CAPACITORS
The two conducting plates separated by an insulant (called dielectric) forms a capacitor. The essential purpose of a capacitor is to store the charge. The capacity of a capacitor to store charge per unit electric potential is named its capacitance. The unit of capacitance is farads (F). However, the unit farad being overlarge , the capacitors are specified. practically in microfarads (µF) or picofarads (pF).
A capacitor may be a component which offers low impedance to a.c. but very high impedance (resistance) to d.c. In most of the electronic circuits, a capacitor has d.c. voltage applied, combined with a way smaller ac. signal voltage. the standard function of the capacitor is to dam the d.c. voltage but pass the a.c. signal voltage, by means of charging and discharging. These applications include coupling, bypassing and filtering for a.c. signal. The schematic symbol of fixed and variable capacitor (C) is shown in figure.
Types of Capacitors
All the capacitors have two main characteristics i.e. their capacitance (C) in farads (or µF or pF) and their operating voltage. The commercial capacitors are generally classified according to the dielectric. Some of the most commonly used capacitors are paper, mica, ceramic, electrolytic and air. Like resistors, capacitors can also either be fixed or variable type. The variable capacitors are mostly air-gang capacitors.
There is no required polarity, since either side can be made positive plate, except for electrolytic capacitor. In electrolytic capacitors, polarity is marked which indicates the side which must be considered as positive. It maintains the internal electrolytic action that produces the dielectric (a thin oxide film) required to form the capacitance.
1. Paper Capacitors : Perhaps, paper capacitors are the most common of all capacitors. For the Construction of these capacitors two metal (aluminium or tin) foils separated by paper inpregnated with a dielectric material such as wax, plastic or oil are rolled into a compact cylinder. Connecing leads are attached to the two metal plate. The entire cylinder is generally placed in a cardboard container coated with wax or encased in plastic.
Paper capacitors are available in a wide range of capacitance values and voltage ratings. Typical capacitance values ranging from 0.0001 µF to 1.0 µF having tolerance of ±10 percent usually with voltage ratings ranging from 200 to 10000 volts and more. Their leakage resistance is of the order of 100 M𝛀. Physical size for 0.05 µF is typically 2.5 cm long with 1 cm diameter. These capacitors should not be used in radio-frequency tuned circuits because they are not electrically stable enough.
A black band at one end of a paper capacitor indicates the lead connected to the outside foil. This be used for the ground or low-potential side of the circuit to take advantage of shielding the outside foil. However, there is no required polarity, since the capacitance is the same no matter which side is grounded.
2. Mica Capacitor : These capacitors consist of alternate thin sheets of metal (aluminium or tin) foils separated by thin mica sheets. Alternate metal sheets are connected together and brought out as one terminal for one set of plates, while the opposite terminal connects to the other set of plates. The entire unit is generally encased in a plastic housing or moulded in bakelite case. In silver-mica capacitors, the opposite faces of the mica sheets are silver coated (which acts as the conducting material).
Mica capacitors are often used for small capacitance values ranging 50 to 500 pF having tolerance of ±2 to ±20 percent usually, with voltage ratings ranging from 200 to 1000 V. The leakage resistance of mica capacitors is of the order of 1000 mega-ohm that is why their leakage current is very small. These capacitors are very small in size having 10 mm length and 3 mm thickness. These capacitors are used extensively in rf tuned circuits, since they are considerably more stable electrically than paper capacitors.
Silver-mica capacitors are more stable electrically than foil-type capacitors and are used in high stability frequency determining circuits. Silver-mica capacitors are available in tolerances ranging from ±5 percent to ±1 percent or better.
3. Ceramic Capacitors : The ceramic is a dielectric material made from earth fired under extreme heat. Titanium oxide or several other types of silicates are used to obtain very high value of dielectric constant of ceramic material. The ceramic capacitors may be of disc type or tubular type. These capacitors are also available in other shapes.
In the disc type, a ceramic disc is coated on two sides with a metal (silver or copper). Tinned copper wire leads are attached to each coating which act as electrodes or plates.
Then the entire unit is encapsulated in a protective coating (plastic) and marked with its capacitance value, either using numerals or a colour code. The colour coding is similar to that used for resistances.
Disc ceramic capacitors are commonly available in capacitance values ranging from 47 pF to 0.05 µF having tolerance of ±2 to ±20 percent usually, with voltage ratings ranging from 200 to 1000 V. The leakage resistance of ceramic capacitors is of the order of 1000 mega-ohm. These capacitors are very small in size.
