Last edited 04 Feb 2019

Alternating current and direct current

The sinusoidal curve that represents the positive and negative phases of AC current.


[edit] Introduction

An electric current is a flow of charge along a conductor, such as a copper wire. When it flows in one direction it is called direct current (DC). When it reverses direction periodically it is termed an alternating current (AC).

AC current is generally used to power homes and businesses, and is also present when audio and radio signals are carried on electrical wires. DC current is typical of batteries that power flashlights and other home appliances and is also used in some industrial applications.

Because AC current reverses direction periodically, it can be characterised by a sinusoidal waveform where the half periods above the x-axis represent the positive phase of the current and the half periods below the x-axis represent the negative phase.

AC current operates in the following way: it will start from a position of zero, build up to its maximum value (top of the positive peak on the sinusoidal curve), reverse to zero, continue to the maximum in the opposite direction (negative, below the x-axis) then reverse back to zero, upon which the cycle begins again. The number of these cycles performed per second is called the frequency and is measured in hertz (Hz).

Domestic and commercial power in the UK and other countries is typically of low frequency (50-60Hz). Far higher frequencies are encountered in other applications such as television (100,000,000 cycles/second (100 megahertz (or 100MHz) where 1MHz is one million cycles/second)). Even higher frequencies of several thousand megahertz are used in microwave and radar applications, while in mobile phones they may be in the order of around 1,000MHz (one thousand million hertz or 1 gigahertz (GHz).

Many electronic devices contain semi-conductors which require low-voltage DC. This means that such devices must convert high voltage AC to low voltage DC. This is usually achieved by the plug-in power plug that is supplied with the device.

[edit] Some advantages of AC over DC

AC current offers numerous benefits over DC. Typically, these include:

  • AC can be relatively easily and economically stepped up or down with a transformer to suit the application. DC cannot be wired through a transformer.
  • Because it can be stepped up (and down), AC can be increased to high voltage levels for transmission over large distances, then stepped down to safer levels for consumer use.
  • High voltages can be generated in AC. This is more difficult with DC.
  • Because of the high voltages that can be generated, AC can be transmitted over long distances.
  • Long-range transmission results in relatively low energy losses resulting from resistance.
  • AC is cheaper to generate than DC.
  • AC can be easily converted to DC if required.

[edit] DC networks

The Energy Saving Trust estimated in 2007 that by 2020, 45% of the electricity consumption in a household would be entertainment, computers and gadgets and LED lighting, all of which are DC-powered. This, in combination with the emergance of DC generation from solar panels, and battery storage, have given rise to the concept of DC (rather than AC) networks.

For more information see: DC electricity networks.

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