Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2002 The McGraw-Hill Companies Grob Schultz

Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2003 The McGraw-Hill Companies 17 CHAPTER Capacitance

Topics Covered in Chapter 17  How Charge Is Stored in the Dielectric  Charging and Discharging Capacitor  The Farad Unit of Capacitance  Typical Capacitors  Electrolytic Capacitors  Capacitor Color Coding

 Parallel Capacitances  Series Capacitances  Stray Capacitive and Inductive Effects  Energy in Electrostatic Field of Capacitance  Troubles in Capacitors Topics Covered in Chapter 17 (continued)

Capacitor Action A capacitor consists of two conductors separated by a dielectric (insulator). Capacitors store energy in the dielectric field. Applying a voltage to a discharged capacitor causes a current to charge the capacitor. Connecting a path across the terminals of a charged capacitor causes current to flow which discharges the capacitor.

Electric Field in a Charged Capacitor Negative plate (conductor) Positive plate (conductor) Dielectric (insulator) } Electric lines of force

Capacitor Charge and Discharge (b) Capacitor charges (c) Charge holds(d) Capacitor discharges (a) No initial charge

The Unit of Capacitance The farad (F) is the basic unit of capacitance. One farad stores one coulomb of charge with one volt applied. Most capacitors have values less than 1 F:  1  F (microfarad) = 1  F  1 nF (nanofarad) = 1  F  1 pF (picofarad) = 1  F

Chip Capacitors Tantalum oxide dielectric (polarized) Ceramic dielectric (non-polarized)

Capacitors Charge and Store Energy Charge on a capacitor, in coulombs: Q = CV Energy stored in a capacitor in joules:  = ½CV 2 Where: Q = electrical charge in coulombs C = capacitance in farads V = voltage in volts  = energy in joules

25 V 1000  F/25 V (polarized) Stored Charge Q = CV = 1000 x x 25 = coulombs

Stored Energy This capacitor can safely store:  = ½CV 2 = ½ 1000 x = J 25 volts is a maximum rating  F/25 V

Capacitor Value Proportional to plate area (A) in meters. Inversely proportional to the spacing (d) between the plates in meters. Proportional to the dielectric constant (K  ) of the material between the plates. A d C = K  x 8.85 x 10 –12 F The value of a capacitor is:

MaterialKK Air or vacuum1 Aluminum oxide7 Ceramic80 – 1200 Glass8 Mica3 – 8 Oil2 – 5 Paper2 – 6 Plastic2 – 3 Tantalum oxide25 Dielectric Constants Plastic Ceramic Paper

Three Ways to Increase Capacitance Increase the dielectric constant Increase plate area Decrease plate spacing

Variable Capacitors The plate spacing is changed to vary the capacitance. Air dielectric variable 1 to 10 pF (trimmer capacitor) Schematic symbol Moveable concentric plates

Total Capacitance of Circuits Series Circuits CCCC etc EQ  . CCCCetc T  123 . Parallel Circuits

Capacitors in Series and Parallel 2  F 5  F 10  F 2  F5  F 10  F C EQ = 17  F C EQ = 1.25  F

Voltage Distribution in a Series Circuit 50 V Q = C EQ V = 1.25 x x 50 = 62.5  C The charge (Q) is constant in a series circuit. 2  F 5  F 10  FC EQ = 1.25  F C1C1 C3C3 C2C2 V C 1 = Q / C 1 = 62.5 x / 10 x = 6.25 V V C 2 = Q / C 2 = 62.5 x / 5 x = 12.5 V V C 3 = Q / C 3 = 62.5 x / 2 x = V KVL check: = 50

Capacitor Defects Open Shorted Excess series resistance Leaky Generally, electrolytic capacitors are the least reliable. +

Safety Capacitors can store lethal charges. Some types can stay charged for months. Connecting polarized capacitors incorrectly can cause them to explode. Applying excess voltage to some capacitors can cause them to explode or ignite. Discharging large capacitors with clip leads or screwdrivers can cause a nasty arc and flying, hot metal.