Any change of a DC level has an Fourier AC frequency associated with it, even if it's in the microHertz region............................
It looks as though your point is based on the concept that DC is absolute and unchanging, so if DC cannot possibly vary any variance must be classified as "AC". But DC does vary and practically all modern electronics relies on varying DC. So if a battery DC voltage is slowly diminishing is that "AC"?
A logic chip can produce a DC squarewave from 0v to 5v. Would you prefer to be forced to say "The logic chip is producing a DC level of 2.5v with a superimposed 5v AC squarewave"? That's not even accurate as the logic chip cannot even produce a 2.5v DC level, it produces DC levels 0v and 5v only, and any waveform produced is a DC waveform. For the purpose of modern convenience and modern equpment we say "a 5v DC squarewave" and eveyone knows what that is.
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actually, i would call it "AC with a DC offset", but i'm an amplifier designer and that's how i see it. the fact that it never changes polarity is irrelevant, because it's a simple matter of the use of a coupling cap to get the waveform to do so and eliminate the offset... in fact many amplifier circuits use this method for coupling the AC waveform to the next stage. other common ways of referring to what you describe is "DC with an AC component" or "DC with AC superimposed". more modern usage would actually replace the "AC" with "audio" or "RF", etc... and be more descriptive. when this is done with logic, this becomes "logic level pulses", and we're primarily looking for 1's and 0's, and maybe a clock rate and various delays in timing, but we're no longer calling it AC or DC, even though it has characteristics of both. in many cases the usage determines the terminology. here in the US, a capacitor is just a capacitor, unless it's used with ignition points in an engine, and then it becomes a "condenser" which is an archaic term for a capacitor. use the word "capacitor" and the old school gearhead will look at you as if you are from Mars. to him it is, and always will be a "condenser". to him a capacitor is the beer can shaped thing in a CDI (capacitive discharge ignition) box, and doesn't belong inside a distributor cap. so the terminology is archaic, but it has a meaning for a specific application. the same with "AC or DC", the original meaning is not really useful anymore, but the terms still exist. "conventional current" is still taught to simplify semiconductor theory, even though we all know (since Edison, who ironically was the first to wage a war between DC and AC) electrons flow from negative to positive.Thank you for your input too, I'm interested in hearing more.
I think part of the problem is the term "AC" is pretty much broken in modern usage.
For instance the AC mains is real alternating current as the current passes through the wire in alternating directions.
But for most of my work (<50v DC devices) almost any time I deal with an "AC" signal or "AC component" in a signal it is not AC at all, it is just a varying DC waveform. For instance a sinewave centred on 2v DC with peaks at 3v and 1v DC.
The term "AC signal" is completely wrong as the current does not alternate at all unless driving a reactive load. And "current" is not even right as what I have is a VOLTAGE sinewave, a DC voltage sinewave. Why should I call it an "AC sinewave"? It's very annoying terminology.
I'm for modernisation of broken archaic terms, so "DC sinewave" is fine as would be "DV sinewave" which may even be more accurate as it is a voltage sinewave not a current one.
One thing I've always wondered. Does anyone know where the 'direct' in DC came from?
...I don't understand why you say modern electronics "relies" on varying DC.
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I have worked as a practicing engineer for 47 years and you are the only one I've known who used the term DC squarewave or DC sinewave so I don't think "everyone" knows what it is.
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No, a square-wave from 0V to 5V is not a DC waveform. It is simply a squarewave. As Eric notes, placing an AC or DC term in front of the words sinewave or squarewave is not common or needed.
ChrisP58 said:...
It seams to me that we are trying to force things into two categories that were defined long before any of the signals that have been mentioned.
When the terms DC and AC were created, electricity meant only one thing. *Power*. Used to run motors, heat a room, or light up the street.
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To me, AC and DC mean power rails.
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That's entirely possible. But have you heard the term "DC ripple", "DC ripple voltage", "DC ripple waveform"? That's definitely a waveform, with a frequency.
so i guess we go back and re-think the whole "War of the Currents" thing. i've been in the electronics field for over 40 years, and have seen some wild concepts on how things work. this is an "interesting" but inaccurate concept. yes, 120Vac gets rectified and then chopped at 20khz (or higher, as some SMPSs work at up to and beyond 1Mhz (but we can't call it khz or Mhz anymore because it's chopped DC?)). this is so we can vastly reduce the size and weight of the transformer. what's on the secondary side of the transformer? AC? high frequency AC, but AC none the less, which needs to be rectified to produce your "pure DC". the thing here is that AC and DC coexist in the real world in various combinations DC can have an AC component (as in ripple), or AC can have a DC component (as in an audio amplifier with offset), or they can be present in equal magnitudes (as in logic levels or SMPS pulses on the primary side of a wall wart).In the case of power electronics, in the old days we used AC transformers in plugpacks. But most modern plugpacks are essentially DC devices, they convert the mains to 170v DC, then that DC is chopped on and off (which is NOT AC as you and others have stated) and the output is DC pulses filtered to pure DC by a cap.
My point is the world is becoming dominated by DC technology, apart from long distance power distribution and audio amps (but which are now going DC too!).
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Also as long as I can remember, electronics has been powered by DC, ie: batteries and rectified mains AC and the terminology is well understood.
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crutschow said:RB if you want to try to redefine the meaning of AC and DC differently from standard engineering practice, have fun, but the only ones likely to use your non-standard definitions are the amateurs. I assume you just enjoy arguing arcane issues, but if you seriously believe that a change in definition for AC and DC is needed then you are even further "Out there" than I realized.
unclejed613 said:So i guess we go back and re-think the whole "War of the Currents" thing. i've been in the electronics field for over 40 years, and have seen some wild concepts on how things work. this is an "interesting" but inaccurate concept. yes, 120Vac gets rectified and then chopped at 20khz (or higher, as some SMPSs work at up to and beyond 1Mhz (but we can't call it khz or Mhz anymore because it's chopped DC?)).
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EricGibbs said:I have never heard a practising engineer use any of the above terms, its more like noob web speak.
I have no argument with that. But RB has a long row to hoe if he actually thinks he can change the common and accepted usage of the terms DC and AC in the engineering world. I think it'll be a cold day in a warm place before the term DC waveform or its variants becomes accepted by any significant number of engineers to signify a time varying signal, DC offset or not. And right now I'm nice and toasty.I think what Mr. RB is trying to say is that technology is moving away from linear and moving towards switched techniques, such as switched mode PSUs rather than transformers, class D amplifiers rather than class a, b, and ab amplifiers.
I have no argument with that. But RB has a long row to hoe if he actually thinks he can change the common and accepted usage of the terms DC and AC in the engineering world.
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So with tongue firmly in cheek I will continue....I don't expect anyone to change based on the simple (and tongue in cheek) points I've put forward.
But in the aim of communication if I'm describing a PIC output squarewave 0v to 5v (especially if communicating to a beginner) then don't be surprised if I go out on a limb to use radical heretic terms like "producing a 5v DC squarewave out of pin4" or even risk the wrath of Hertz by saying "a squarewave at 10kHz".
I'm just not going to say "the PIC is producing is a 2.5v DC signal with 5v AC squarewave overlaid operating at a repetition rate of 10 thousand times a second".
Maybe I was hoping someone would be brave enough and put their hand up and say "Hey, why do we have to say "alternating" if the signal is all DC??".
Unless it's followed by the term "offset".That's cool. We'll just outlaw the term "DC" altogether anytime there's a waveform present.
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