Download Introduction to Capacitors and Capacitance

Introduction to Capacitors and
Capacitance
Online Resource for ETCH 213
Faculty: B. Allen
Capacitor
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 load.
This device was originally called a condenser.
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Charging a capacitor
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Electrostatic or electric field
A force field produced by static electric
charges. Also called a voltage field, it is a field
or force that exists in space between two
different potentials or voltages.
Current generates a magnetic field.
Voltage generates an electric field.
Online Resource for ETCH 213
Faculty: B. Allen
Field strength
The strength of a electromagnetic field at a
given point.
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Field strength (V/m) =
Charge difference (V) volts
Distance between plates (d) meters
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Faculty: B. Allen
Electric polarization
Electric Polarization – A displacement of
bound charges in a dielectric when placed in an
electric field.
Dielectric – Insulating material between two
(di) plates in which the electric field exists.
Online Resource for ETCH 213
Faculty: B. Allen
Unit of Capacitance
Farad (F) –
A one farad capacitor stores 6.24 × 1018
electrons
or
A 1 F capacitor stores 6.24 × 1012 electrons
Online Resource for ETCH 213
Faculty: B. Allen
Capacitance, C (farads) =
Charge, Q (coulombs)
Voltage, V (volts)
Q =C × V
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Factors determining
capacitance
Area of the plates (A)
Distance between the plates (d)
Dielectric constant (K)
C=
8.85 × 10-12 × K × A
d
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Faculty: B. Allen
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.
Online Resource for ETCH 213
Faculty: B. Allen
Capacitors in parallel
CT = C1 + C2 + C3 +…
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Capacitors in series
1
=
1
CΤ C1
+
1
C2
+
1
+
C3
1
+ …..
C4
Online Resource for ETCH 213
Faculty: B. Allen
Voltage across a capacitor in
series
VX =
CT
CX
VT
Online Resource for ETCH 213
Faculty: B. Allen
Types of capacitors
Fixed-Value Capacitor – A capacitor whose
value is fixed and cannot be varied.
Mica Capacitor – A fixed capacitor that uses
mica as the dielectric between its plates.
Ceramic Capacitor – A capacitor in which the
dielectric material used between the plates is
ceramic.
Online Resource for ETCH 213
Faculty: B. Allen
Coding of capacitors
Alphanumeric labels
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Color coding of capacitors
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Capacitor time constant
DC charging
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Time constant τ = R × C
The time constant is 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.
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Faculty: B. Allen
Capacitor charging
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Faculty: B. Allen
AC charge and discharge. (a)
Capacitor will block DC. (b)
Capacitor will pass AC.
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Phase relationship between
capacitor current and voltage
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End of
Introduction to Capacitors and
Capacitance
Online Resource for ETCH 213
Faculty: B. Allen