A capacitor is a very useful electronic component, which is widely used in devices which require some sort of timing.
Think of a capacitor as two metal plates which are very close to one another, but which do not actually touch, this can be seen from the electronic circuit symbol for a capacitor below. the space between the two plates is filled with an insulator called a dielectric. This layer of insulation can be ceramic, polystyrene, polyester, polycarbonate, mica, air, paper or electrolyte. In larger capacitors these are in the form of long ribbons wound into a cylinder.

How does it work?

When the capacitor is connected to a voltage supply, a current will flow and the capacitor will start to collect charge. When a battery is connected to a capacitor the voltage does not reach the plates straight away. To achieve this difference in potential between the plates of the capacitor, electrons have
to be supplied to the more negative plate and removed from the more positive one. This takes time, the time is dependent on the size of the current
carrying the electrons. A capacitor stores an electrical charge.

This is a little like a bucket filling with water.
When the bucket, or capacitor is full the flow stops.

Capacitance is measured in Farads. The unit was named after Michael Faraday, a famous scientist who did a lot of work with electricity. Farads are very large units and most capacitors have a capacitance of less than 1 Farad, the usual values for capacitors is measured in microfarad (µF) or even smaller units.capacitors_parts250.png
1F = 1000millifarad (1000 mF)
1mF = 1000 microfarad (1000µF) where µ is a Greek letter pronounced 'mu''

1 microfarad (µF) = 1/1 000 000 F
1 nanofarad (nF) = 1/1 000 000 000 F
1 picofarad (pF) = 1/1 000 000 000 000 F

The smallest capacitors are usually ceramic plate or disc types with their value measured in pF. Electrolytic
capacitors are the largest in terms of their value which is measured in μF. Polyester capacitors can be in nf
or μF and fall between ceramic types and electrolytic types.

How a capacitor is identified
The capacity of a capacitor is usually printed on its case. A capacitor also has a stated working voltage which must not be exceeded.

Non-electrolytic capacitors normally have a value of less than 1 µF and are available in many shapes and sizes. They can be connected either way around in a circuit.
Electrolytic Capacitors typically have much higher values, but must be connected the correct way around in a circuit. The leg connected to 0V or the -ve side of the battery is normally marked
How a capacitor charges and discharges
If a large capacitor is connected to a battery through a high value resistor, it will charge up gradually as shown in the graph below
When it is fully charged the voltage measured across its legs will be the same as for the battery. If it is then discharged by connecting it across a resistor and an LED, its voltage will gradually fall, as shown in the graph below
The LED will be bright at the beginning and then will gradually become dim. The time taken for the capacitor to charge up depends on the value of the resistor and the value of the capacitor.
A good working rule states that the time taken (in seconds) for a capacitor to charge is:
  • Charging Time T (in seconds) = 5 x capacitance (in Farads) x Resistance (in Ohms)
  • T = 5 x R x C
  • T = 5 x 1000 x 1/1000
  • T = 5 seconds
The circuit shown below is the basis of many timing circuits because the time taken for the capacitor to charge to a particular voltage is always constant
The "time constant" (T) of a resistor capacitor circuit is calculated by taking the circuit resistance and multiplying it by the circuit capacitance. For a 1 kΩ resistor and a 1000 µF capacitor, the time constant should be 1 second. This is the amount of time it takes for the capacitor voltage to increase approximately 63.2% from its present value to its final value: the voltage of the battery.

What are capacitors used for?
As you have already discovered a capacitor takes time to achieve a difference in potential between the plates. So a capacitor can be used to create a time delay. But capacitors can be used for other applications. Another feature of a capacitor is that it allows AC voltage to pass through it but stops DC voltage from doing so. You will find capacitors used extensively in circuits because of this especially in audio equipment and in power supplies.
Capacitors are also used in filters to get rid of unwanted signals or frequencies. An example of this is in tone controls or graphic equalisers.

Michael Faraday (1791- 1867)
  • Faraday was a British chemist and physicist.
  • Faraday made many large contributions to natural philosophy.
  • He discovered electro-magnetic rotations in 1821, electro-magnetic induction in 1831 and the laws of electrolysis in the early 1830s.
  • In 1831, Faraday discovered electromagnetic induction, the principle behind the electric transformer and generator. This discovery was crucial in allowing electricity to be transformed from a curiosity into a powerful new technology.
  • During the remainder of the decade he worked on developing his ideas about electricity.
  • He was partly responsible for coining many familiar words including 'electrode', 'cathode' and 'ion'.
  • Later on he discovered the magneto-optical effect and diamagnetism in 1845 and, between 1845 and the late 1850s, he established the field theory of electro-magnetism.