23. Electrostatic Energy and Capacitors. 2 Topics Capacitors Electrostatic Energy Using Capacitors.
Capacitors
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Transcript of Capacitors
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
EGR 101 2
Parallel plates separated by a dielectric layer
Fixed Value Capacitors
• Polarized Electrolytic Capacitors• Most electrolytic capacitors are polarized
EGR 101 3
Variable Capacitors
Interleaved-Plate Capacitors
EGR 101 4
Charging a Capacitor
EGR 101 5
Electrostatic Charge Develops on the Plates
Electrostatic Field Stores energy
Discharging a Capacitor
EGR 101 6
Apply a discharging component (here a short circuit) across the capacitor. (Safer to use a resistor!)
Capacity (Capacitance) of a Device
EGR 101 7
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
EGR 101 8
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
EGR 101 9
Capacitor Ratings
EGR 101 10
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
EGR 101 11
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
EGR 101 13
d
A.C r
1210858
EGR 101 14
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)
EGR 101 15
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
EGR 101 17
nT CCCC 21
Cn = the highest-numbered capacitor in the parallel circuit