In tubular ceramic capacitors, the inner and outer surfaces of a hollow ceramic tube are coated with silver and form the two plates of the capacitor. These capacitors are available in capacitance values ranging from 1 to 500 pF.
Special low-voltage, high capacitance ceramic capacitors are also available for use in transistor circuits. These Capacitors are available in capacitance values ranging from 0.05 µF to 0.47 µF and higher with voltage ratings ranging from 3 to 100 V.
Ceramic capacitors are used primarily as coupling and bypass portions of radio frequency circuts rather than frequency detemining elements. Specially designed ceramic capacitors are used in resonant circuits.
4. Electrolytic Capacitors : An electrolytic capacitor contains two aluminium electrodes. Between the two electrodes, absorbent gauze soaks up electrolyte (borax, phosphate or carbonate) to provide the required electrolysis that produces an oxide film (a molecular-thin layer of aluminium oxide) at the positive electrode when d.c. voltage is applied. The oxide film acts as an insulator and forms a capacitance between the positive aluminium electrode and the electrolyte in the gauze separator. The negative aluminium electrode simply provides a connection to the electrolyte. Usually, the metal can itself act as the negative terminal of the capacitor.
Electrolytic capacitors have a high capacitance to size ratio since the aluminium oxide layer is molecular- thin. These capacitors are available in capacitance values ranging from less than 1 µF to 10 000 µF or more. Common voltage ratings range from 1 to 700 volts. The leakage current of these capacitors is as high as 0.1 to 0.5 mA/µF. These capacitors are commonly used in situations where a large capacitance is required. Generally, these capacitors are used in the filter section of d.c. power supplies and transistor circuits. These are also used as audio coupling capacitors in transistor amplifiers.
These capacitors must be connected in the circuit as per polarity marked on the capacitor. If they are connected in opposite polarity, the reversed electrolysis forms gas in the capacitor. It becomes hot and may explode. Non-polarized electrolytic capacitors are also available which are applied in the starting winding of a.c. single-phase motors. Basically, a non-polarized electrolytic capacitor contains two capacitors, connected internally in series-opposing polarity. The capacitance is one-half either capacitance, but the oxide film is maintained.
In the new form of electrolytic capacitors tantalum (Tₐ) is used instead of aluminium. Sometimes titanium (Tᵢ) is also used. For the same capacitance, the size of the tantalum capacitor is very small since the dielectric constant for tantalum oxide is 25 as compared to dielectric constant of 7 for aluminum oxide The other important features of these capacitors are that they have longer life and less leakage current.
Variable Capacitors
The electronic circuits, in which frequency is to be changed as per requirement such as tuning circuits, variable capacitors are used. Air-gang capacitor is the most common variable capacitor. These capacitors are available in single, dual and three-section units.
These capacitors are generally available in capacitance ranges from 10 pF to 365 pF. The capacitance of the capacitor depends upon the common area between the movable and fixed plates. The common area can be changed by rotating a shaft to which movable plates are attached. The greater the common area the larger is the capacitance.
Another type of variable capacitor is called trimmer (sometimes also called padder). These capacitors are used in the electronic circuits where the variation of capacitance is not frequent. Once a setting is obtained to match he circuit, the capacitance is not to be changed after that. This capacitor consists of two or more metal (aluminium) sheets separated by sheets of mica or ceramic. The metal sheets are under spring tension so that as they are sequeezed together by the turning of an adjusting screw, their physical spacing and hence, trimmer capacitance is varied.
CAPACITOR COLOUR CODING
Colour-coding is used to indicate the capacitance value of mica and tubular ceramic capacitors since they are smaller in size. The capacitance value of these capacitors is usually in pF units. The colours used are the same as for resistor coding, from black for 0 upto white for 9.
A six dot system is generally used for mica capacitors. Reading the top row first from left to right, then the bottom row, in reverse order i.e. from right to left. The first dot indicates the material of capacitor i.e. White colour for mica and Siver for paper. The value of capacitance is read from dots 2, 3 and 4. For example, if the colours of dots 2, and 4 are red, green and brown, the capacitance is 430 pF. Dot 5 specifies tolerance, while dot 6 gives Case of capacitor, specifying its temperature co-efficient.
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