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> Ac Versus Dc Question, Clarification Required Please
Bunkysdad
Posted: January 19, 2018 09:02 pm
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I am having some difficulty understanding the difference or the ‘actual distinction’ between fluctuating dc current and ac current, say with a microphone input into a small audio amplifier...Both are sinusoidal, but ac appears to regularly reverse it’s direction or polarity (+ to -) while dc appears to merely vary its voltage potential from a position of more positive to one of less positive...Of the many sources I’ve searched I find that the term ‘alternating current’ is ‘routinely interchanged’ for both an ac source and a dc source, strangely without any added or apparent clarification...

Thank You for Your Help,
Bunkysdad
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MacFromOK
Posted: January 19, 2018 11:05 pm
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Forrest Mims (from Getting Started in Electronics): "An electrical current can flow either of two directions through a conductor. If it flows in only one direction, whether steadily or in pulses, it's called direct current (DC)."

"Forrest Mims is the most widely read electronics author in the world. His sixty books have sold over 7.5 million copies and have twice been honored for excellence by the Computer Press Association."

Some folks maintain that pulsed DC is actually AC, but I personally don't agree. The term "AC" means "alternating current," and originally meant alternating polarity, not alternating intensity.

YMMV. beer.gif


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Sch3mat1c
Posted: January 20, 2018 03:06 pm
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There's no such thing as "pulsating DC", only AC superimposed upon DC. smile.gif

Often, "pulsed DC" is used to mean a DC source that's being switched on and off, creating such a combination. It's regrettable but often used.

Tim


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Ratch
Posted: January 21, 2018 03:28 am
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QUOTE (Bunkysdad @ January 19, 2018 09:02 pm)
I am having some difficulty understanding the difference or the ‘actual distinction’ between fluctuating dc current and ac current, say with a microphone input into a small audio amplifier...Both are sinusoidal, but ac appears to regularly reverse it’s direction or polarity (+ to -) while dc appears to merely vary its voltage potential from a position of more positive to one of less positive...Of the many sources I’ve searched I find that the term ‘alternating current’ is ‘routinely interchanged’ for both an ac source and a dc source, strangely without any added or apparent clarification...

Thank You for Your Help,
Bunkysdad


For either voltage or current, I look upon AC as meaning "alternating cycle" and DC as "defined constant". You can mix and match the two waves, but usually the result is then considered an arbitrary waveform.

Ratch


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CoulombMagician
Posted: February 10, 2018 08:25 pm
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The terms are certainly used loosely however not quite as badly as misuse of the term "Ground".
In general DC refers to zero Hertz which is constant in time and AC refers all nonzero frequencies or the time varying component of a signal.
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Ratch
Posted: February 11, 2018 03:55 pm
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QUOTE (CoulombMagician @ February 10, 2018 08:25 pm)
The terms are certainly used loosely however not quite as badly as misuse of the term "Ground".
In general DC refers to zero Hertz which is constant in time and AC refers all nonzero frequencies or the time varying component of a signal.

A constant signal value is not the same as a zero hertz value. A zero hertz signal is is ambiguous and nonsensical. For instance, what is the value of a zero hertz signal at A*Sin(w*t)? What about the value at A*Cos(w*t)? Both terms are sinusoidal representations, yet the values at w=0 differ.

Ratch


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Sch3mat1c
Posted: February 12, 2018 02:51 pm
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QUOTE (Ratch @ February 11, 2018 09:55 am)
QUOTE (CoulombMagician @ February 10, 2018 08:25 pm)
The terms are certainly used loosely however not quite as badly as misuse of the term "Ground".
In general DC refers to zero Hertz which is constant in time and AC refers all nonzero frequencies or the time varying component of a signal.

A constant signal value is not the same as a zero hertz value. A zero hertz signal is is ambiguous and nonsensical. For instance, what is the value of a zero hertz signal at A*Sin(w*t)? What about the value at A*Cos(w*t)? Both terms are sinusoidal representations, yet the values at w=0 differ.

Ratch

The sin value is trivially zero so only the cos term matters for w=0.

You can do a Fourier series just fine including zero; the coefficient c_0 applies to both sin and cos terms, and since sin is 0 and cos is 1, c_0 simply equals the average of the time-domain function, in other words, the DC offset.

Tim


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Ratch
Posted: February 13, 2018 05:36 am
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QUOTE (Sch3mat1c @ February 12, 2018 02:51 pm)
QUOTE (Ratch @ February 11, 2018 09:55 am)
QUOTE (CoulombMagician @ February 10, 2018 08:25 pm)
The terms are certainly used loosely however not quite as badly as misuse of the term "Ground".
In general DC refers to zero Hertz which is constant in time and AC refers all nonzero frequencies or the time varying component of a signal.

