Capacitors Chapter 12 EGR 101 1 Definition Capacitance

Capacitors Chapter 12 EGR 101 1

Definition • Capacitance – the ability of a component to store energy by accumulating charge • A capacitor is a circuit component designed to store charge • Practical applications with capacitors: Camera flash – Charges up and then quickly discharges Power storage – Solar collectors charge up capacitors so that energy can be used after EGR 101 2 dark

Capacitor Construction • 2 Plates • Separated by a Dielectric EGR 101 3

Variable Capacitors – Interleaved-Plate Capacitors EGR 101 4

Fixed Value Capacitors • Polarized Electrolytic Capacitors • Most electrolytic capacitors are polarized EGR 101 5

Capacitance • Amount of charge that a capacitor can store per unit volt applied where C = the capacitance of the component, in Coulombs per Volt defined as Farad (F) [C] = [Q]/[V]=C/V = F. Q = the total charge stored by the component V = the voltage across the capacitor corresponding to the value of Q EGR 101 6

Capacitance Examples C= C= C= EGR 101 7

• Unit of Measure – farad (F) = 1 coulomb per volt (C/V) • Typical ranges – Most capacitors fall in the picofarad (p. F) to microfarad ( F) range – Tolerance • Usually fairly poor • Variable capacitors used where exact values required EGR 101 8

Capacitor Value Codes • Physically large capacitors usually have their values printed directly on the case • Smaller capacitors are generally labeled using a code: – 2 -digit code: the number represents the value of the component in p. F Example: 15 = 15 p. F – 3 -digit code: the code is interpreted like the first three digits of a resistor code Example: 473 = 47 x 103 p. F = 47 n. F – The numbers 6 and 7 are not used as multiplier values – The numbers 8 and 9 are decoded as follows: 8 = 0. 01 and 9 = 0. 1 Example: 158 = 0. 15 p. F EGR 101 9

Physical Characteristics of Capacitors A d where C A d = the capacity of the component, in farads (F) = permittivity of the dielectric = the area of either plate, in square meters (m 2) = the distance between the plates, in meters (m) What are the units of ? EGR 101 10

Comparison to Resistance • For resistance, R = r. L/A • For capacitance, C = A/d • As r increases, R increases; as increases, C increases • As L increases, R increases; as d increases, C decreases • As A increases, R decreases; as A increases, C increases EGR 101 11

Permittivity • Permittivity of a capacitor dielectric is = o x r - Permittivity of a vacuum: o = 8. 85 x 10 -12 F/m MULTIPLIED BY - The relative permittivity of the material r e. g. : Material r air 1 paper 2. 5 mica 5 glass 7. 5 EGR 101 12

Team Activity 1 • If you have a capacitor with the following parameters, what is its capacitance? • Plate cross-sectional area = 1 cm 2 Dielectric material = air distance between plates = 2 cm • What happens to the capacitance if you change the dielectric to oil and the distance between plates to 1 cm? • For the original dielectric material and plate distance, what would the cross-sectional area need to be to create a 1 F capacitor? EGR 101 13

Series Capacitors Where CT = the total series capacitance Cn = the highest-numbered capacitor in the circuit EGR 101 14

Team Activity 2 Determine the total capacitance EGR 101 15

Parallel Capacitors A 1 A 2 where Cn = the highest-numbered capacitor in the parallel circuit EGR 101 16

Team Activity 3 Determine the total capacitance EGR 101 17

Demonstration • http: //www. howstuffworks. com/framed. htm ? parent=capacitor. htm&url=http: //micro. ma gnet. fsu. edu/electromag/java/capacitor/ EGR 101 18

Relationship between Capacitor Voltage and Current • Capacitor Current i + vc _ where i = the instantaneous value of capacitor current C = the capacity, in farads = the instantaneous rate of change in capacitor voltage EGR 101 19

Team Activity 4 • If the voltage across a 2 F capacitor is what is the current through the capacitor? EGR 101 20