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CAPACITOR
ENGR. VIKRAM KUMAR
B.E (ELECTRONICS)
M.E (ELECTRONICS SYSTEM ENGG:)
MUET JAMSHORO1
Capacitor
• A capacitor is a device for storing charge and
electrical potential energy.
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Capacitor
• All capacitors consists of two metal plates
separated by an insulator. The insulator is
called dielectric. (e.g. polystyrene, oil or air)
• Circuit symbol:
+
Dielectric
_
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Structure of capacitor
• Capacitor
= conductor (metal plate)
+
insulator (dielectric)
+
conductor (metal plate)
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Capacitors
• Basic capacitor construction
Plate 2
Dielectric
material
The dielectric
material is an
insulator therefore
no current flows
through the
capacitor
Plate 1
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Formation of a Capacitor
• Capacitors are formed all of
the time in everyday
situations:
– when a charged
thunderstorm cloud induces
an opposite charge in the
ground below,
– when you put your hand near
the monitor screen of this
computer.
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Capacitor
Storing a charge between
the plates
• Electrons on the left plate
are attracted toward the
positive terminal of the
voltage source
• This leaves an excess of
positively charged holes
• The electrons are pushed
toward the right plate
• Excess electrons leave a
negative charge
+
+
+
_
_
-
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Charged Capacitor
• A capacitor is said to be charged when there
are more electrons on one conductor plate than
on the other.
When a capacitor is
charged, energy is
stored in the dielectric
material in the form of
an electrostatic field.
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Capacitance
• Consider any isolated pair of conductors with charge
Q
Capacitance is defined as
Q
C
V
Unit : farad (F)
Where Q = charge on one conductor
V = potential difference between two conductors9
Capacitance
• The capacitance of a conductor is the charge required
•
•
•
to cause unit change in the potential of the conductor.
A one-farad capacitor stores one coulomb of charge
when a potential of 1 volt is applied across the
terminals of the capacitor.
The smaller the change in potential of the conductor
when a certain charge is transferred to it, the more
charge it can store before breakdown occurs.
In electronics, the microfarad (μF) and the picofarad
(pF) are usually used to measure capacitance.
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Capacitance of a Capacitor
Q
C
V
• Note that Q is not the net charge on the capacitor,
which is zero.
• Capacitance is a measure of a capacitor's ability to
store charge.
• The more charge a capacitor can hold at a given
potential difference, the larger is the capacitance.
• Capacitance is also a measure of the energy storage
capability of a capacitor.
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Capacitance
 Fringing – At the edge of the capacitor plates the
flux lines extend outside the common surface area
of the plates.
Capacitance
 Dielectric – Insulator of the capacitor
 The purpose of the dielectric is to create an electric
field to oppose the electric field setup by free charges on
the parallel plates.
 Di for “opposing” and electric for “electric field”
Dipoles – Formed within the insulator of a
capacitor when the electrons of the insulating
material are unable to leave the parent atom and
travel to the positive plate of the capacitor
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Capacitance
With different dielectric materials between the
same two parallel plates, different amounts of
charge will deposit on the plates.
 Permittivity – The ratio of the flux density to the
electric field intensity in the dielectric. A measure
of how easily the dielectric will “permit” the
establishment of flux lines within the dielectric.
 Relative permittivity – Often called the dielectric
constant, it is the ratio of the permittivity of any
dielectric to that of a vacuum.
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Capacitors
Working voltage – the voltage that can be applied
across a capacitor for long periods of time with out
breakdown
 Surge voltage – The maximum dc voltage that can
be applied for a short period of time
Leakage current – The current that results in the
total discharge of a capacitor as the capacitor is
disconnected from the charging network for a
sufficient length of time.
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Voltage Rating of Capacitors
• If the voltage applied across
the capacitor is too great, the
dielectric will break down and
arcing will occur between the
capacitor plates.
• The voltage rating of the
capacitor is the maximum
voltage that can be steadily
applied without danger of
breaking down the dielectric.
