Talk:Direct current

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

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)

[edit] Does DC have a frequency of zero?

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.

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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)

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

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 202.124.177.50 (talk) 11:02, 5 December 2006 (UTC).

ĶЇŖŤÏ —Preceding unsigned comment added by 210.214.91.105 (talk) 16:42, 1 October 2007 (UTC)

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

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)

[edit] DC is not the same that CC

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)