A constant signal value is not the same as a zero hertz value. A zero hertz signal is is ambiguous and nonsensical. For instance, what is the value of a zero hertz signal at A*Sin(w*t)? What about the value at A*Cos(w*t)? Both terms are sinusoidal representations, yet the values at w=0 differ.

Ratch

The sin value is trivially zero so only the cos term matters for w=0.

You can do a Fourier series just fine including zero; the coefficient c_0 applies to both sin and cos terms, and since sin is 0 and cos is 1, c_0 simply equals the average of the time-domain function, in other words, the DC offset.

Tim

Can you really do a valid Fourier series on a constant signal? The period is an infinite amount of time and the signal returns to its value in an infinitesimal amount of time. I think there are some theoretical considerations that have not been fully explained. The sin function is just as valid as the cos, varying only in phase. Also, both sin and cos have zero offset. When considering all of the above, I still don't believe that a DC signal can be represented as a zero omega sinusoidal.

Ratch


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Sch3mat1c
Posted: February 14, 2018 04:31 am
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QUOTE (Ratch @ February 12, 2018 11:36 pm)
Can you really do a valid Fourier series on a constant signal?  The period is an infinite amount of time and the signal returns to its value in an infinitesimal amount of time.  I think there are some theoretical considerations that have not been fully explained.  The sin function is just as valid as the cos, varying only in phase.  Also, both  sin and cos have zero offset.  When considering all of the above, I still don't believe that a DC signal can be represented as a zero omega  sinusoidal.

Ratch

If you're talking nonperiodic signals, then it's Fourier transform, not series, but yes, it exists for any analytic signal.

The FT of 1 is delta(w), i.e., all the energy at 0Hz, with a phase of 0 degrees. smile.gif

Tim


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Ratch
Posted: February 14, 2018 05:14 am
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QUOTE (Sch3mat1c @ February 14, 2018 04:31 am)
QUOTE (Ratch @ February 12, 2018 11:36 pm)
Can you really do a valid Fourier series on a constant signal?  The period is an infinite amount of time and the signal returns to its value in an infinitesimal amount of time.  I think there are some theoretical considerations that have not been fully explained.  The sin function is just as valid as the cos, varying only in phase.  Also, both  sin and cos have zero offset.  When considering all of the above, I still don't believe that a DC signal can be represented as a zero omega  sinusoidal.

Ratch

If you're talking nonperiodic signals, then it's Fourier transform, not series, but yes, it exists for any analytic signal.

The FT of 1 is delta(w), i.e., all the energy at 0Hz, with a phase of 0 degrees. smile.gif

Tim

A FT is not a real time representation of a DC signal using sinusoidal functions.

Ratch


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dmg
Posted: February 14, 2018 01:39 pm
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i used to think you got 2 points where the power comes from, and if the flow of electrons is allways pointing to the same direction then its DC.
no switching of polarity.

surely one might consider when voltage goes up and down, say jumps from 6 to 9 volts DC, but thats still DC. it ay evenhappen periodicly, and you can count that and tell the freqveny of that happening, but as long as electrons flow from point A to B, and allways do so, its is certainly DC.
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Sch3mat1c
Posted: February 14, 2018 07:57 pm
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QUOTE (Ratch @ February 13, 2018 11:14 pm)
A FT is not a real time representation of a DC signal using sinusoidal functions.

Ratch

Ah! Well if the analysis must be real time, then there can be no such thing other than the real time function v(t). There is nothing else to transform! Therefore, it is not meaningful to speak of the decomposition into sines and cosines (which exist for all time -- i.e., are noncausal and non-finite-time).

You need a special case, like a hard window, on a DFT, or FT, or any of the wavelet transforms, etc.

In that case, the kernel function won't be as simple as a sine, and you've lost whatever value there was to apply such methods. wink.gif

Tim


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Ratch
Posted: February 15, 2018 12:34 am
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Sch3matic,

I am not the one who brought transforms into this discussion. All I am saying is that you cannot represent a DC signal in real time by a sinusoidal function of zero frequency.

Ratch

This post has been edited by Ratch on February 15, 2018 12:41 am


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Ratch
Posted: February 15, 2018 12:39 am
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dmg

Does that mean that a full wave rectified sinusoidal is a DC signal? As long as it does not cross the x-axis? I consider DC to mean "defined constant". If it jumps around, it is an arbitrary waveform.

