Capacitor - Physics Assignment Angstrom Edu Pvt Ltd

 

Capacitor

 

 A capacitor is an electrical component capable to store electrical energy in the form of voltage existing across it. The voltage applied across the capacitor sets up an electric field in it and energy is stored in the electric field. Various capacitors are shown in Fig. 1.

Fig. 1 Various Capacitors

Capacitor consists of two conducting plates separated by insulating (non-conducting) or dielectric material when the voltage is applied across the two plates it stores the voltage in the form of electric field, as one plate becomes positive charged plate correspondingly other plate becomes negative charged and it releases energy when load is connected across it at the other side. Capacitance is the ability of capacitor to store charge measured generally in micro Farads (µF) and pico Farad (pF).

 A capacitor offers low impedance to ac (alternating current) and high impedance to dc (direct current) means it easily allow ac to pass through it but at the same time it blocks dc.

Capacitors are used for waveform generation, filtering, bypassing and blocking applications. They are used in ICs like integrators, differentiators, and in many wave shaping circuits and in TV remote control and radio also we can see the direct use of it.  

Capacitor can be fixed capacitor or variable capacitor. Variable capacitors are mostly air-gang capacitors which are used in radio telecast work. Symbolic representation of capacitors are shown in Fig. 2.

                         

                                               

Fig. 2 Symbolic representation of Fixed and Variable Capacitors

Types of Capacitor- Capacitors are categorized based on insulating material used as

1.     Mica capacitors

2.     Paper capacitors

3.     Plastic film capacitors

4.     Electrolytic capacitors

5.     Non polarized capacitors

6.     Ceramic capacitors

1.     Mica Capacitors- Mica capacitors are constructed by sandwiching mica with metal foil.  Fig. 3 shows the Mica capacitors.

Fig. 3 Mica capacitors

They are of round, rectangular, or irregular shape as shown above. It is having very small leakage current (as leakage resistance in the order of 1000MΩ) and dissipation factor. Available range of mica capacitors are 1pF to 0.1 µF with ± 1 to ± 20 percent.

The structure of mica capacitors cannot be reduced because mica is not flexible enough to be rolled in tubes. Mica capacitors are used as precision capacitor because of their small tolerance and high stability under temperature therefore used in high frequency applications as oscillator tuning and filter circuits. It also gives best service in high voltage and high current applications.

Mica capacitors have no polarity preference and their capacitance value are indicated by a colour code scheme printed on their package similar to resistance colour coding.

          2. Paper Capacitors- Paper capacitor are usually of cylindrical shape because they are made by    a sandwich of metal and impregnated paper sheet into a tube. Axial leads are attached to each metal sheet and the tube is encapsulated in waxed paper or plastic. Fig. 4  shows the picture of Paper capacitor.

Fig. 4 Paper capacitor

It is most widely used capacitor because of it’s low cost and broad range of capacitance value (500pF to 50µF). It can withstand high voltage, however the leakage current of paper capacitor is high and tolerance is relatively low therefore restricted for some applications.Paper capacitors are generally used in certain power supply and transmitting circuits.

        3. Plastic film capacitors- Plastic film capacitors are constructed in the same technique as paper capacitor, except that a thin sheet of plastic (such as Mylar, Teflon, or Polyethylene) is employed as dielectric. This dielectric material improves the properties of the capacitor by minimizing leakage currents at high temperature as 150-200degree centigrade. Other properties are similar to those of paper capacitor but the cost is high, generally used when paper capacitor cannot meet the design specifications. The range of capacitor is between 500pF to 10µF. Fig. 5 shows the image of Plastic film capacitor.

Fig. 5 Plastic film capacitor

         4. Electrolytic capacitors- Electrolytic capacitors are shown in Fig. 6. The basic structure of aluminium electrolyte capacitor consists of two aluminium foils, one of which is coated by an extremely thin oxide. The oxide is grown on the metal by a process of applying a voltage to the capacitor called forming.

 

Fig. 6 Electrolytic Capacitor

The thickness of oxide depends on the forming voltage. Between the foils is an electrolyte solution soaked into the paper. This electrolyte is a conductor and serves as an extension of the non-oxidized metal foil. The two oppositely charged plates are effectively separated by only an extremely thin oxide film which possesses extremely high dielectric constant. Once the oxide is formed, the foils are rolled into a tube and the piece of foil without the oxide is connected to the capacitor’s exterior package which is the negative connection to the capacitor. The other lead marked as positive terminal.

It is important to give strong emphasis on polarity connections and rated voltage must not exceed the limit. It is available in various ranges as 1µF to 0.5F.

        5. Non-polarized capacitor- These are the capacitors which has no polarity constrain issue, it can be connected either way in the circuit. Fig. 7 shows the image of non-polarized capacitor.

 

Fig. 7 Non-polarized capacitor

        6. Ceramic capacitor- Ceramic capacitor is the most frequently used capacitor. It uses ceramic material as insulating material. Ceramic capacitors are of disc or tubular type. Fig. 8 shows the appearance of ceramic  

Fig. 8 Ceramic Capacitor

It is divided into two types as low loss low dielectric constant, which has high leakage resistance used in high frequency applications almost as mica capacitor is used. Second type is low loss high dielectric constant type provides large capacitance in small volume but capacitance varies as temperature, voltage and frequency varies, so used in the places where exact capacitance is not required like circuit coupling and bypass capacitors.

 

FAQ

1)     What are various types of capacitors?

2)     What is capacitor?

3)     On what category we can classify capacitors?

4)     What is the symbolic representation of capacitor?