Capacitor

Capacitor, Capacitance, Symbol of capacitors

Capacitor: 

                 The capacitor is a component which has the ability or “capacity” to store energy in the form of an electrical charge producing a potential difference across its plates, much like a small rechargeable battery.

Capacitance is the ability of a component or circuit to collect and store energy in the form of an electrical charge. 

Unit of capacitance is measured in farad (f). 

Construction of capacitor: A capacitor consists of two conducting plates separated by an insulator (or dielectric). 

Working of Capacitor

                  A capacitor collects energy (voltage) as current flows through an electrical circuit. Both plates hold equal charges, and as the positive plate collects a charge, an equal charge flows off the negative plate.

 When the circuit is switched off, a capacitor retains the energy it has gathered, though slight leakage usually occurs. 

To understand the working of capacitor; please click the link below

 https://www.youtube.com/watch?v=5hFC9ugTGLs 

https://www.youtube.com/watch?v=f_MZNsEqyQw

Types of Capacitors:

                     Based on the construction and material used capacitors are widely classified into different types some are

               1. Paper Capacitor 

               2. Ceramic capacitor 

               3. Mica capacitor 

               4. Plastic film capacitor 

               5. Electrolytic capacitor

Paper capacitor: 

            Paper capacitors are made of paper or oil-impregnated paper and aluminum foil layers rolled into a cylinder and sealed with wax. These capacitors were commonly used but are now replaced by the plastic or polymer type of capacitors. The paper capacitors are bulky, highly hygroscopic and soaks moisture which causes loss to the dielectric degrading its overall performance is the major drawback with this type of capacitors.

Ceramic capacitor: 

                Ceramic Capacitors or Disc Capacitors are generally made by coating two sides of a small porcelain or ceramic disc with silver and are then stacked together to make a capacitor. For very low capacitance values a single ceramic disc of about 3-6mm is used. Ceramic capacitors have a high dielectric constant and are available so that relatively high capacitance’s can be obtained in a small physical Size. 

Mica capacitor: 

                 Mica capacitors are manufactured by plating silver electrodes directly on to the mica film dielectric. To achieve the required capacitance, several layers are used. Wires for the connections are added and then the whole assembly is encapsulated. The values of silver mica capacitors range in value from a few Picofarads up to two or three thousand Picofarads. This type of capacitor is not as widely used these days. However they can still be obtained and are used where stability of value is of the utmost importance and where low loss is required. 

Plastic film capacitor: 

                   These use polystyrene, polycarbonate or Teflon as their dielectrics are sometimes called “Plastic capacitors”. The construction of plastic film capacitors are similar to that for paper film capacitors but use a plastic film instead of paper. The main advantage of plastic film capacitors compared to impregnated-paper types is that they operate well under conditions of high temperature, have smaller tolerances, a very long service life and high reliability.

Electrolytic capacitor: 

                     Electrolytic Capacitors are generally used when very large capacitance values are required. Here instead of using a very thin metallic film layer for one of the electrodes, a semi-liquid electrolyte solution in the form of a jelly or paste is used which serves as the second electrode (usually the cathode).

To understand the working of types of capacitor; please click the link below https://www.youtube.com/watch?v=e3W0kdLodXo&t=49s 

Factors affecting Capacitance of Capacitors 

1. Plate Area: 

           All other factors being equal, greater plate area gives greater capacitance; less plate area gives less capacitance. Explanation: Larger plate area results in more field flux (charge collected on the plates) for a given field force (voltage across the plates). 

2. Plate Spacing: 

           All other factors being equal, further plate spacing gives less capacitance; closer plate spacing gives greater capacitance. 

Explanation: Closer spacing results in a greater field force (voltage across the capacitor divided by the distance between the plates), which results in a greater field flux (charge collected on the plates) for any given voltage applied across the plates. 

3. Dielectric Material: 

                    All other factors being equal, greater permittivity of the dielectric gives greater capacitance; less permittivity of the dielectric gives less capacitance. 

Explanation: Although it’s complicated to explain, some materials offer less opposition to field flux for a given amount of field force. Materials with a greater permittivity allow for more field flux (offer less opposition), and thus a greater collected charge, for any given amount of field force (applied voltage).

Specification of Capacitors 

                    Capacitor specification is always mentioned with values and voltage rating.

All capacitors have a maximum voltage rating and when selecting a capacitor consideration must be given to the amount of voltage to be applied across the capacitor. The maximum amount of voltage that can be applied to the capacitor without damage to its dielectric material is generally given in the data sheets as: WV, (working voltage) or as WV DC, (DC working voltage). 

If the voltage applied across the capacitor becomes too great, the dielectric will break down (known as electrical breakdown) and arcing will occur between the capacitor plates resulting in a short circuit. The working voltage of the capacitor depends on the type of dielectric material being used and its thickness. 

However, if the dielectric material becomes damaged due excessive voltage or over temperature, the leakage current through the dielectric will become extremely high resulting in a rapid loss of charge on the plates and an overheating of the capacitor eventually resulting in premature failure of the capacitor. Then never use a capacitor in a circuit with higher voltages than the capacitor is rated for otherwise it may become hot and explode. 

Characteristics of capacitor: 

                  It blocks flow of DC and permits AC 

                  It is used for coupling two devices 

                  It bypasses the unwanted frequencies 

                  It feeds the desired signal to any section

                  It is used for phase shifting, filtering, motor starter 

                  It is used to get tuned frequency 

Capacitors in Series and Parallel