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Symbol of Capacitor
Capacitors store electric charge. They
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. |
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Capacitance
This is a measure of a capacitor's
ability to store charge. A large capacitance means that
more charge can be stored. Capacitance is measured in
farads, symbol F. However 1F is very large, so prefixes
are used to show the smaller values.
Three prefixes (multipliers) are used,
µ (micro), n (nano) and p (pico):
 | µ means 10-6
(millionth), so 1000000µF = 1F
| n means 10-9
(thousand-millionth), so 1000nF = 1µF
| p means 10-12
(million-millionth), so 1000pF = 1nF |
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Capacitor values can be very difficult
to find because there are many types of capacitor with
different labeling systems!
There are many types of
capacitor but they can be split into two groups,
polarized and unpolarized. Each group has its own
circuit symbol. |
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Tantalum Bead
Capacitors
| Tantalum
bead capacitors are polarized and have low voltage ratings like
electrolytic capacitors. They are expensive but very small, so they
are used where a large capacitance is needed in a small size.
Modern tantalum bead capacitors are printed with
their capacitance, voltage and polarity in full. However older ones
use a color-code system which has two stripes (for the two digits) and
a spot of color for the number of zeros to give the value in µF. The
standard color 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 color 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, black spot
means 68µF
For example: blue, grey, white spot
means 6.8µF
For example: blue, grey, grey spot
means 0.68µF
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Unpolarized capacitors
(small values, up to 1µF)
Small value capacitors are unpolarized
and may be connected either way round. They are not
damaged by heat when soldering, except for one unusual
type (polystyrene). They have high voltage ratings of at
least 50V, usually 250V or so. It can be difficult to
find the values of these small capacitors because there
are many types of them and several different labeling
systems! |
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Capacitor
Color Code
A color code was used on polyester
capacitors for many years. It is now obsolete, but of
course there are many still around. The colors should be
read like the resistor code, the top three color bands
giving the value in pF. Ignore the 4th band (tolerance)
and 5th band (voltage rating).  For example: brown, black, orange
means 10000pF = 10nF = 0.01µF.
Note that there are no gaps between
the color bands, so 2 identical bands actually appear as
a wide band.
For example: wide red, yellow
means 220nF = 0.22µF.
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ColorCode
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| Color |
Number |
| Black |
0 |
| Brown |
1 |
| Red |
2 |
| Orange |
3 |
| Yellow |
4 |
| Green |
5 |
| Blue |
6 |
| Violet |
7 |
| Grey |
8 |
| White |
9 |
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Polystyrene
Capacitors
This type is 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.
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