Ratch

This post has been edited by Ratch on February 15, 2018 12:41 am


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CoulombMagician
Posted: February 15, 2018 09:02 pm
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As I recall DC stands for direct current and AC stands for alternating current, not that that helps with this discussion.
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Ratch
Posted: February 16, 2018 01:36 am
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QUOTE (CoulombMagician @ February 15, 2018 09:02 pm)
As I recall DC stands for direct current and AC stands for alternating current, not that that helps with this discussion.

How many times have you looked on the identification plate of an appliance and saw "120 volts AC"? It doesn't say 0.8 amps AC, does it? Doesn't it make more sense for it to be interpreted as 120 volts alternating cycle? Then it is correct for both voltage and current specifications.

How about direct current? What does that mean? Does that mean current goes directly from one point of a circuit to another point? Doesn't it make more sense to say the current or voltage is designated or defined to be constant (DC)? What do you think?

Ratch


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Sch3mat1c
Posted: February 16, 2018 08:04 pm
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"DC current" is redundant but accepted phrasing in English.

I wonder if other languages solve that more elegantly. Probably.

Tim


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dmg
Posted: February 16, 2018 10:52 pm
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QUOTE (Ratch @ February 15, 2018 12:39 am)
dmg

Does that mean that a full wave rectified sinusoidal is a DC signal? As long as it does not cross the x-axis? I consider DC to mean "defined constant". If it jumps around, it is an arbitrary waveform.

Ratch

actually if you take a sine wave and rectify it, then i consider the output to be dc.
electrons flow only in one direction.
there is no alternation in it.
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Ratch
Posted: February 16, 2018 11:46 pm
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QUOTE (dmg @ February 16, 2018 10:52 pm)
QUOTE (Ratch @ February 15, 2018 12:39 am)
dmg

Does that mean that a full wave rectified sinusoidal is a DC signal?  As long as it does not cross the x-axis?  I consider DC to mean "defined constant".  If it jumps around, it is an arbitrary waveform.

Ratch

actually if you take a sine wave and rectify it, then i consider the output to be dc.
electrons flow only in one direction.
there is no alternation in it.

You would call it DC? The amplitude sure is not designated constant (DC), is it? How about calling it pulsating mono-polarity?

Ratch



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MacFromOK
Posted: February 17, 2018 02:16 am
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QUOTE (CoulombMagician @ February 15, 2018 03:02 pm)
As I recall DC stands for direct current and AC stands for alternating current, not that that helps with this discussion.

This.

Ya'll can jump up and down and point out discrepancies if/as you wish, but this has been the definition of DC and AC for well over a century.

And it remains so today: beer.gif

define:"direct current"


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Ratch
Posted: February 19, 2018 02:08 am
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QUOTE (MacFromOK @ February 17, 2018 02:16 am)
QUOTE (CoulombMagician @ February 15, 2018 03:02 pm)
As I recall DC stands for direct current and AC stands for alternating current, not that that helps with this discussion.

This.

Ya'll can jump up and down and point out discrepancies if/as you wish, but this has been the definition of DC and AC for well over a century.

And it remains so today: beer.gif

define:"direct current"

The old definition has been wrong for so long. Let's make it right starting tonight. Correct and explain, or ask anyone who uses that abbreviation incorrectly what they mean.

Ratch


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jlsgp
Posted: February 20, 2018 07:44 am
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QUOTE
  "DC current" is redundant but accepted phrasing in English.

I wonder if other languages solve that more elegantly. Probably.

Tim 

If you are curious to know how the Chinese word it: DC translates to "straight flow of electricity" and AC is "interchanging flow of electricity".

So their concept is that if the flow of electricity is in one direction then it is DC, while the flow of electricity that changes in opposite directions is AC. The amplitude of flow, or current, is not in the expressions - and so whether it is fluctuating or constant is out of scope.


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MacFromOK
Posted: February 21, 2018 12:17 am
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QUOTE (jlsgp @ February 20, 2018 01:44 am)
If you are curious to know how the Chinese word it: DC translates to "straight flow of electricity" and AC is "interchanging flow of electricity".

So their concept is that if the flow of electricity is in one direction then it is DC, while the flow of electricity that changes in opposite directions is AC. The amplitude of flow, or current, is not in the expressions - and so whether it is fluctuating or constant is out of scope.

Pretty much the same as our DC/AC definition then.

Some folks just keep trying to read more into it than was intended. beer.gif


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