Given my fondness for the band of the same name, it is strange that I have not used this title for a post before. AC/DC refers in this case to alternating and direct current. Mains electricity in the UK is 240 volt alternating current that switches polarity (flows the opposite way) 50 times a second (50 Hz). The mains in other countries varies (60 Hz in Japan), 110 volts in the USA but it is always alternating current. Direct current, such as you would get from a battery, has fixed positive and negative poles.
Alternating Current
You can see from the picture above how the current changes over time. This kind of wave is called a sine wave. It is generated by rotating a coil of copper wire within a magnetic field. Huge generators in power stations rotate to produce the domestic supply.
Electrical devices that are designed to be plugged into the mains either use the alternating supply directly or convert the alternating current into direct current. Mains electricity has to be transported along very long wires, and alternating current does this far more efficiently, so that is why the National Grid uses alternating current. Alternating current is also more dangerous. Thomas Edison was so concerned about the safety of alternating current versus the direct current that he favoured, that he infamously executed an elephant named Topsy in 1903 to try to prove his point.
Direct Current
Direct current is produced by electric cells, solar cells and dynamos. A dynamo is an electric generator fitted with a device called a commutator that turns the alternating current into direct current. The word dynamo is only applied to a generator that produces direct current.
Alternating current can be converted into direct current with a simple arrangement of diodes called a rectifier. A diode will only allow current to flow in one direction (in the direction of the arrow in its symbol) so the route through the circuit switches as the direction of the flow changes. The diagram above shows this rather nicely. Try to follow the path around the circuit to see how it works. The smoothing capacitor ensures that you get a reasonably constant voltage at the load (which is the output).
Direct current can be turned into alternating current with a device called an inverter. There are many different designs, and they tend to be quite complicated, so I will leave you to research them for yourselves.
Questions…
- What is the difference between a cell and battery?
- From the sine wave above, what property is governed by the amplitude of the wave and what property by the wavelength?
- Suggest a reason LEDs are used instead of filament lamps in modern torches.
- If your bedside light has alternating current flowing through it, why doesn’t it flash on and off when it is being used?
- In the rectifier diagram above, what component is providing the load?
- Why is it essential to have a load in a circuit?
- Into what form of energy does the load convert the electrical energy?
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