DC Wave Questions

[ehsjr]

---
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.

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

 

[operator jay]

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.

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?

j

 

[ehsjr]

It is equally a shame that there are those that are sometimes
incapable of offering correction gracefully, eh, John? If it pains
you so much to engage in your ungracious edifying, perhaps you would
do well to bugger off, and leave the stress of educating imbeciles to
those with more patience.

Interesting! I thought John's response to the op was
called for. The OP is going to get himself into trouble
with the attitutde he's exhibited. In my opinion, John
saw through the BS and called a spade a spade. I don't
know whether the OP got it or not - but John made it
clear that the BS wasn't fooling anybody.

I'll have to go back and read it again in light of
your post.

Ed

 

[NSM]

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

True but pointless. We know what we mean. Even 'current' is a borrowed term
used as an analogy as is 'potential' or even 'pressure'. If we have voltage
surely we should only speak of amperage.

N

 

[Kitchen Man]

Oh, I agree. I just think it helps to be a bit patient. Not that I'm
all that good at patience, myself. But we should try. We have all
been rookies, once or twice.

 

[Kevin Aylward]

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.

That's going a bit far. "Meaningless" means no meaning, and that is not
really an accurate description for the terms AC and DC. They have a
pretty well understood meaning, despite some suggestions in this thread.

"quotes with no meaning, are meaningless" - Kevin Aylward


Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

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:

Since we are quibbling her on terms, lets get this bit straight shall
we.

"Current flow" is wrong. Its simply "current" or "charge flow".
"Current" already contains the notion of "flow".


Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kitchen Man]

You are the one with the requirements, assertions, and definitions, not me.

Actually, the ones with the requirements, assertions, and definitions
are codes and organizations such as the NEC and the IEEE, and the
bothersome universities that teach the stuff.

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.

You are talking about transients, and if you intend for the questions
to be rhetorical, then I think you should demonstrate some expertise
in the subject matter that shows why the questions' answers must be
obvious. I don't think they are, so I will answer the questions:

The behavior of the flashlight in your example is neither AC nor DC,
it is transient. The first case is the instantaneous step function
caused by the closing of a source to a circuit. The second case is a
long-term curved ramp caused by the decay of a voltage source. AC and
DC analyses are steady-state. AC analysis will never apply to the
example. DC analysis must be performed prior to the transient
analysis in order to provide a steady state model for the application
of time-sensitive mathematics.

There is quite a bit of information available on the web about circuit
analysis. Your curiosity is to be commended; you might consider a web
crawling adventure, or even an education in the field.

And hey operator jay, what do you operate? Not electrical
substations, I wouldn't guess.

 

[--]

And, according to what you've said in other posts, if that were a
0.6 volt peak sinewave with 1.0 volt dc, it wouldn't be.

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.

No, both do not - only one of the 1 volt/.6 volt examples given has an
_alternating_ direction component - both examples do have a _variation_ in
their magnitude component.
( This is not a new discussion - and all of the dozen or so engineering
and physics texts and training manuals I have researched on the matter
adhere to the "alternating is reversing" definition of AC. It has been
custom and practice for at least 40 years.)

1) the 1 volt dc with the .6 sine variation does not alternate its
direction of flow. Its flow only varies in the magnitude of the charge
flowing always in one direction.
It has no alternating current ( i.e, it has no regularly reversing, i.e.
_alternating_, charge flow direction)

2) the 1 volt sinewave with the .6 volt dc does reverse charge flow
direction. It is alternating in its flow direction.
It also varies in its magnitude.

The direction of the description vector must alternate in order to have
Alternating Current. If it does not change direction but only varies in
magnitude, the descriptive vector is not alternating, it is merely varying
in magnitude.

3) Impedance laws apply equally to varying DC and to AC.

 

[daestrom]

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.

'Alternating' is not the same as 'altering'. "Alternating current" is an
electrical current where the magnitude and *direction* [emphasis added]
varies cyclically.

http://en.wikipedia.org/wiki/Alternating_current

One may 'alter' the magnitude of a DC current without it becoming
'alternating current'

daestrom

 

[daestrom]

That isn't true.

It is true by most definitions of 'Alternating Current'.
http://en.wikipedia.org/wiki/Alternating_current

'Alternating' means both magnitude and direction vary over time. A current
that varies in magnitude but not direction is not 'alternating'.

If it varies, it's AC.

Only by your apparent definition. But your definition does not agree with
the established industry.

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.

By adding a capacitor in series, you have altered the circuit. The
capacitor filters out the DC component of a the original varying DC voltage
applied. The capacitor has a varying DC voltage across it, but it never
changes polarity (you can use an electrolytic capacitor that is polarity
sensitive without damage).

The current through the resulting series circuit *does* alternate in
magnitude and *direction*, even though the voltage applied to the circuit
varies in magnitude only. So yes, the 'AC came from somewhere'. But that
doesn't mean the applied voltage is AC. Such logic is flawed. There is no
'law of conservation of AC' that says it can't be 'generated by the
capacitor'.

