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Capacitors

Introduction

Capacitors appear in many areas of science and technology and have a variety of uses. In GCSE Electronics capacitors are used with resistors to make a timing circuit. They are also used in power supplies. In A level Electronics capacitors are used to make frequency dependant circuits such as audio tone controls. In A level physics their structure and relationship to electric fields is considered.

Capacitors are passive components - they don't need a power supply to operate. Their function is to store electrical charge (and hence energy) and they can store and release this charge over a period of time. Usually in electronics everything happens (almost) instantly and so it is useful to find a component that functions over an extended period of time - capacitors enable us to make timing circuits and oscillators.

Caution must be exercised when handling large value capacitors as they can store a lethal charge, even when disconnected from the supply voltage.


Capacitance

The capacitance (C) of a capacitor (a measure of how much charge it can store) is measured in Farads (F). A Farad is a very BIG capacitance indeed and so we usually use capacitors that have a smaller value. The standard abbreviations are:


Therefore

It is important to be able to convert between these different units


Working Voltage

Capacitors have another specification called the working voltage. The working voltage is the maximum voltage that can be applied to a capacitor. If the maximum working voltage is exceeded then the insulation inside the capacitor is damaged and it may conduct causing the capacitor to heat up with undesirable consequences. Electrolytic capacitors are particularly prone to failing spectacularly when used at too high a voltage. In short, capacitors that exceed their safe working voltage tend to blow up. You have been warned.

This short VIDEO shows what happens. In this video the capacitor with a 16v working voltage was attached, backwards, to a 40v power supply - the heating caused a build up of gas and the resulting failure.


Types of Capacitor - Electrolytic

symbol large electrolytic capacitor large electrolytic capacitor
Electrolytic capacitors have the following properties:


IMPORTANT: They have a white stripe down one side which indicates the negative side/connection.

Electrolytic capacitors usually have the capacitance and safe working voltage printed on them.


Types of Capacitor - Non Electrolytic

symbol non electrolytic capacitor
Non-Electrolytic capacitors have the following properties:

Non-Electrolytic capacitors usually have the capacitance and safe working voltage printed on them in number code as shown below.


Reading Capacitor Values

High capacity capacitors are usually big enough to have their value written on them (usually in µF). The working voltage is also shown on the capacitor.
Smaller value capacitors also tend to be physically small and so a code is used to identify their value.

The code is a three digit code that gives the value in pF.

The first two digits give the value, the last digit gives the number of zero's

Eg:

Storing Charge

Capacitors store charge in the same way that a bucket stores water. A voltage is needed to force the charge onto the plates of the capacitor. Therefore a charged capacitor must have a voltage across it and/or a capacitor with a voltage across it must be charged. The amount of charge stored depends on the voltage used to push the charge in ... the greater the voltage, the greater charge stored ... and the "capacity" or size of the capacitor. The capacity is called the Capacitance and is a measure of how much charge can be stored per volt. To continue our analogy, the capacitance of a capacitor is like the width of the bucket whereas the voltage is like the depth to which the bucket is filled. If the bucket is very wide then even a shallow depth stores a large volume of water. In the same way, a large value capacitor stores a lot of charge at just a few volts.

Q=CV Charge (Q) - measured in Coulombs (C)
Voltage (V) - forces charge to move into the capacitor
Capacitance (C) - a measure of how much charge a capacitor can store per volt

The relationship is thus Q = C V

If the charge stored on a capacitor is plotted as a function of voltage then we expect a linear relationship
Note: Don't get Capacitance (C) and the unit of charge - Coulombs (C) - mixed up!

E.g. The charge stored on a 1000µF capacitor charged to 12v is: Q = C V = 1000 x 10-6 x 12 = 0.012 C

If the charging current were 10mA then this would take 1.2 seconds to charge fully