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Energy Stored by a Capacitor
 The ideal capacitor does not dissipate any energy supplied to it. It
stores the energy in the form of an electric field between the
conducting surfaces.
Charging of a capacitor always involves some expenditure of energy
by the charging agency.
This energy is stored up in the electrostatic field set up in the
dielectric medium.
• On discharging the capacitor, the field collapses and the stored
energy is released.
 when the capacitor
is uncharged,
• little work is done in transferring charge from
one plate to another.
• Let us find the energy spent in charging a capacitor
of capacitance C to a voltage V.
1
E  QV
2
1
 CV 2
2
1 Q2

2 C
Schematic of simple circuit with capacitor
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Capacitor Charging
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Capacitor Charging
Current stops when capacitor is fully charged
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Capacitor Charging
Capacitor remains charged when power source is removed
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Charging of Capacitors
• As a capacitor becomes charged, the current flow
decreases because the voltage developed by the
capacitor increases over time and opposes the source
voltage.
R
R
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Transients in Capacitive Networks:
Charging Phase
 The placement of charge on the plates of a
capacitor does not occur instantaneously.
Transient Period – A period of time where the
voltage or current changes from one steady-state
level to another.
 The current ( ic ) through a capacitive network is
essentially zero after five time constants of the
capacitor charging phase.
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Initial Conditions
 The voltage across a capacitor at the instant of the start of
the charging phase is called the initial value. Once the
voltage is applied the transient phase will commence until
a leveling off occurs after five time constants called steadystate as shown in the figure.
Charging a Capacitor
• Voltage-charge
characteristics
• Current flow
I
Vc
or Q
VC  V0 (1  e
t
RC
)
t
I  I oe
t
RC
t
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Capacitor Discharging
Capacitor discharges when a current path is applied
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Capacitor Discharging
Capacitor discharges when a current path is applied
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Capacitor Discharging
Capacitor discharges when a current path is applied
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Capacitor Discharging
Capacitor discharges when a current path is applied
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Capacitor Discharging
Capacitor discharges when a current path is applied
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Capacitor Discharging
Capacitor discharges when a current path is applied
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Capacitor Discharging
Capacitor is fully discharged when plates are neutral
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Discharging of Capacitors
R
R
• The charged capacitor
is the source of voltage
for the current flow. The
current will cease
flowing when the
charges of the two
plates are again equal,
meaning that the
capacitor is completely
discharged.
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Discharging a Capacitor
• Voltage-charge
characteristics
• Current flow
t
VC
or Q
Q  Q0e
t
I  I oe
t
RC
RC
I
t
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Time Constant
• Resistance is unavoidable in circuits, through the wire itself or
through components such as resistors.
• Resistance introduces the element of time into charging and
discharging a capacitor
• The voltage across a capacitor cannot change instantaneously
because it takes finite time to move charge from one point to
another.
• The rate at which a capacitor charges or discharges is determined by
the time constant t = RC (seconds)
• During one time constant interval, the charge on a capacitor changes
approx. 63%
• Five time constant intervals is accepted as the time to fully charge or
discharge a capacitor and is called the transient time.
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Charging Capacitor Voltage
applied
voltage
charge
voltage
Time
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Charging Capacitor Voltage
applied
voltage
resistance added
charge
voltage
Time
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Discharging Capacitor Voltage
applied
voltage
charge
voltage
Time
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Discharging Capacitor Voltage
applied
voltage
resistance added
charge
voltage
Time
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Applications
Capacitors find applications in:
Electronic flash lamps for
cameras
Line conditioners
Timing circuits
Electronic power supplies
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Applications of Capacitors
• The capacitance is varied by
altering the overlap between
a fixed set of metal plates
and a moving set. These are
used to tune radio receiver.
• Press the key on a computer
keyboard reduce the capacitor
spacing thus increasing the
capacitance which can be
detected electronically.
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Applications of Capacitors
• Capacitive touch-screens use a layer of capacitive
material to hold an electrical charge; touching the
screen changes the amount of charge at a specific
point of contact.
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THE END
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