Capacitors

Capacitance is the ability of a component to store energy in the form of an electrostatic charge.

A Capacitor is a component designed to provide a specific measure of capacitance.

EGR 101 1

Capacitor Construction

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Parallel plates separated by a dielectric layer

Fixed Value Capacitors

• Polarized Electrolytic Capacitors• Most electrolytic capacitors are polarized

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Variable Capacitors

Interleaved-Plate Capacitors

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Charging a Capacitor

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Electrostatic Charge Develops on the Plates

Electrostatic Field Stores energy

Discharging a Capacitor

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Apply a discharging component (here a short circuit) across the capacitor. (Safer to use a resistor!)

Capacity (Capacitance) of a Device

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Capacity is the amount of charge that a capacitor can store per unit volt applied.

Capacity is directly proportional to charge and inversely proportional to voltage

CVQV

QC or

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CVQV

QC or

whereC = the capacity (or capacitance) of the component, in coulombs per volt, or Farads

Q = the total charge stored by the component

V= the voltage across the capacitor

Example

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Capacitor Ratings

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Most capacitors rated in the picofarad (pF) to microfarad (F) range

Capacitors in the millifarad range are commonly rated in thousands of microfarads: 68 mF = 68,000 F

Capacitors in the nanofarad range are also commonly rated in microfarads: 68 nF = 0.068 F

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Capacitors in the nanofarad range are also commonly rated in microfarads: 68 nF = 0.068 F

Tolerance

Usually fairly poor

Variable capacitors used where exact values required

• 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 pFExample: 15 = 15 pF

– 3-digit code: the code is interpreted like the first three digits of a resistor code

Example: 473 = 47 x 103 pF = 47 nF

– 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 pF

Capacitor Value Codes

Capacitance of a Parallel Plate Capacitor

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d

A.C r

1210858

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d

A.C r

1210858

C = the capacity of the component, in farads(8.85 X 10-12)= the permittivity of a vacuum, in farads per meter (F/m)

r = the relative permittivity of the dielectric

A= the area of either plate, in square meters (m2)

d = the distance between the plates, in meters (m)

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Plate Area: capacitance is directly proportional to plate area

Dielectric Thickness: capacitance is inversely proportional to dielectric thickness

Dielectric Permittivity: the ease with which lines of electrical force are established in the dielectric material

Relative Permittivity: the ratio of a material’s permittivity to that of a vacuum

Capacitors in Series

EGR 101 16

n

T

CCC

C111

1

21

CT = the total series capacitanceCn = the highest-numbered capacitor in the circuit

Capacitors in Parallel

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nT CCCC 21

Cn = the highest-numbered capacitor in the parallel circuit