Also see: Capacitance
Capacitors store electric charge and their capacitance is a measure of how much charge they can hold. Capacitance is measured in farads, symbol F, but 1F is very large so these prefixes (multipliers) are used to show smaller values:
Capacitors are used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They are used to smooth varying DC supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals.
There are many types of capacitor but they can be split into two main groups: polarised (generally 1µF and greater) and unpolarised (generally less than 1µF). Each group has its own circuit symbol.
Rapid Electronics: Capacitors
Polarised capacitors must be connected the correct way round as shown by their circuit symbol on the right. Markings on their body identify the leads and for radial style capacitors the longer lead is +. Polarised capacitors are not damaged by heat when soldering.
These are the most widely used type of polarised capacitor and they are available in two styles: radial with both leads at the same end (10µF in picture) and axial with leads at each end (220µF in picture). Radial capacitors tend to be a little smaller and cheaper.
Electrolytic capacitors are large enough to be clearly labelled with their capacitance, voltage rating (see below) and polarity so they are usually easy to identify. Always take care to connect electrolytic capacitors the correct way round because they may explode when reversed.
Electrolytic capacitors have a voltage rating which can be quite low and it should always be checked when selecting an electrolytic capacitor. If a project parts list does not specify a voltage, choose a capacitor with a rating which is greater than the project's supply voltage. 25V is a sensible minimum for most battery circuits.
Rapid Electronics: Electrolytic Capacitors
Tantalum bead capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small and are used in special situations where their small size is important.
Modern tantalum bead capacitors are printed with their capacitance, voltage and polarity. Older ones use a colour-code system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V). The positive (+) lead is to the right when the spot is facing you: 'when the spot is in sight, the positive is to the right'.
For example: blue, grey, white spot means 6.8µF
Rapid Electronics: Tantalum Capacitors
Small value capacitors are unpolarised and may be connected either way round. There are various types but ceramic is the most widely available and suitable for most purposes. Unpolarised capacitors are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V so these can be ignored for most projects suitable for beginners.
Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be.
For example 0.1 means 0.1µF = 100nF.
Sometimes the multiplier is used in place of the decimal point:
For example: 4n7 means 4.7nF.
A number code is often used on small capacitors where printing is difficult:
For example: 102 means 1000pF = 1nF (not 102pF)
472J means 4700pF = 4.7nF (J means 5% tolerance).
Rapid Electronics: Ceramic Capacitors
A colour code was used on polyester capacitors for many years, it is now obsolete but colour-coded capacitors may still be found.
The colours should be read like the resistor code, the top three colour bands giving the value in pF. Ignore the 4th band (tolerance) and 5th band (voltage rating).
brown, black, orange means 10000pF = 10nF = 0.01µF.
Note that there are no gaps between the colour bands, so two identical bands appear as a wide band, for example:
wide red, yellow means 220nF = 0.22µF.
Polystyrene capacitors are rarely used now. Their value in pF is normally printed without units. Polystyrene capacitors can be damaged by heat when soldering (it melts the polystyrene) so you should use a heat sink, such as a crocodile clip. Clip the heat sink to the lead between the capacitor and the joint.
You may have noticed that capacitors are not available with every possible value, for example 22µF and 47µF are readily available, but 25µF and 50µF are not.
Why is this? Imagine that you decided to make capacitors every 10µF giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits and capacitors cannot be made with that accuracy.
To produce a sensible range of capacitor values the size of the 'step' between values must increase as the value increases. The standard capacitor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.
The E3 series has 3 values for each multiple of ten: 10, 22, 47, ... then it continues 100, 220, 470, 1000, 2200, 4700, 10000 etc. Notice how the step size increases as the value increases (values roughly double each time).
The E6 series has 6 values for each multiple of ten: 10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc. Notice how this is the E3 series with an extra value in the gaps.
The E3 series is the one most frequently used for capacitors because many types cannot be made with very precise values.
Variable capacitors are mostly used in radio tuning circuits and they are sometimes called 'tuning capacitors'. They have very small capacitance values, typically between 100pF and 500pF. Some have trimmers built in (for making small adjustments - see below) as well as the main variable capacitor. Note that many have very short spindles unsuitable for the standard knobs used for variable resistors.
Variable capacitors are not normally used in timing circuits because their capacitance is too small to be practical and the range of values available is very limited. Instead timing circuits use a fixed capacitor and a variable resistor.
Variable Capacitor Symbol
Photograph © Rapid Electronics
Trimmer capacitors (trimmers) are miniature variable capacitors. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built. They are the capacitor equivalent of presets which are miniature variable resistors.
A small screwdriver or similar tool is required to adjust trimmers. The process of adjusting them requires patience because the presence of your hand and the tool will slightly change the capacitance of the circuit in the region of the trimmer!
Trimmer capacitors are only available with very small capacitances, normally less than 100pF. It is impossible to reduce their capacitance to zero, so they are usually specified by their minimum and maximum values, for example 2-10pF.
Trimmer Capacitor Symbol
Photograph © Rapid Electronics