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
Parallel plates separated by a dielectric layer
EGR 101
2
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
Electrostatic Charge Develops on the Plates
Electrostatic Field Stores energy
EGR 101
5
Discharging a Capacitor
Apply a discharging component (here a short
circuit) across the capacitor. (Safer to use a
resistor!)
EGR 101
6
Capacity (Capacitance) of a Device
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
Q
C
V
or
EGR 101
Q  CV
7
Q
C
V
or
Q  CV
where
C = 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
EGR 101
8
Example
EGR 101
9
Capacitor Ratings
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
10
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
EGR 101
11
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 pF
Example: 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
Capacitance of a Parallel Plate Capacitor

C  8.85  10
EGR 101
12

A
r
d
13

C  8.85  10
12

A
r
d
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
14
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
EGR 101
15
Capacitors in Series
CT 
1
1
1
1

  
C1 C2
Cn
CT = the total series capacitance
Cn = the highest-numbered capacitor in the circuit
EGR 101
16
Capacitors in Parallel
CT  C1  C2      Cn
Cn = the highest-numbered capacitor in the
parallel circuit
EGR 101
17