Talk:Direct current

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--Some Old Man (talk) 00:56, 22 May 2009 (UTC)

Does DC mean one-way or constant?[edit]

I do not agree with the section claiming DC sometimes refers to "constant polarity." DC is really the constant component of a voltage or current signal. The rectified signal has a DC component, but it also has many frequencies added to the signal. I think the point that is desired to be made is the conversion of AC to DC through a rectifier. However, a full-wave rectified signal is not DC. Mak17f 05:11, 10 December 2005 (UTC)

Please see the second to last paragraph of this article. The bigger problem here is that the term 'DC' has been overloaded. For example, DC orginated as an abreviation for direct current - a current that does not periodically reverse direction. Thus, a phrase like 'DC voltage' becomes 'direct current voltage' which doesn't make much sense at all. Further, a 'DC current' becomes 'direct current current' which is redundant. We EEs commonly refer to the DC component of a signal thereby equating the term 'DC' to the time average of a time varying signal or, if we are in the frequency domain, the 0 Hz component of the signal. Additionally, we talk of DC voltage or current sources when we mean constant voltage or current sources. Finally, we refer to DC circuits when we mean a circuit without memory (the voltages and currents in the circuit do not depend on past or future values of voltages and currents). There are probably other examples where the term 'DC' has become shorthand for something other than a uni-directional current. Alfred Centauri 15:14, 10 December 2005 (UTC)
Perhaps my concern is more that I think the most common use for the “DC” terminology is being a synonym for “constant;” be it constant voltage or current. Because of this, I feel the last couple paragraphs that you mention should be placed in the more prominent position in the beginning of the article. If in some circles DC is a synonym for constant polarity, it should be as an aside rather than the main definition. I agree that it seems the DC term is thrown around in many situations. Possibly in this article the different definitions should be clearly presented? I’ve rearranged the article such that uses and definitions are separated. Maybe this way the debate can be focused in these areas.Mak17f 18:50, 10 December 2005 (UTC)

DC means low frequency but not necessarily constant polarity. As an example, a DC motor can be rotated clockwise using a positive DC voltage and counterclockwise using a negative DC voltage. One meaning is with respect to power with constant polarity, like batteries and generators. The second meaning is baseband signal with frequency less than a threshold.Nanoatzin (talk) 08:19, 11 September 2012 (UTC)

Does DC have a frequency of zero?[edit]

does dc has frq.

If you mean "Does DC have frequency?", no, I don't think it does. —Zootm 12:19, 28 September 2005 (UTC)
Actually (and mathematically), it does have a frequency: 0 Hz.--Lenilucho 22:50, 1 April 2006 (UTC)
Really? Consider a sinusoidal voltage source:
v(t)=cos(\omega t) V\,
The rms value of this voltage is:
v_{rms}=\frac{1}{\sqrt{2}} V\,
This result is independent of the frequency. If we set the frequency to zero then:
v(t)=cos(0 t) V = 1 V\,
Thus, it would appear that this source should deliver 1W to a resistance of 1 ohm yet, according to the rms value calculated above, it should deliver an average power of 1/2W. So, is a 1V DC voltage source really equivalent to a 1V peak sinusoidal source of frequency zero? Alfred Centauri 04:50, 2 April 2006 (UTC)
Ok, may be mathematically that is not right. Or I don't know what's wrong on what you wrote.But conceptually, a 0 Hz sinusoidal (or any waveform) would complete a cycle 0 times per second. So either it is not a periodic function (but a sine is continuous, so that's not right), or it is a continous waveform (so it wouldn't ever change, like DC). That's why it is sometimes considered of 0 Hz.Actually, if you take a function generator and you set its frequency at 0 Hz, with any waveform, you'll get DC.--Lenilucho 09:08, 2 April 2006 (UTC)
The mathematical discrepency is due to the product of zero and infinity being indeterminate. In other words, we define DC to be a constant voltage or constant current (e.g., 1V or 1A). There is no product of zero and time in the definition. On the other hand, setting the frequency to zero for a sinusoidal function of time gives a constant for any finite value of t but, in the limit as t goes to infinity, the behaviour of this function is different from that of a constant.
Here's an analogous problem. Is an ideal wire the same as an ideal zero ohm resistor? According to one definition, an ideal wire is identical to a zero volt voltage source. That is, the voltage across the ideal wire is identically zero regardless of the current through the wire. On the other hand, the voltage across a resistor is the product of the resistor current and the resistance. If a resistor is connected to a 1V source, the voltage across the resistor is 1V. In the limit as the resistance is reduced to zero ohms, the voltage is still 1V but the current is unbounded. In other words, the product of zero ohms and infinite current equals 1V in this case. However, if we connect an ideal wire across the 1V source, we have an inconsistent circuit that says 1V = 0V. Thus, a zero ohm resistance is not quite the same as a short circuit. Alfred Centauri 13:24, 2 April 2006 (UTC)

