What is Alternating Current

Alternating Current:
A battery provides a constant voltage.   This is called a Direct Current or ‘DC’ source of power.   When a circuit is connected to a battery, the positive rail is always positive and the negative rail is always negative.

If you connect a battery to a circuit through a double-pole changeover switch as shown here:

When the changeover switch is operated, the battery is effectively turned over or inverted.   This circuit is called an ‘inverter’ because it repeatedly inverts the supply voltage.   If the switch is operated on a regular, rapid basis, the graph of the output voltage is as shown on the right.   This is a ‘square wave’ voltage and is used extensively in electronic equipment.   It is called alternating current or ‘AC’ for short.   SCRs and Triacs can be used conveniently with supply voltages of this type.   Mains voltage is also AC but is rather different:

Mains voltage varies continuously in the form of a sine wave.   In Britain, the mains voltage is described as ‘240 Volts AC’ and it cycles up and down 50 times per second, i.e. 50 positive peaks and 50 negative peaks in one second.   It would be reasonable to assume that each voltage peak would be 240 Volts but this is not the case.   Even though the supply is described as 240 Volts, it peaks at the square root of 2 times greater than that, i.e. 339.4 Volts.   The actual supply voltage is not particularly accurate, so any device intended for mains use should be rated to 360 Volts.   In America, the supply voltage is 110 Volts AC and it cycles 60 times per second, peaking at plus and minus 155 Volts.   Later on, you will see how one or more diodes can be used to convert AC to DC in a unit which is sold as a ‘mains adapter’ intended to allow battery operated equipment be operated from the local mains supply.

Electronics Tutorial

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