Capacitors contain conducting plates, usually a thin metal film, separated by an insulator called the dielectric. They are able to store electric charge - the larger the capacitance the greater the charge at a given voltage.
Electrolytic capacitors usually come in an aluminium can with a plastic sleeve. The illustration gives and idea of the shape of the capacitor but some types (axial leads) have a lead leaving each end. These capacitors are polarised - they must be connected the correct way in a circuit.
The important information is written on the can and includes:
Non - electrolytic capacitors may be connected either way in a circuit because they are not polarised.
Some types used to have a colour coding printed onto the outside of the case of the capacitor. Polyester types had a flattish rectangular shape but even older versions used to be tubular.
You may still come across colour coded capacitors in older equipment and the chart on the separate sheet will help in their identification.
Current non-electrolytic capacitors have a numeric code which is explained on a separate sheet.
Most non-electrolytic types will work at the voltages used in electronic circuits but for power applications, particularly when used with the mains, the voltage rating becomes important. A 1000 V capacitor is needed at mains voltages. Suppressor capacitors or capacitors used to start mains motors need to be carefully selected.
The farad is the unit of capacitance but it is far too large for practical purposes.
Practical electrolytic capacitors are rated in microfarads.
In the power supply section expect to find capacitors with values up to 10,000 μF These are physically large but smaller ones with values in the tens or hundreds of microfarads are found at various other places on the board, with values up to thousands available. The largest non-electrolytic types are only a few μF and most are measured in picofarads
1 pf = 1/1,000,000,000,000F
1nF = 1000pF = 1/1,000,000,000F
Electrolytic capacitors are used to smooth the rectified mains supply for use in electronic equipment. Smaller ones will be used to allow a.c. signals to pass but to block d.c. signals.
The diagram opposite shows and electrolytic capacitor that may be found in a power supply.
To help identify it correctly fill in the details inside the box.
Non-electrolytic types have a variety of applications. Ceramic types can be effective at removing 'spikes' from supplies or motors, polyester types may be used as suppressors and polystyrene is used at radio frequencies. When there are many integrated circuits there is likely to be a number of very small capacitors used for decoupling purposes - this prevents unwanted triggering of the electronic switching circuits.
The first two digits are written down (colours are the same as the resistor colour code). The third band is the multiplier. Multiply the first two digits by the multiplier value in the table below.
|Colour||First digit||Second digit||Multiplier|
(value in pf)
Newer capacitors have a numerical code. This may be difficult to read without a magnifying glass but gives the capacitance by the following method.
Note: Other information gives the working voltage and dielectric material of the capacitor.
Find the value of each of the capacitors in the box opposite. Express the left capacitor in picofarads, the middle one in nanofarads and the capacitor on the right in microfarads.