The two charged plates forms a device for concentrating and storing an electric charge. We refer to this device as a capacitor and its ability to store a charge is called capacitance ( also known as condenser ). Put simply, capacitance is a measure of the ability of a capacitor to store an electric charge when a potential difference is applied. Thus a large capacitance will store a larger charge for a given applied voltage.
A simple parallel plate capacitor is shown in fig.1.In practice and although air-spaced capacitors are used in some radio frequency (RF) applications, the space between the plates of most capacitors is filled with an insulating material, known as a dielectric. Typical dielectric materials are polyester, mica, or ceramic. Note that a dielectric material must be a good insulator (it must not conduct electric current) and also that it must be able to retain its insulating properties when a high voltage is applied to it.
If we plot charge, Q, against potential difference, V, for a capacitor we arrive at a straight line law. The slope of this graph is an indication of the capacitance, C, of the capacitor, as shown in Fig.1. From Fig.2, the capacitance is directly proportional to the slope of the graph, as follows:
In symbols this relationship is simply
C = Q/V
Or we can write above equation as
Q = CV and V = Q/C
where the charge, Q, is measured in coulombs (C) and the potential difference, V, is measured in volts (V).The unit of capacitance is the Farad (F)
Where one Farad of capacitance produces a charge of one Coulomb when a potential difference of one volt is applied. Note that, in practice, the Farad is a very large unit and we therefore often deal with sub-multiples of the basic unit such as microFarad (1 × 10-6F), nF (1 × 10-9F), and pF (1 × 10-12F).
Factors Determining Capacitance
Capacitance The capacitance of a capacitor depends upon the physical dimensions of the capacitor (i.e., the size of the plates and the separation between them) and the dielectric material between the plates. The capacitance of a conventional parallel plate capacitor is given by:
where C is the capacitance (in Farads), E0 is the permittivity of free space, Er is the relative permittivity (or dielectric constant) of the dielectric medium between the plates), A is the area of the plates (in square metres), and d is the separation between the plates (in meters). The permittivity of free space, E0, is 8·854 × 10-12 F/m.
In order to increase the capacitance of a capacitor, many practical components employ interleaved multiple plates (see Fig. 3) in which case the capacitance is then given by:
where C is the capacitance (in Farads), E0 is the permittivity of free space, Er is the relative permittivity of the dielectric medium between the plates), n is the number of plates, A is the area of the plates (in square metres), and d is the separation between the plates (in metres).
Capacitors in Practical
The specifications for a capacitor usually include the value of capacitance (expressed in microF, nF, or pF), the voltage rating (i.e. the maximum voltage which can be continuously applied to the capacitor under a given set of conditions), and the accuracy or tolerance (quoted as the maximum permissible percentage deviation from the marked value).
Other practical considerations when selecting capacitors for use in a particular application include temperature coefficient, leakage current, stability and ambient temperature range. Electrolytic capacitors require the application of a DC polarising voltage in order to work properly.
This voltage must be applied with the correct polarity (invariably this is clearly marked on the case of the capacitor) with a positive (+) sign or negative (–) sign or a coloured stripe or other marking. Failure to observe the correct polarity can result in over-heating, leakage, and even a risk of explosion!
The typical specifications for some common types of capacitor are shown in Table 1.
Working voltages are related to operating Temperatures and capacitors must be de-rated at high temperatures . Where reliability is important capacitors should be operated at well below their nominal maximum working voltages. Where the voltage rating is expressed in terms of a direct voltage (e.g. 250V DC) unless otherwise stated, this is related to the maximum working temperature.
It is, however, always wise to operate capacitors with a considerable margin for safety which also helps to ensure long term reliability. The working DC voltage of a capacitor should be no more than about 50% to 60% of its rated DC voltage.Where an AC voltage rating is specified this is normally for sinusoidal operation.
Performance will not be significantly affected at low frequencies (up to 100kHz, or so) but, above this, or when non-sinusoidal (e.g. pulse) waveforms are involved the capacitor must be de-rated in order to minimise dielectric losses that can produce internal heating and lack of stability.
Special care must be exercised when dealing with high-voltage circuits as large value electrolytic and metalised film capacitors can retain an appreciable charge for some considerable time. In the case of components operating at high voltages, a carbon film bleed resistor (of typically 1M ohm 0·5W) is often connected in parallel with the capacitor to provide a discharge path.
Some typical small capacitors are shown in Photos below.
Capacitor Markings and Colour Codes
The vast majority of capacitors employ written markings which indicate their values, working voltages, and tolerance. The most usual method of marking resin dipped polyester (and other) types of capacitor involves quoting the value (in ?F, nF or pF), the tolerance (often either 10% or 20%), and the working voltage (using _ and ~ to indicate DC and AC respectively). Several manufacturers use two separate lines for their capacitor markings and these have the following meanings:
First line: capacitance (in pF or microF) and tolerance (K=10%, M=20%)
Second line: rated DC voltage and code for the dielectric material.
A three-digit code is sometimes used to mark monolithic ceramic capacitors.
The first two digits correspond to the first two digits of the value whilst the third digit is a multiplier that gives the number of zeroes to be added to give the value in pF.
The colour code shown in Fig.4 is used for some small ceramic and polyester types of capacitor. Note, however, that this colour code is not as universal as that used for resistors and that the values are marked in pF (not F).
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