Capacitors

What are they ?

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.

Identifying Capacitors

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:

• the value of capacitance in μF
• the maximum voltage it should withstand
• the polarity - a negative or positive sign will be drawn

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.

Measurement of Capacitance

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

or nanofarad

1nF = 1000pF = 1/1,000,000,000F

What are they used for ?

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.

Colour Coding of Capacitors

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.

Numerical Coding of Capacitors

Newer capacitors have a numerical code. This may be difficult to read without a magnifying glass but gives the capacitance by the following method.

1. Find the three digit number.
2. Write down the first two digits as in the order on the body of the capacitor.
3. Note the third digit. This is the multiplier and gives the number of zero's to put at the end of the value.
4. The number is the value in picofarads.

1. The first two digits are 4 and 7 so write down 47.
2. The digit 3 gives the number of 0's which is 3. Add these to the end of 47 to give 47 000.
3. The capacitance is 47 000 pF (picofarads). Since 1000 pF is equal to 1 nF (nanofarad) it could also be written as 47 nF.

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.