Transcript Capacitor Construction A capacitor is a device that sores electrical charge. It is constructed of two parallel conductive plates separated by an insulating.

Capacitor
Construction
A capacitor is a device that sores electrical charge.
It is constructed of two parallel conductive plates separated by
an insulating material called the dielectric.
How a capacitor stores charge
No electrons flow through the dielectric because it is an insulator.
Basic Idea
• The Voltage across a capacitor
CANNOT change instantaneously
• The charge/discharge current,
however, can change
instantaneously!
• If the capacitor is disconnected from
the source, it retains the stored charge
for a long.
• Becareful when touching or handling
capacitors in or out of a circuit. If you
touch the leads, you may be in for a
shock as the capacitor discharges
through you.
• C:The amount of charge that a
capacitor can store per unit of voltage
across its plates.
– Simply put, capacitance is a measure of a
capacitor’s ability to store charge
– C=Q/V
• Units:
– The farad (F) is the basic unit of capacitance.
– Examples: MicroFarads, PicoFarads
How a capacitor stores energy
A capacitor stores energy in the form of
an electric field that is established by the
Opposite charges stored on the two plates.
The electric field is represented by lines of
forces between the positive and negative charges.
• What affects a capacitor’s ability to
store charges?
– Area
– Distance between plates
– Dielectric
Overlap area
If you can move the effective plate area, you can make a variable capacitor.
Plate distance
Dielectric constant
The presence of the dielectric material
weakens the electric fields between the plates.
So V is reduced .
Since C=Q/V and Q=CV, C is increased for fixed Q.
Dielectric strength
• The dielectric also determines the
maximum field strength that can be
handled by a capacitor.
– Air: 80 V/mil
– Glass: 2000 V/mil
Dielectric properties
Material
Dielectric Strength (V/mil)
Dielectric constant
(Relative Permitivity)
Air
80
1
Paper (paraffined)
1200
2.5
Oil
375
4
Mica
1500
5.0
Glass
2000
7.5
Ceramic
1000
1200
If you wish to conserve space, you may wish to use ceramic, but you might not be able to
use it in high voltage applications..
If you wish to build capacitors for high voltage applications, you may choose glass,
but your capacitors may be bulky..
Capacitance formula
• C=Aε/d
Capacitors
•
•
•
•
Mica Capacitors
Ceramic Capacitors
Plastic Film Capacitors
Electrolytic Capacitors
Mica Capacitors
Typical range: 1 pF to 0.1 uF
Stack capacitors in parallel to increase effective capacitance.
Ceramic capacitor
More compact than Mica capacitors since ceramic dielectric has high
dielectric constant.
Range: 1 pF to 100 uF
Voltage rating: up to 6000 V
A surface mount ceramic
capacitor
Note that the capacitors are connected in parallel
Plastic film capacitors
You roll up the capacitor to increase capacitance
Electrolytic capacitor
Straight plate is positive, the curved plate is negative.
1 uF to 200,000 uF.
Comparison
Category
Range
Mica capacitors
1 pF to 0.1 uF
Voltage rating: 100 V to 2500 V dc.
Ceramic capacitor
1 pF to 100 uF.
Voltage rating: up to 6000 V.
Plastic Film capacitors
Less than 1 uF.
Electrolytic capacitors
1 uF up to 200,000 uF.
350 V is a typical maximum.
Caution
• http://www.youtube.com/watch?v=9j
DsNe_bmtE
• How does it happen?
– The reverse bias voltage destroys the
dielectric material.
– The capacitor will conduct short circuit
current
– The oxide will get oxidised, converted into
oxygen gas, generating high pressure,
and bursting open the capacitor.
Series Capacitance
Parallel Capacitance