10 - Chargers and
Mains Power Adapters

 

In the U.K., the mains electricity is supplied to buildings with a voltage of 230 volts. Unlike batteries (with a steady voltage), mains voltage alternates between + volts and - volts at a rate of 50 times a second (or frequency of 50 Hertz (Hz)).

In other countries this differs: for example, in America, mains power is supplied at a lower voltage of 120 volts and with a higher frequency of 60 Hz.

The alternating "+" and "-" values, are known as the voltage's polarity and this can be shown on a diagram called a waveform.

 

As the voltage rises and falls the current direction also changes. When we describe mains power, we say it is 230 volts (A.C.) - where A.C. stands for "alternating current".

 
230-Volt Mains Waveform
Showing Voltage Rising and Falling with Time

 

(The white dot indicates no polarity, or
O volts, when it crosses the dotted line).


Click the switch to see
the + and - polarity of the rising and falling mains voltage in slow motion.

 

Appliances such as electric kettles, cookers, fires or toasters, need a lot of power and they run directly from the 230-volt mains supply.

Equipment, such as DAB radios, Blue Ray players, or TV's, need much less power. These use internal power supplies/power adapters to reduce the mains voltage down to a few, or tens of volts.

Power Adapters / Chargers

External power adapters and chargers are commonly used to reduce the high mains voltage to safe, useable voltages of 3, 5, 9 or 12 volts.

 

 

 

At the same time, the power adapters/chargers will usually convert the alternating current (AC), to direct current (DC). This can be seen in the graph on the left - where the flat waveform shows a constant voltage.

 

A standard phone charger, or usb computer socket, typically provides a constant voltage of 5 volts and up to 500mA (0.5 amps) of current.

 

It is important for an internal fuse, or cut-out, to be included in a power adapter. This will automatically provide protection from an overload. This could happen if a circuit connecting to the adapter attempts to draw more current/power than is available.