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

Repeating yourself doesn't make you correct.

daestrom

 

[Floyd L. Davidson]

'Alternating' means both magnitude and direction vary over time. A current
that varies in magnitude but not direction is not 'alternating'.

Then you'll have a very difficult time explaining now a
transistor amplifier works if it uses AC coupling that involves
capacitors.

Only by your apparent definition. But your definition does not agree with
the established industry.

Which industry? You can't do AC circuit analysis with any other
definition.

By adding a capacitor in series, you have altered the circuit. The

Capacitors don't generate voltage or current. The circuit
alteration merely demonstrates that the voltage and current on
one side meets all of your requirements, while the identical
charge flow on the other side does not, which indicates a flaw
in your specification.

capacitor filters out the DC component of a the original varying DC voltage
applied.

Yes, which leaves the AC that was there all along. It's AC
after, and it was AC before. If you do circuit analysis the
treatment is exactly the same on both sides of the capacitor.

The capacitor has a varying DC voltage across it, but it never
changes polarity (you can use an electrolytic capacitor that is polarity
sensitive without damage).

Exactly. Yet there *is* current through the capacitor, which
only passes AC. That AC current isn't generated inside that
capacitor. It comes out one side, so it *had* to be coming in
the other side.

The current through the resulting series circuit *does* alternate in
magnitude and *direction*, even though the voltage applied to the circuit
varies in magnitude only. So yes, the 'AC came from somewhere'. But that
doesn't mean the applied voltage is AC. Such logic is flawed. There is no
'law of conservation of AC' that says it can't be 'generated by the
capacitor'.

That's hilarious. DC applied to a capacitor generates AC????

I don't think so.

Repeating yourself doesn't make you correct.

Won't help your point either. And it makes no difference how many
places you find it ill defined either.

 

[Floyd L. Davidson]

Picture my flashlight, battery powered. Generally this is considered a dc
circuit. When I turn it on or off, there is 'change'.

You were doing pretty good up to that point.

So is it in fact an
AC flashlight?

No. But that doesn't mean there is never any AC present in the
circuit.

It happens that with a battery powered flashlight that is rare
(but predictable too), and of no consequence whatever. It can
be ignored in design and operation of the flashlight. But that
doesn't mean there is never any AC in the circuit, or that there
are no circuits where it is significant.

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.

Every time you flip the switch on or off, there is AC in that
circuit.

You can probably prove it too, relatively easy. Tune an AM
receiver to a frequency where no station is being received, and
hold the flashlight up close to the antenna. Flip it on and off
a few times. I suspect, though I haven't actually tried this,
that you'll hear a pop in the radio's speaker almost every time
you flip the switch. That is because some of the AC produced by
flipping that switch is RF.

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

It's been 24 hours of daylight for quite some time now. The sun
hasn't actually gone down for a month (May 10th), but of course
we had 24 hours of light long before that. The next time it
gets below the horizon will be August 1, and it will be late
August before it gets "dark".

The temperature is 30F right now, with a reported 18 mph wind and
fog. It was gusting up to 30 mph last night. It probably won't get
much warmer than maybe 36F today.

That is actually very comfortable weather, mostly because it is
unlikely to rain. I hate getting wet...

 

[Floyd L. Davidson]

That's going a bit far. "Meaningless" means no meaning, and that is not
really an accurate description for the terms AC and DC. They have a
pretty well understood meaning, despite some suggestions in this thread.

Given the significant experience of several people involved in
this discussion, and the wide variety of interpretations they
are giving to those terms, it would seem that just about the
*only* thing one could positively take away from this particular
discussion is that, as Don says, those terms are meaningless.

 

[John Fields]

Is this an AC ammeter, or a DC ammeter? (And isn't that just a voltmeter
anyway, in most actual cases????) Hmmm...

 

[Floyd L. Davidson]

No, both do not - only one of the 1 volt/.6 volt examples given has an
_alternating_ direction component - both examples do have a _variation_ in
their magnitude component.
( This is not a new discussion - and all of the dozen or so engineering
and physics texts and training manuals I have researched on the matter
adhere to the "alternating is reversing" definition of AC. It has been
custom and practice for at least 40 years.)

1) the 1 volt dc with the .6 sine variation does not alternate its
direction of flow. Its flow only varies in the magnitude of the charge
flowing always in one direction.
It has no alternating current ( i.e, it has no regularly reversing, i.e.
_alternating_, charge flow direction)

2) the 1 volt sinewave with the .6 volt dc does reverse charge flow
direction. It is alternating in its flow direction.
It also varies in its magnitude.

The direction of the description vector must alternate in order to have
Alternating Current. If it does not change direction but only varies in
magnitude, the descriptive vector is not alternating, it is merely varying
in magnitude.

3) Impedance laws apply equally to varying DC and to AC.

Item 3 is correct. That is because "varying DC" *is* AC.

It is AC even if the axis is shifted far enough to avoid
polarity reversals relative only to some specifically defined 0
current.

The reversals are relative... to the steady state condition,
not to some magical 0 current where supposedly no electrons are
flowing.

