Electronics Forums > Re: DC Wave Questions

Re: DC Wave Questions

Rich Grise
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 06-11-2005, 12:34 AM
On Fri, 10 Jun 2005 14:55:02 -0700, jackbruce9999 wrote:

> 2 questions about a fully DC Sine Wave..

If you think that the term "fully DC Sine Wave" even means anything,
then you have not understood the coursework. Either your teacher is
incompetent, or you have been spending too much time partying and not
enough time studying.

Good Luck!
Rich

NSM
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Posts: n/a

 06-11-2005, 04:12 AM

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...

> Again, is the term "DC Sine Wave" problematic because it is
> fundametnally wrong

Yes. DC by definition is zero frequency.

N

Pooh Bear
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Posts: n/a

 06-11-2005, 05:59 AM

(E-Mail Removed) wrote:

> Again, is the term "DC Sine Wave" problematic because it is
> fundametnally wrong

Yes !

Graham

Tam/WB2TT
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Posts: n/a

 06-11-2005, 02:36 PM

"Bob Penoyer" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Sat, 11 Jun 2005 04:12:52 GMT, "NSM" <(E-Mail Removed)> wrote:
>
>>
>><(E-Mail Removed)> wrote in message
>>news:(E-Mail Removed) groups.com...
>>
>>> Again, is the term "DC Sine Wave" problematic because it is
>>> fundametnally wrong

>>
>>Yes. DC by definition is zero frequency.

>
> Um, no. DC is Direct Current, i.e., current that flows in one
> direction. For example, the output from a rectifier is DC but it
> certainly isn't "zero frequency."

The output of a rectifier contains both AC and DC. You put a filter on it to
get close to pure DC.

Fred Abse
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Posts: n/a

 06-11-2005, 06:22 PM
On Fri, 10 Jun 2005 20:47:59 -0700, jackbruce9999 wrote:

> Again, is the term "DC Sine Wave" problematic because it is fundametnally
> wrong OR is it problematic because it is at odds with conventional
> terminology and nomenclature.

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".

> ....if it is fundamentally wrong, then please
> show how.....however, if we're just talking about convention,

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

> then why
> break balls? (Wait, I'm sorry, I don't mean literally "breaking balls",
> that's just nomenclature).....if you were given a piece of paper a week
> ago with just the words "A Fully DC Sine Wave" on it and you were asked to
> come up with as many possible things it could realistically mean, how many
> things could you come up with? If you were being truthful I think you
> could only think of one thing (and think of it very quickly)

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

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)

NSM
Guest
Posts: n/a

 06-12-2005, 05:00 AM

"Bob Penoyer" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...

> 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.

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

N

operator jay
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Posts: n/a

 06-12-2005, 01:31 PM

"Floyd L. Davidson" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...

> 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
>
>

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

j

operator jay
Guest
Posts: n/a

 06-12-2005, 05:21 PM

"Floyd L. Davidson" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> "operator jay" <(E-Mail Removed)> wrote:
> >"Floyd L. Davidson" <(E-Mail Removed)> wrote in message
> >news:(E-Mail Removed)...
> >
> >> 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
> >>
> >>

> >
> >I think "zero" is a good reference for current flow, and that the actual

>
> Sure... now, can you define "zero"?
>

Put an ammeter there and it says zero. That's zero. Electrons bouncing
around in the conductor have an average net displacement, over time, of 0.

> >(absolute) direction can be measured. Voltages have the reference

issues.
>
> E =IR
>
> Since our resistance is fixed, it's the exact same issue, though
> perhaps easier to understand, with voltage. (I gave some
> consideration as to whether to post that with voltage or current
> references, and since "AC" and "DC" use the term "current",
> decided to go with current to avoid the easier path to the same
> statement you are making.)
>

Current is a different issue from voltage because voltage is a relative
quantity. It is a type of measurement of a change in field between two
locations. Current is a rate of flow of charge at a single location (well,
typically, through a single Gaussian surface), and is measurable at that
location, and does not have the ambiguity that voltage has. It does not
need a reference. If I say that my toaster is running at 120V and 8A, you
may ask "120V relative to what" and I'll answer "neutral". You would not

j

Kitchen Man
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Posts: n/a

 06-12-2005, 08:08 PM
On Sun, 12 Jun 2005 12:21:01 -0500, "operator jay" <(E-Mail Removed)>
wrote:

>
>Put an ammeter there and it says zero. That's zero. Electrons bouncing
>around in the conductor have an average net displacement, over time, of 0.

"Put and ammeter there" and if it says +300mA to +800mA back and
forth, then it's Alternating Current, innit?

Kitchen Man
Guest
Posts: n/a

 06-12-2005, 08:32 PM
On Sun, 12 Jun 2005 04:55:45 -0800, (E-Mail Removed) (Floyd L.
Davidson) wrote:

>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.

Thanks, Floyd, for that excellent and understandable post.

--
Al Brennan

"If you only knew the magnificence of the 3, 6 and 9,
then you would have a key to the universe." Nicola Tesla

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