation) as a single tone at about the average of the two frequencies and an AM modulation at the difference of the two frequencies. Obviously, if the beat is very long period, you have to hear the two beating tomes for a cycle or two of the beat period to detect that effect. Harmonically related tones just produce a repeating pattern at some integer multiples of each of the component frequencies. This can produce a very pleasing effect. You hear sound from one musical source as a fundamental and several harmonically related frequencies. If a second musical source (a harmonizing voice, for example) has its fundamental at one of the harmonics of the other signal, your brain recognizes this simple multiple relationship as a pleasing musical harmony. For some ratios. This page shows some of the approximate ratios between notes that sound interesting together:

Not really. since linear circuit components react to many frequencies by the addition if the effect of each frequency, it is a very powerful analytical procedure to break a signal down into its harmonics and evaluate the response of a circuit to each of those harmonics, and add all the effects together to get the total response.

Symmetrical distortion of a sine wave (shape of positive half cycle is a mirror image of that on the negative half cycle) can be shown to be made up of only the fundamental and odd harmonics (3 times. 5 times, etc.). If the distortion peaks up one half cycle and flattens the other or shifts the zero crossing so that one half cycle lasts longer than the other, there are even harmonics in the wave shape. There may also be odd ones, too. Got to do that Fourier analysis to quantify that.

You can with a resistor. From a Fourier perspective, that flat part just represents a time when the curve of some frequencies is nearly canceled by the curve from other frequencies. You need an infinite number of harmonics to make a truly flat square wave with perfectly square corners.

I hate it when that happens.

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One statement (quoted in full above) that (even though wrong) at least has something to do with the topic, Six out of seven comments are piddly attempts a childish and gratuitous insults.

No discussion John. I don't waste time teaching basics to grown men who have temper tantrums in public.

You're confusing a temper tantrum with getting a dressing down you thoroughly deserve. You're a poser pretending to knowledge and skills you don't have, for the purpose of elevating yourself to a station which you yearn to occupy, but can't.

If you wish to end the discussion or exit the thread, then just do it. There's really no need for parting shots unless you find it necessary to have the last word before you run away.

^^^^^^^^^^^^

Posting public articles like the above may well cost you employment in the future.

The problem with that definition is that it is unnecessarily limiting. You can find other sources where the definition reads "magnitude *or* direction," the latter which I believe to be more correct. If the signal is steady state, then the current that changes magnitude but never direction is simply an AC signal with a DC component greater in positive amplitude than the negative peak of the AC component.

Well, "intrinsic" may be a bit to strong of a word. Boiling and freezing points very considerably with pressure.

--Mac

me.

If you have definitions of AC and DC handy from IEEE or someone, stick them on here. I'd say that the (apparent) widespread disagreement means that, functionally, there is no single pervasive definition for these terms, but it would be interesting to see if some of these bodies have published definitions. It would be really interesting if they had definitions, and they didn't quite agree with one another, or if they were "wishy-washy".

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"show why the answers must be obvious" sounds like a peculiar concept. An obvious answer inherently needs no explaining.

You feel that neither AC nor DC is correct as a description for the flashlight behavior. I wonder whether there is a sufficiently definitive (and also agreeable) meaning of "AC" or of "DC" that would merit this position. I infer from other posts that there are people who would say it is DC. There may be others who would say it is AC. So a statement of fact that it is neither AC nor DC is suspect. By the way, my rhetorical questions were actually whether it is an AC flashlight and whether it is an AC battery. The point of this was (I thought obviously) to illuminate the difficulty in declaring some things to be AC or to be DC. There is a certain ridiculousness (I thought obviously) in calling a flashlight AC or in calling a battery AC. Yet it would be awkward (I thought obviously) in adhering to calling it DC if one's description of DC was that the (voltage / current) would essentially remain constant. Thus my (possibly obvious) point and my rhetoricals. I'll work on them. Your response did not seem to conradict my point.

The web has lots of information and lots of misinformation, I think you'd agree. I'm not sure that I have displayed curiosity in these posts. An education in the field of circuit analysis? No, thanks, you go ahead.

That's remarkably funny. What do you operate? Not your brain I wouldn't guess.

I simply feel it's prudent to carefully consider all responses, as I've grown rather un-fond of the taste of my foot in my mouth. I agree that the poster's attitude is obtuse at best, but flinging off-hand insults is risky. I should know.

Besides, right now I'm the new guy in a new job after 30 years in electronics, so I have to practice restraint. Practice, practice, practice! :-)

And purity and other factors.

The difficulty is understanding just what Don was getting at. His first post in its entirety was:

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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.

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

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If he thinks "AC" and "DC" are gross and meaningless oversimplifications, why would he offer alternative definitions of AC and DC to those given by Penoyer which Don thinks are "Total and utter horseshit"? Why offer definitions of AC and DC at all if he thinks so poorly of the terms? Because when people see him first disparaging someone else's definitions and then offering definitions of his own, they're going to think he believes his own definitions are good ones.

On Mon, 13 Jun 2005 21:28:49 -0500, operator jay wrote: [snip]

There are really only two definitions put forth in this thread. One is that AC refers to all non-zero frequency components of a signal, and DC refers to the zero frequency (average) component of a signal.

The other camp believes that DC means a current whose direction doesn't change, and AC means a current whose direction does change.

For me, the fact that a Voltage can be called DC proves that the other camp is not right, or not entirely right. In fact, the OP was talking about a DC Voltage. There was no mention of current at all.

--Mac

When the context is clear, people sometimes use the same word (especially, informally) to mean one of several different things in different contexts. The problem with the original post was that the poster meant the opposite of what most of us would have assumed the context implied, so we tried to explain that to him. Then he told us either we were wrong or that it didn't matter, if we were capable of figuring out what he meant. Some people are harder to help than others. ;-)

Not to mention with *what* it is!

I knew a fellow one time who put together a nice little experiment where he bolted everything together with nice shiny nickel-cadmium plated screws.

He then proceeded to boil the ni-cad coating off the screws, which plated the expensive ceramic insulators and shorted out his thermionic diode.

Danged, several weeks of work shot because it just hadn't occurred to him that ni-cad would boil at room temperature.

He eventually made it work though, using stainless steel hardware, and when he wrote a thesis about what his diode did and didn't do, they awarded him a PhD in Nuclear Engineering.

I doubt that it *boils* at room temperature; evaporates slowly, maybe. At least, not at the temperature of any rooms I've been in.

Horse pucky. I think I can afford to miss the tutorials on your website.