1. Chapter 11 Capacitance

2. 2 Objectives –After completing this chapter, the student should be able to: Explain the principles of capacitance. Identify the basic units of capacitance. Identify different types of capacitors.

3. 3 Determine total capacitance in series and parallel circuits. Explain RC time constants and how they relate to capacitance.

4. 4 Capacitance –the ability of a device to store electrical energy in an electrostatic field. –The letter C stands for capacitance.

5. 5 Capacitor –A device that possesses a specific amount of capacitance. –Made of two conductors separated by an insulator. The conductors are called plates. The insulators are called dielectric.

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7. 7 Treat all capacitors as though they were charged. –Never touch both leads of a capacitor with your hands. –A capacitor can hold a potential indefinitely if it does not have a discharge path.

8. 8 The basic unit of capacitance is the farad (F). A farad is the amount of capacitance that can store 1 coulomb (C) of charge when the capacitor is charged to 1 volt. –1 microfarad (  F) = 0.000,001 or 1/1,000,000 farad. –1 picofarad (pF) = 0.000,000,000,001 or 1/1,000,000,000,000 farad.

9. 9 A capacitor is either fixed or variable. –A fixed capacitor has a definite value that cannot be changed. –A variable capacitor is one whose capacitance can be changed. by varying the space between the plates or by varying the amount of meshing between two sets of plates (tuning the capacitor).

10. 10 Factors affecting capacitance –Area of the plate. –Distance between the plates. –Type of dielectric material. –Temperature.

11. 11 Dielectric constant –A measure of the effectiveness of a material as an insulator. Some examples of dielectric constants: –Paper = 2 - 3 –Mica = 5 - 6 –Titanium = 90 - 170

12. 12 Electrolytic capacitors –Large capacitance for size and weight. –Consists of two metal foils separated by fine gauze saturated with a chemical paste called an electrolyte. –Polarized, having a positive and negative lead.

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14. 14 Paper and plastic capacitors –Constructed by a rolled foil technique. –Plastic film is used more often.

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16. 16 Ceramic disk capacitors –Popular because they are inexpensive to produce. –Used for 0.1 microfarad and smaller. –Ceramic is dielectric. –Reliable general purpose capacitor.

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18. 18 Variable capacitors –Padders –Trimmers –Tuners

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20. 20 Formula for total capacitance in series circuits: 1/C T = 1/C 1 + 1/C 2 + 1/C 3... + 1/C n Formula for total capacitance in parallel circuits: C T = C 1 + C 2 + C 3... + C n

21. 21 RC Time Constants –Reflects the time required for a capacitor to charge up to 63.2% of the applied voltage or to discharge down to 63.2%.

22. 22 –Time constant is expressed as t = RC t = time in seconds R = resistance in ohms C = capacitance in farads

23. 23 In Summary –Capacitance –Capacitor –Fixed capacitor symbol –Variable capacitor symbol

24. 24 –Capacitance is measured in farads (F). –Microfarads (  F) and picofarads (pF) are used most often. –The letter C represents capacitance. –Four factors affecting capacitance Area of plates Distance between plates Types of materials Temperature

25. 25 –Capacitor types Electrolytic Paper Plastic Ceramic Variable –The formula for total capacitance in a series circuit is: –1/C T = 1/C 1 + 1/C 2 + 1/C 3... + 1/C n

26. 26 –The formula for total capacitance in parallel circuits is: C T = C 1 + C 2 + C 3... + C n –The formula for the RC circuit time constant is: t = RC –Time constants