DC Wave Questions

I can't vouch for the historical facts, but as far as zero frequency and non-zero-frequency goes, you are pretty much correct.

Another point to note is that many signals have both AC and DC. It is not a dichotomy. The signal you mentioned at the start of this thread has both AC and DC.

Historically, I think what happened is that the terms originally were used to describe two competing power sources (the war between those who wanted a DC power grid and those who wanted an AC power grid was surprisingly fierce). Later, the terms started getting used to describe signals, and that is probably when the shift to the ZF- and NZF-meaning happened.

Also, there may be people out there who still think of AC and DC in the original sense (I'm not sure about this, but maybe people who work with power stuff exclusively), but among electrical engineers, the signal perspective prevails.

--Mac

Well, look at it this way: almost all of us had to read your post twice and think about it to make sure we understood the most likely meaning of it.

If you worded it differently, the meaning would be crystal clear, and we would only have to read it once. So in a sense, it is inconsiderate and a waste of our time to post it in such a way that we can't immediately understand. You can easily be forgiven for doing this once out of ignorance.

Anyway, I agree that some people were rude to you. But you haven't exactly showed yourself to be receptive to advice, either.

--Mac

I object to all of the above. Go read Scroggie's "Second thoughts on Radio Theory".

Pretty much. Even in other languages I believe that terminology is basic - certainly German is the same (Gleichstrom. Wechselstrom.)

It is fundamentally meaningless. A sine wave is a defined mathematical function:

y = a sin(x)

I think what you are talking about is a sine wave with an added DC component:

y = (a sin(x) + b)

which is not purely a sine wave, nor is it purely DC. It's "a sine wave plus DC" if you like.

Referring to "a DC sine wave" is analogous to referring to "a curved straight line".

No, not just convention, this is engineering. The language we speak is mathematics. I think I just showed you why.

I could only think of only one thing: "This guy is not an engineer, or scientist, or mathematician!"

In this case, the OP actually used the words "sine wave", which would appear to preclude any such flexibility, anyway.

Where it matters, I prefer the use of "steady" or "varying"

"AC" and "DC" have overtones of Edison, Tesla, Ol' George Westinghouse, and the unfortunate Mr. Kemmler.

If you learn the math the definition of AC and DC is totally irrelevant. Your original Question #1 indicates that you don't know the math. That is fine; it is something you can learn, but no amount of quibbling over semantics is going to change the principles of circuits.

Forget the encyclopedia, learn differential equations instead. Encyclopedia are for junior high school kids.

terminology ("DC sine wave")....

plenty of rubbish spouted on the net.

Once in a while someone coins a new term like Heinleins' waldo... but I don't think its going to happen here.

What has been described is similar to the waveform one would see on the grid of a class A tube circuit. (but negative voltage of course)

There are a number of waveforms that go in only one direction relative to ground such as sawtooth waves, square waves, and triangle waves. To refer to these as say a "DC triangle wave" would be equally confusing without further qualification.

The closest descriptor that I can think of offhand that might meet with general acceptance would be "bias signal".

------------- Don't freak out, a sine wave with a DC offset so that it is all pulsating DC is not at all unusual, every transistor amp has one since transistors can only accomodate one polarity of current.

-Steve

A rectified AC waveform contains DC and AC components but if the current isn't changing direction, it isn't alternating current. And, if it isn't AC, it's DC.

You have a 10V Dc ssignal with a superimposed 5V p-p signal. The load's impedance to DC is determined by the load's DC resistance (steady state ) The load's impedance to AC will be determined by its R (ac not Dc) ,L,C combination (steady state) The two are different and independent values.

You truly are an idiot. Bluegrass is growing all over kentucky, and some gets planted every year. Notr only that, but you can listen to it on the radio or TV.

Another waveform that's very similar ( albeit more of a sawtooth waveform rather than sine wave ) is power line ripple on a DC supply rail.

Graham

It's DC with a ripple riding on top of it. With no filtering the ripple runs down to zero.

I like Jack's terminology. The wave itself isn't DC, but I think "fully DC" is an acceptable way of describing its location.

AC generators and transformers are usually designed to produce sine waves with no DC, but sine waves were known long before those inventions.

A wave is a succession of curves. A sine wave is a wave whose displacement follows the form of a sine. A pure acoustic tone is a sine wave regardless of ambient pressure. A ripple on a pond is a sine wave regardless of the water level.

As not all voltage variations are curves, our generic term was "waveforms". If the plate voltage of an amplifier tube varied from 998 to 1000 volts in the form of a sawtooth, we'd call that two-volt variation a sawtooth waveform. If it was sinusoidal we'd call it a sine wave.

To call a waveform an AC sine wave implies that there is no DC, but this thread is the first time I've read the claim that all sine waves are AC sine waves.

Actually, DC from a rectifier *is* "zero frequency", to the degree that it is DC. Of course until the AC is filtered out, it has both AC and DC components.

That is *precisely* correct. (It just doesn't tell enough of the story to explain the confusion of this "flows in one direction" definition of DC.)

The output of a rectifier until filtered *does* have both AC and DC, which actually is another way of saying that yes it *does* change directions.

What? you say!

The problem is that "direction" only has meaning when measured in comparison some specific point of reference. If you have three different reference points, one at the DC level, one at the peak positive swing and one at the peak negative swing, you have three very different views of "direction" for current flow:

Reference Direction Point of flow ========= =====================================

Peak Pos All Negative

DC level Equal cycles of Positive and Negative

Peak Neg All Positive

Obviously the only point of reference you are thinking about is the Peak Negative swing, and just as clearly that is *not* appropriate. The correct reference point is necessarily the DC level, and measured from that point of reference the current is going to go alternately in each direction.

Bingo, there is your AC.

This is very similar to confusing the negative side of a rectifer circuit with ground. Rectifiers are most often grounded on the negative side, but that is absolutely arbitrary. They can be grounded on the postive side, and with some effort can have ground at other levels either between those two, or even beyond either of them.

But rather than look at DC as current going all going in one direction, and AC as anything else, it is *far* easier to view it as AC is any current that is changing, and DC is anything else (i.e., the current is steady).

Technically those definitions are exactly the same, but one leads to a lot of confusion.

I think "zero" is a good reference for current flow, and that the actual (absolute) direction can be measured. Voltages have the reference issues.

This type of problem is analyzed using the superposition theorem.. One source is +10 VDC The second source is a 10 V pk-pk sine wave.

Just add the two results for the answer, into a common load. For the DC, allow time for the transient to settle....

If you can use PSPICE, or the equivalent, you can do this easily , with additional experiments which may improve your understanding......

Andy

PS It's a shame you have to weed thru all the crap from some of the posters here who have a lot of time on their hands and have no tolerance for those who are just learning their craft....