The frequency of a pure DC signal is generally considered to be  1 \div (sampling \ period) , but this depends upon the situation. DC power that contains ripple has a spectrum with tones at multiples of the ripple frequency having different amplitudes. Nanoatzin (talk) 08:22, 11 September 2012 (UTC)

Length of the cable to be used for Arc welding machine[edit]

I want to know what are the limitations for the length of the cable to be used for Positive connection for a DC Welding machine. Let me explain more we want to do welding for one equipment which is at a distance of 15metres from the welding machine we can get the power supply for the welding machine there only. For this i want to know is it safe to keep welding machine at 15metres distance and do the welding. In this case what is the earthing procedure we need to follow is it necessary to earth nearer to the equipment or we can earth the common earth of the machine which is nearer to the welding machine. Please suggest me what to do —The preceding unsigned comment was added by (talk) 11:02, 5 December 2006 (UTC).

ĶЇŖŤÏ —Preceding unsigned comment added by (talk) 16:42, 1 October 2007 (UTC)

Recommend you purchase the most recent edition of the National Electrical Code and follow the guidelines.Nanoatzin (talk) 08:26, 11 September 2012 (UTC)

What is DC? (as commonly accepted in EE)[edit]

Is it a steady state current (voltage) with no perturbations (ie no frequencies above zero), or is it just a unidirectional flow of charge the magnitude and frequency of which could vary as any old function of time you care to think up?--TreeSmiler (talk) 00:02, 6 January 2008 (UTC)

The DC component of a signal (voltage, current, or whatever) is the mean value, or the zero-frequency Fourier component (which has an rms value higher than that of an AC component of the same amplitude, as pointed out above). The DC component thus does not fluctuate. But the term is also used for voltages or currents where the mean is much greater than the fluctuations; that is, the fact that a signal has fluctuations does not prevent it being called DC. And a DC voltage source (power supply) can have even negative fluctuations of the current, and still be DC. It's a broad concept. Dicklyon (talk) 00:26, 6 January 2008 (UTC)
I would argue that the concept isn't broad at all but that the usage of the term is sloppy. That fact that it is 'understood' that DC is often used to mean 'relatively constant' doesn't change the definition of DC (in EE) as a non-fluctuating, i.e., constant with time signal. A unidirectional flow of charge that varies significantly with time is, at the technical level, sometimes called pulsating DC but this is a contradiction. To be precise (something we should all strive to be), a unidirectional current contains, in general, a component that has a non-zero time average as well as fluctuating component(s) with zero time average. Alfred Centauri (talk) 02:34, 6 January 2008 (UTC)
If you prefer a strict definition, please do give us a source for it. What you call "sloppy usage" has been a topic of discussion at least since 1893, as we see here. Even a book with a strict definition follows immediately with "In practice, ...". And Vannevar Bush was flexible about different meanings, too. Dicklyon (talk) 04:22, 6 January 2008 (UTC)
The acronym DC (direct-current) refers to a unidirectional current. Thus, strictly speaking, a 'DC voltage' (where DC is understood to be an acronym for direct-current) is a 'unidirectional current voltage' which is obviously nonsense . However, the term DC, where DC is no longer understood to be an acronym, has come to mean constant. I quote from 13th edition of "The Standard Handbook for Electrical Engineers":
"DC Sources. Some sources, such as batteries, deliver electric energy at a nearly constant voltage and thus they are modeled as constant voltage sources. The term dc sources basically means direct-current sources, but it has come to stand for constant sources as well."
Thus the terms "DC circuit analysis", "DC solution", "DC component", "DC coupled", "DC restoration", etc. I stand by my assertion that the concept of the term DC, used by most EEs today to mean (ideally) constant, isn't broad at all. As additional support for my assertion, consider this where we find:
"Direct current (DC) is defined as a current that is constant due to a steady, unidirectional flow of electrical charge" (emphasis is mine) Alfred Centauri (talk) 17:04, 6 January 2008 (UTC)

DC is not the same that CC[edit]

CC constant. DC sign constant. --Nicoguaro (talk) 14:08, 18 March 2008 (UTC)

Within the world of EE, DC is a synonym for constant, e.g., DC (non-timevarying) circuit analysis, DC (non-time varying) source, DC (non-time varying) component etc. Alfred Centauri (talk) 01:27, 19 March 2008 (UTC)

Use of direct current[edit]

It would be helpful if someone who knows about such things could amend this article to discuss how it is that much if not most of the world relies on direct current. If the disadvantages of direct current are so great, why does all of Europe have DC systems? —Preceding unsigned comment added by Ahistorian (talkcontribs) 02:36, 26 August 2008 (UTC)

