May 282012
 

A capacitor also known as a condenser is a passive electrical component that received its name from the Latin condensare – compression, accumulation, indicating what the component is used for. Capacitors allow electric current to pass through it but store and accumulate electrical charge. In general, a capacitor contains two electrical conductors separated by a substance that is slightly conductive or completely non-conductive– a dielectric.

C simbol

The capacitor can have constant or variable capacitance. Capacitance is a basic parameter of capacitors and characterizes their ability to accumulate electrical charge. It is indicated on the capacitor and is measured in farads. One farad is the capacitance of a capacitor between whose electrodes there is a potential difference of 1 volt under load of electricity of 1 coulomb and is denoted by F, i.e. 1F = 1C/1V. This physical quantity is named after the English physicist and chemist Michael Faraday. The value of the capacitance equal to 1 farad is a fairly large quantity, and there is practically no capacitors whose capacitance is measured only in farads, except some super-capacitors, therefore the most often used quantities are picofarad (10-12 F), nanofarad (10-9 F) and microfarad (10-12F). The capacitance, indicated on the capacitor, however, is nominal or planned and the actual one may differ from it significantly and depends on many factors, therefore it is necessary to use the concept tolerance, which shows the largest percent difference between the planned and the actual capacitance.
Other basic parameters of the capacitor are the rated voltage which should not be exceeded because this can lead to breakdown of the capacitor’s dielectric, the breakdown voltage – this is the voltage at which irreversible breakdown occurs. Capacitor losses, tangent of the dielectric loss angle, temperature coefficient showing the capacitance alteration depending on the temperature, specific capacitance, showing the relation of the capacitance to the volume of the capacitor, the work temperature – this is the ambient temperature and is given in degrees Celsius, the test voltage between the terminals, the test voltage for the insulation of the housing, etc.

Depending on the type of the dielectric capacitors may be:

• Liquid dielectric capacitors (electrolytic capacitors) – their specific capacitance is very large, their polarity should be observed.

• capacitors with solid inorganic dielectric (ceramic, glass, mica, glass-enamel)
• capacitors with solid organic dielectric (paper, metal-paper, foil-stiroflex)
• gaseous dielectric capacitors
• vacuum dielectric capacitors (there is vacuum between the capacitor’s plates)

Depending on their form capacitors may be flat, cylindrical or spherical.

Applications of capacitors

Capacitors are used virtually everywhere in electrical engineering for a variety of reasons:
• they can be used in electrical circuits to compensate the inductive power and for filtering higher harmonics.
• as start capacitors – they are used only during the start-up of electric motors. They are designed to compensate the reverse component of a rotating magnetic field in start mode, thereby increasing the starting torque. When the rated rotation frequency of the rotor is reached, the capacitor is switched off by a centrifugal switch, a time relay or an electric relay. The work of a three-phase induction motor connected to a single-phase voltage can be used for rotation as well.
• in DC circuits, capacitors are used to even out the pulsations of the rectified voltage
• the ability of the capacitor to store the electric charge for a long time allows the component to be used as a type of memory device.
• in AC circuits capacitors can be used as a ballast for limiting the current strength.
• the capacitor can be very quickly discharged when connected to a circuit with low-ohmic resistance. The rapid discharge leads to an impulse with high momentary power, which is used in pulsed lasers, in photoflash, in electromagnetic accelerators, in pulse high-voltage generators, in a Cockcroft–Walton voltage multiplier for accelerating particles and breaking the atomic nucleus.
• the process of charging and discharging a circuit with a capacitor and resistor (CR circuit) takes a specified time, which allows the capacitor to be used as a timing device when high accuracy is not required, considering that it is dependent on temperature.
• as a source of powerful electric discharge (Van der Graaf generator)
• capacitor for starting a car engine. There the capacitor is used for fast compression of the magnetic field and for preventing arcing between the contact terminals.
• in various measuring devices. For example, for measuring the moisture in the air – when hygroscopic wood is used as a dielectric, by changing its humidity the capacitance of the capacitor also changes. Measuring the level of a fluid – when the level of a conductive fluid increases, reaching the two poles of the capacitor, this leads to the capacitor’s discharging. Measuring very small displacements – when moving the plates of a flat capacitor, its capacitance changes.
• The latest application of capacitors with very high capacitance and very long period of discharge (super-capacitors) is in electric vehicles and hybrids.

Capacitor connection

Capacitors can be connected in parallel, in series and mixed. The calculation of the total capacitance of capacitors that are connected in parallel is done by simply summing them up: C = C1 + C2 + … + Cn.

C paralel
The calculation of capacitors that are connected in series is done through their reciprocal values: 1 / C = 1/C1 1/C2 + … + 1/Cn.

C serial
If the connection is mixed, the total capacitance is calculated by dividing the circuit into sections and calculating the capacitance using the above formulas. Parallel connection is used when we want to reduce the risk of breakdown because each capacitor takes only part of the potential difference. Parallel connection is used when we want to achieve higher capacitance than the capacitance of each single capacitor.

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