Otherwise, instead of two types, you are dividing circuit analysis
into three types, two of which are identical in all significant
respects other than an arbitrary definition that is meaningless.

It makes no sense to say that "Impedance laws apply equally" and
then claim that the two are not identical.

 

[Choreboy]

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.

N

With high frequency and amplitude, a sine wave could be very steep at 0
and 180 degrees. It could also turn sharply at 90 and 270, like the
corner of a square wave. You would need low frequency and amplitude for
a sine wave to approximate the flat peaks of a square wave.

That part is simple enough for me, but I don't understand harmonics. If
you overdrive an amplifier with a sine wave, the output will resemble a
square wave. I know the output can be broken down into the input
frequency and its odd multiples. I'll have to accept it on faith.

 

[John Fields]

That isn't true.

---
Yes, it is. If you have proof, instead of just a statement to the
effect that it isn't, I'd love to see it.
---

A non-sequitor.

---
"Non sequitur." No. A non-sequitur is an inference or a conclusion
that does not follow from the premises, or a comment that is unrelated
to a preceding one. My error was the omission of a reference, Mr.
Lancaster's: "1 volt peak sinewave with a 0.6 volt dc term"
---

If it varies, it's AC.

---
No, it isn't. What's necessary is the polarity reversal before it can
be considered 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.

---
It most certainly was!

Consider a DC coupled audio amplifier running from a single 12V
supply, with its output set to Vcc/2 and feeding an 8 ohm load with a
4VPP sinusoidal signal. Like this:

+12
|
+--o--+
| AMP |---+-->Vout
+--o--+ |
| [4R]
| |
GND GND

Now, with phi being equal to the phase angle of the signal and zero
degrees corresponding the voltage halfway between the most positive
and least positive output voltage, the output voltage excursions will
look like this:

phi Vout
-----+------
0° 6V
90° 8V
180° 6V
270° 4V
360° 6V

Now, connect that magical capacitor between the amp and the load, as
shown below, and watch what happens:

+12
|
+--o--+
| AMP |---[cap]--+-->Vout
+--o--+ |
| [4R]
| |
GND GND


phi Vout
-----+------
0° 0V
90° +2V
180° 0V
270° -2V
360° 0V

Why?

Well,for starters, consider that under quiescent conditions the
left-hand side of the cap will be charged to 6V and the right hand
side will be at zero volts since there is no galvanic path to the
output of the amp through the cap.

Now, imagine that the voltage at the amp's outout starts to go
positive. What will happen is that the amp will start sucking
electrons out of the cap, generating a potential difference across the
cap's plates which causes electrons to flow through the resistor,
making the top of the resistor more positive than the bottom.

Continuing in time, a point will be reached where the output of the
amp will start forcing electrons _into_ the resistor, at which point
the direction of travel of the electrons will be reversed. This
periodic reversal will cause the polarity of the signal into the
resistor to alternate. This alternating voltage will then give rise
to an _alternating current_ in the resistor.
---

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

---
Poppycock. It's precisely the alternations in the direction of charge
flow which cause it to be called "Alternating Current".

Your way would have it be called AC by assigning some arbitrary rate
of change, irrespective of direction as the delineation point, which
makes no sense at all. That is, what would you specify as the rate of
change which would delineate between between AC and DC? 0.5A/s?
0.001A/s? 0.1V/s?

 

[--]

Strictly speaking, I believe the reactance (part of impedance)
equations apply to any variation in current magnitude. Their appropriate
application does not in any way require reversing the charge.

1) I think one needs to define the term "alternating current" by its
phenomena rather than define it by what applies to "AC". In other words,
define AC as alternating current -rather than defining AC as "anything
requiring an impedance calculation because of its magnitude variation".
( OK, all scientific definitions require definitions in terms of other
defined concepts; thus voltage and charge are defined in terms of force.
And yes, any phenomena in its purest defined form uses the fewest of the
core units, and only the core units, of the measuring system. And yes,
since, unlike in the British ft-sec-lb system, force is not a core unit of
the metric kg-sec-m system, one cannot be as "pure" in the metric system
with many definitions as one can be in the British system, "decile"
convenience notwithstanding)

2) There are two phenomena and two descriptive words if one uses the
mathematical description of the changes associated with current: changes in
current _direction_ and changes in current _magnitude_.

There are three (or more) phenomena if one uses only the two descriptive
terms _AC_ and _DC_, well evidenced in this thread: changes in direction and
magnitude, changes in magnitude only, or no changes in either magnitude or
direction. Three phenomena defined using only two words for those three
cannot be specific and exclusive enough for a rigorous definition. The
middle condition, the overlap as it were, ends up wanting.

3) In the definition approach to a phenomena, one deals with the
descriptive term and the phenomena itself and ignores the present attached
effects. Once the definition is had, then the phenomena's interaction with
other phenomena can be determined. Yes, having such rigor in a definition
can be more complicated in its application.

In the application approach to defining a phenomena, one defines by
addressing what equations, etc., apply to the condition. In this approach,
you end up in circular arguments, chasing your tail. Something always will
not fit. Like changes in magnitude without changes in direction.

 

[John Fields]

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.