Transcript on Capacitors.

Chapter 9
Capacitors
1
Capacitor
Previously called a condenser, it is a device that
stores electrical energy in the form of an electric field
that exists within a dielectric (insulator) between two
conducting plates, each of which is connected to a
lead.
2
Charging a Capacitor
3
Discharging a Capacitor
4
Field Strength
The strength of an electric, magnetic, or
electromagnetic field at a given point.
5
Electric Polarization
Electric Polarization – A displacement of bound
charges in a dielectric when placed in an electric field.
Dielectric – Insulating material between two plates in
which the electric field exists.
6
Unit of Capacitance
Farad (F) – The unit of capacitance, equal to 1
coulomb/volt.
A 1 F capacitor stores 6.24  1018 electrons when
1 V is applied across its plates.
A 1 F capacitor stores 6.24  1012 electrons when
1 V is applied across its plates.
7
Calculating Capacitance
Charge, Q (coulombs)
Capacitanc e, C (farads) 
Voltage, V (volts)
Q
C
V
Q  C V
Q
V
C
8
Capacitance Formula
(8.85  1012 )  K  A
C
d
where
C = capacitance, in farads (F)
A = plate area, in square meters (m2)
K = dielectric constant
D = distance between the plates, in meters (m)
9
Capacitors in Parallel
CT = C1 + C2 + C3 + …
10
Capacitors in Series
CT 
1
1 1
1
   ...
C1 C2 C3
or
C1  C2
CT 
C1  C2
11
Capacitive Voltage-divider
VCX
CT

 VT
CX
where
VCX
CT
CX
VT
= voltage across desired capacitor
= total capacitance
= desired capacitor’s value
= total supplied voltage
12
Dielectric Breakdown and
Leakage
Breakdown Voltage – The voltage at which
breakdown occurs in a dielectric or insulator.
Leakage Current – Small, undesirable flow of
current through an insulator or dielectric.
13
Common Fixed-Value Capacitors
Fixed-Value Capacitor – A capacitor whose value
is fixed and cannot be varied. These capacitors are
classified by dielectric material.
Mica
Ceramic
Surface-mount ceramic
Paper
Plastic
Electrolytic
14
Coding of Capacitors
Alphanumeric labels
15
Testing a Capacitor Using an
Ohmmeter
16
Capacitance Analyzer
An instrument used to
measure the capacitance
of a capacitor or a
circuit.
17
Capacitor DC Charging
18
Time Constant ()
The time needed for either a voltage or current to
rise to 63.2% of the maximum or a fall to 36.8% of
the initial value. The time constant of an RC circuit
is equal to the product of R and C.
  RC
19
Capacitor Charge Curve
20
Capacitor Discharge Curve
21
AC Charge and Discharge.
(A) Capacitor Will Block DC.
(B) Capacitor Will Pass AC.
22