CAPACITORS
| In its
simplest form, a capacitor consists of an insulator
sandwiched between two conductors. The insulator is
called a "dielectric" and may consist of
almost any insulating material ranging from paper, glass, ceramic,
air, oil, plastic and so on.
Often the 'sandwich' is rolled into a cylindrical shape to save space.
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Exploded view of a typical capacitor - An insulator between two conductors. |
When a capacitor (also called a "condenser") is connected into an electrical circuit and a voltage is applied, electrons flow onto the metal plates. If the circuit is then disconnected the 'charge' will remain on the plates. A capacitor therefore acts as a 'storage device' for electricity. The capacitors used for tuning in radio applications, often have moveable 'plates', but in by far the majority of applications, the positions and sizes of the plates and dielectric are fixed. (Hence the capacitance remains constant.)
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The unit of capacitance is
the Farad. (After Michael Faraday) The symbol is the Greek letter
"mu" or: 
(For simplicity, the lower case letter "u" will be
used in these notes from now on.)
Until very recently, it would be true to say that a 'one-farad' capacitor would be the size of a dinner plate. The 'farad' is a very large unit. The majority of capacitors in general use have values in the range of: micro-farads, nano-farads, or pico-farads!
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(One farad = one million micro-farads = one thousand million nano farads = one million million pico farads.) |
Large capacity capacitors are used in domestic appliances such as some video recorders as an emergency memory back-up power supply, rather than using batteries. Typically, these capacitors may have values of five or six farads at 3 volts. This 'storage' feature of capacitors makes them useful in AC to DC power supplies where they are used to help 'smooth' the resulting DC output.
The key to understanding the other main use of capacitors in circuits is that they are basically insulators.
If a battery is connected to a capacitor, electrons will flow from the battery onto one of the plates and from the other plate into the battery. This flow of current will continue until the charge on each plate is at its maximum. (As determined by the construction of the capacitor.)
For the brief time this process is taking place, there appears to be a complete circuit. If a globe were connected into this circuit, it would glow during the time current was flowing and charging the plates.
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When the switch is closed electrons flow into the capacitor. While the capacitor is charging the globe will light up. |
If the battery is removed and the switch closed, electrons flow out of the capacitor. The globe lights up. |
Consider then, if alternating current (AC) is applied to the capacitor, the plates will be continually charging and discharging as current flows into and out of the plates. A globe connected into the circuit will now remain alight for as long as the varying voltage is applied.
A capacitor will therefore conduct changing current, but block D.C. (It is important to appreciate that the voltage need not necessarily alternate from positive to negative to be 'passed'; a changing d.c. current will pass 'through' a capacitor.)
ELECTRICAL CHARACTERISTICS OF CAPACITORS
As outlined previously, one use of capacitors is where the designer wishes to pass changing voltage ( the 'signal') while blocking unwanted DC effects. The choice of capacitor depends upon two factors - the signal voltage and its frequency. (In some cases, current is also significant.)
If the applied voltage exceeds the rating of the capacitor, current may 'punch' through the dielectric from one plate to the other. Maximum voltage rating is usually marked on the capacitor in some way.
The size of the plates and the thickness of the dielectric determines the efficiency with which the capacitor will pass a particular signal. A small, thin capacitor is more effective for higher frequency signal than one with large plates and thick dielectric.
Physical construction and the type of materials used will also have an effect on the frequency response of capacitors.
Once the general type of capacitor has been chosen (refer to the following table), the designer must select the capacitance needed to 'pass' the specific signal. As a general guide, if audio signals are involved (several thousand Hz), typical values would be tens of micro-farads, while radio frequency signals would necessitate the use of pico, or 'nano' value.
In reality, consideration must be given to the effects of other components, but the general rule applies.
CAPACITORS- TYPICAL ELECTRICAL CHARACTERISTICS:
| Capacitor type | Max. voltage | Frequency |
| ceramic | 50 volts typical | high (radio) |
| greencap | 100 volts typical | low to medium |
| tantalum | up to 50v typical | low |
| electrolytic | up to 100v typical | low |
NOTE: This table is meant to provide a guide only. There are many exceptions to the 'voltage' figures given. In general, a higher voltage rating results in much higher cost.
CAPACITOR MARKINGS
The capacitance of capacitors may be marked in one of several ways.
 |
 A 100 uF Electrolytic Capacitor |
Where there is sufficient room on the body of the device a number and the units will be printed e.g. 100uF 25 VW, which indicates that this capacitor has a capacitance of 100 micro-farads and a breakdown voltage of 25 volts. (approximately)
Smaller capacitors, such as 'greencaps' use a numerical system where the first place represents the first digit, the second place; the second digit and the third place is the number of zeros. (the multiplier) The capacitance so indicated is in picofarads!
104 K = 100,000pF or 0.1uF
Colour codes follow a similar pattern to that used for resistors, but they tend to become rather confusing at times. A good set of 'data' sheets should be consulted when decoding is needed.