All of Europe does not have DC systems. The United Kingdom uses 240VAC at 50Hz. Direct current is used most often in modern commercial electrical power systems to transmit power over 1,000 miles. One such DC line delivers power from Bonneville Dam to Sylmar, Los_Angeles using +/-500,000 volt DC lines. Power lines operating at 240VDC can carry power just a few miles, which tends to be impractical.Nanoatzin (talk) 08:42, 11 September 2012 (UTC)

Description of the filled parameters in the WikiProject Electronics template and the WikiProject Energy template.[edit]

I added the WikiProject Electronics template since this article pertains to this WikiProject; the WikiProject Energy template was already added, but none of the parameters were filled. I referred to the templates in the talk page of the article for AC current.

WikiProject Electronics Class and WikiProject Energy Class: I rated the article as being Start-Class because it has one reference; from and, under the description for Start-Class articles, the rating of Start-Class should be applied for "An article that is developing, but which is quite incomplete and, most notably, lacks adequate reliable sources". If an article has only one listed source, it is, at best, start-class, regardless how informative, well-written, or long it is. This is simply because such articles need to be more verifiable, which Wikipedia holds in very high regard. The editors did do a good job, especially in formatting and organization; the article only needs more information and references for verification.

WikiProject Electronics Importance: Direct current (D.C. [or just DC]) circuits are absolutely fundamental to electronics, since every standard electronics class begins with the properties and equations that apply to a DC circuit. An understanding of DC circuits is the first step towards an understanding of higher-level concepts in electronics. Since knowledge of electronics requires knowledge of DC circuits, I rated this article as being of top importance.

WikiProject Energy Importance: Direct circuits form a paradigm of electric information and energy transfer. Indeed, networks of energy transfer can often be simplified (conceptually) into a DC circuit with components for those who do not know much about physics or when details are neglected. However, unlike in electronics, general knowledge of the dynamics of energy transfer in physics does not require knowledge of DC circuits. For these reasons, I rated this article as being of high-importance.

This article is technically lacking an infobox, so I noted that.

--Some Old Man (talk) 01:23, 22 May 2009 (UTC)

Suggest merge[edit]

DC circuit contains very little not already desecribed here, and this unreferenced stub definition could usefully be merged here to give it context. --Wtshymanski (talk) 20:52, 17 April 2012 (UTC)

DC circuit is not the same as DC. A DC circuit is one or electrical device that are physically connected. Direct current is a form of energy. It would be better to add DC circuit to the DC disambiguation page. Merging the two articles is likely to make both inaccurate.Nanoatzin (talk) 08:55, 11 September 2012 (UTC)

What, exactly, would you discuss only at "Direct current" that is not appropriate for "DC circuit" or the reverse? These aren't even two successive chapters in the physics textbook. --Wtshymanski (talk) 21:56, 17 September 2012 (UTC)
Which argument are you using? You have one for and one against there. (talk) 11:44, 18 September 2012 (UTC)
  • Strongly Oppose Wtshymanski has a long history of merging articles with the sole intent of (during the 'merge') deleting large quantities of either or both articles (that don't meet his idea of what should be in what he regards as 'his' encyclopedia). (talk) 11:44, 18 September 2012 (UTC)


The graphic for the article that shows "pulsing" is incorrect. The correct term is ripple.Nanoatzin (talk) 08:55, 11 September 2012 (UTC)

Also (less commonly) known as 'pulsating DC' (even though the term is technically inaccurate). The standard symbol used actaully shows pulsating DC rather than DC with ripple. (talk) 11:47, 18 September 2012 (UTC)

Current Flow[edit]

Current is defined as the flow of charge, in articles that deal the concept of current specifically, using the expression current flow when talking about charge flow creates an implicit definition which is never defined. --Kyohyi (talk) 16:59, 11 October 2013 (UTC)

At the moment the lede uses a definition for Direct Current as the unidirectional flow of charge. In the third sentence of the lede says "Direct current may flow in a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams.". "current may flow" implies a flow of flow of charge. I would like to change this sentence so that we don't imply a flow of a flow of charge. --Kyohyi (talk) 20:52, 15 October 2013 (UTC)
Please, learn a level of English grammar, syntax and sentence parsing appropriate to the level of pedantry that you're pushing. "Current flow" is a commonplace idiomatic expression that is very widely used in an unambiguous sense. Yet in deference to you and your claims of tautology, we are not strongly resisting your goal to rework uses of it. However "current may flow" is simply a different construct; there is no tautology here, even to your nitpicking level. There is no error here and we should not "fix" it merely to make an article inaccessible to normal readers. Andy Dingley (talk) 22:13, 15 October 2013 (UTC)