A.C. or Variable Voltage or Something Else?

I would call that DC with AC ripple. Same for both cases.

Thanks for reply, thats interesting. I wonder if there is a formal definition of when we call it DC with AC ripple and when we call it something else. If it was 200 volts AC with a 10 volt DC component then that must , I suppose, be called something else.

Do not ridicule yourself with the terminology, learn the science.

If it is superimposed then it might be called a "noise" riding over the other signal in order to check the power filter cleaning the signal in pure form???? Long ago, I did a project using a SNSR or signal noise rejection ratio test :)

It needs filtering. Seriously, its called unfiltered DC power supply.

Both waveforms are the sum of just two sinusoidal voltages. Specifically your first voltage waveform is:

v1(t)=5V*cos(0*t)+10*cos(ω*t)

Your second waveform is:

v2(t)=4.9V*cos(0*t)+10*cos(ω*t)

Welcome to your first step in understanding the Fourier Theorem.

I see that DC might be represented as a sine wave with zero angular velocity.. I suppose...I think. Or is that a trick to model it mathematically?

I was looking at waveforms on my scope last night and wondering how to describe some of them and feeling inept because I could not describe some as DC or AC or other category and it seemed very basic to me that I should be able to put a label on them. Thanks for your reply.

Thank you for your explanation and it does seem clear.

So a current could wobble arbitrarily or in square wave or anything else but its DC so long as current does now reverse. The moment it reverses its AC regardless of the amplitude or duration of reversal.

Thanks.

I meant "does not reverse"

This is the classic definition of AC and DC but I find it to be a misleading misnomer. An AC generator is closer to an ideal voltage source than an ideal current source. A battery is closer to an ideal DC voltage source than an ideal DC current source. The OP uses the common lexicon of calling a sine wave voltage as an AC device even though the only way an alternating voltage source can be an alternating current source by this strict definition is that one must know that the load is a linear, time invariant, load. A 120 VAC RMS generator driving a 10k ohm resistor in series with a power diode will draw direct current from an AC voltage source.

Yes, one can always take the Norton or Thevenin equivalency to make any source a current or voltage source for the appropriate analysis.

Thankyou Redfred this is giving me some things to think on. You are right, I was being sloppy talking of dc/ac voltages and had I stopped to think a bit more its true that I had a resistive load in mind.

You are very welcome. Remember, most people will sloppily call a voltage source as AC or DC without ever recognizing the contradiction. (Is that a current or voltage source?) Most of the time this contradiction will not matter. In your original question it does matter.

JRaef: I agree from an absolute definition of AC/DC, but it becomes semantics when these are used with test equipment. e.g. DMMs and Oscilloscopes. These use AC/DC without regard for current flow or alternating sine wave rotating equipment power source. I know this is second nature to you. But the OP asked a question, which to me was analog voltages where DC had AC superimposed.

I agree that the question needs considerable more detail. Like what frequency of the source, and it's impedance, and the transmission lines, and the load. You may measure negative voltages, but the current may never go negative given energy storage in capacitors and inductors which are ignored at low frequency. Most non EEs don't understand why you can't parallel connect 75/300 ohm antenna wire to multiple TVs, or leave un terminated stubs on this wiring. Try to make under 100 picosecond rise time measurements on fast pulses with an Oscilloscope. Most EEs would not have a clue about the scope probe and special ground connections needed.

I'm not arguing your answer, just that the OP should not go away with an absolute definition that AC/DC is current only.

Maybe I'm sloppy in my use. But this is how analog/digital mentors used it while training me.

The first answer is going to depend on what you reference the AC component to.

For the purpose of my answer, I will assume that the DC voltage is positive.

If you reference it to the DC voltage, then you have a normal AC sine wave going both positive 5v and negative 5v.

If you reference it to the original base line, then you have an AC sine wave ripple superimposed on a DC base. The ripple will vary the magnitude of the voltage between 10v positive and 0v (or 10v -ve and 0v if the DC is -ve).

The second answer again depends on the reference point of the AC component.

If you reference it to the DC voltage, then you have an AC sign wave going both positive 5v and negative 5v.

If you reference it to the original base line then you have an AC ripple superimposed on the DC which will vary from 9.9v positive to 0.1v negative (or 9.9v -ve to 0.1v +ve if the DC is -ve).

Thankyou for your answer Spades.

I'd call the first example a demodulated receiver signal.The second I'd call a demodulated overmodulated signal.

That sounds fine if the signals were in a radio. Thanks for reply.

Call it whatever you like.

DC

In the RN, we simply called it "lumpy" DC.....we all knew what that meant!

Remember that AC after passing through a rectifier, often looks quite similar.....but its not AC if it does not "alternate"!! Goes negative I mean!!

Some audio signals can also look quite similar if not biased around the 0 volts, but have a DC component....

At the risk of sounding condescending, your answers are in any Basic Electronics textbook.

Your first waveform could be described as:

* A 10v p-p signal with a +5v offset

* A +5v signal with a 10v p-p ripple

* A 3.5v RMS generator in series with a 5v battery.

All of these say the same thing in different ways. the main thing is which component you consider 'important.'

Analog amplifiers, which take in 'sinusoidal' signals(1) and increase their power, are concerned with the sine wave, so the DC component is the 'offset' shifting the sine wave all into one polarity for the amplifiers. Silicon-based amplifiers cannot work with AC, since transistors react differently when 'forward biased' or 'reversed biased'(2).

Digital repeaters(3) look at the information coded in the voltage levels(4), so the sine wave 'riding' on the DC would be 'noise' that could interfere with decoding the signal.

If you are looking at 'simple power,' just driving a motor, bulb, or solenoid(5), then you'd be looking at the power levels in the waveform, thinking of it in terms of generators and batteries. RMS stands for 'Root-Mean-Square,' which is the way to determine the effective 'DC strength' of an AC signal.(6)

Notes:

1) By which I mean 'incredibly complex waveforms.' All waveforms, no matter how complex, can be mathematically converted into a collection of equivalent sine waves, through a process called Fourier Transformation, so all analog signals are treated as a single sine wave in textbooks, since once you know how to handle a sine wave, you'll be ready for when you learn how to break complex signals down into their sines.

2) Transistors also only work as good amplifiers when they are in their 'linear zone' of conduction, so the signal can't even go down to 0v.

3) The digital equivalent to amplifiers. Repeaters (also called Regenerators) do not 'amplify' the signal, they decode the signal back into 1's ans 0's, then re-encode the signal, that way any noise the signal picked up is removed (as long as it wasn't bad enough to corrupt the decoding), and the new signal is considered 'repeated' or 'regenerated' from the old signal.

4) We're looking at simple signals here for example. Basic Amplitude Modulation, not Frequency Modulation, Phase Modulation or QAM. Besides, once the data gets decoded down, it's all voltage levels at the 1's and 0's stage anyway.

5) Yes, most solenoids are DC only, I'm trying to keep things simple and easy to understand. Go read the next chapter in the textbook if you know this section well enough to heckle the instructor.(7)

6) The exact formula is in the textbooks, but the 'simple shortcut' for a signal that is a 'pure' sinewave is [ peak voltage * 0.707 ]. That's peak voltage, remember, not peak to peak.

7) Anyone else feel like they're back in college trying to help the first-sememster students get a grasp of the material? That's a weird vibe to get from a forum.