I wonder whether alternating current is produced from DC current or whether AC and DC are entirely different concepts. Is there any relationship between AC and DC?

  • $\begingroup$ Hi justin, I've edited your post since each post should only contain one question. $\endgroup$ – David Z Oct 27 '14 at 8:06

AC can be made from DC, and vice versa. DC is a steady voltage, with AC the voltage fluctuates from negative to positive and back, many times per second.

Most electricity generators generate AC. The reason for preferring AC is that, historically, it is easier change voltage with AC: all you need is a transformer. The generators generate a high voltage, although usually a transformer is used to get it even higher.

The reason for the high voltage is to reduce losses in the cables (at higher voltages less current is needed for the same power, and less current means less heat waste). This high voltage gets sent down the powerline to a substation, where a transformer converts it to the 240V (110V in the US) used in the home. Most electronic equipment requires low voltage DC to operate. Converting AC to DC is easy. In essence, all you need is a rectifier - just a diode will do, although you probably need a transformer first, to reduce the voltage down further. In practice more complex circuitry is used to make sure the DC is smooth and stable.

Modern electronics make it fairly easy to convert DC back to AC. In fact, a normal powerpack (wall wart) first rectifies 50 Hz 240V AC to DC (at over 300V), then converts that to AC at several hundred kHz, and finally back to 5V DC, all within a small black cube.

To avoid AC losses in powerlines, some run DC instead of AC, at hundreds of thousands of volts. That kind of conversion is not exactly easy!

  • $\begingroup$ could you please see why when ac flows energy increases.reference:answers.yahoo.com/question/index?qid=20080726170922AAGgGMU. $\endgroup$ – justin Oct 27 '14 at 10:56
  • $\begingroup$ @justin. AC does not increase energy. However, when using high voltage you need less current for the same power (power = current x voltage). Less current means less loss (loss = cable_resistance x current x current). With AC it's easy to increase the voltage (just use a transformer); with DC you need advanced electronics. Hence the preference for AC. Ignore the 2nd answer in your yahoo link, it's meaningless; AC does not work that way. $\endgroup$ – hdhondt Oct 27 '14 at 23:03
  • $\begingroup$ Does this less current happen because of rmi value of current. $\endgroup$ – justin Oct 28 '14 at 4:07
  • $\begingroup$ @justin Do you mean RMS instead of RMI? RMS is the "average" value of the fluctuating AC current. And if I understand your question correctly then no, the reduction in current does not happen "because of rms". Because power=VxI, you get the same amount of power at, say, double the voltage with half the current. And high voltages are used because halving the current reduces the losses by a factor of 4. $\endgroup$ – hdhondt Oct 29 '14 at 2:44
  • $\begingroup$ yes I meant RMS. $\endgroup$ – justin Oct 29 '14 at 4:23

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