capacitor. construction a capacitor is a device that sores electrical charge. it is constructed of...
TRANSCRIPT
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.
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 ofan 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.
Dielectric constant
The presence of the dielectric materialweakens 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 propertiesMaterial 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 touse 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..
Mica Capacitors
Typical range: 1 pF to 0.1 uFStack capacitors in parallel to increase effective capacitance.
Ceramic capacitor
More compact than Mica capacitors since ceramic dielectric has highdielectric constant.Range: 1 pF to 100 uFVoltage rating: up to 6000 V
Electrolytic capacitor
Straight plate is positive, the curved plate is negative.
1 uF to 200,000 uF.
ComparisonCategory Range
Mica capacitors 1 pF to 0.1 uFVoltage 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=9jDsNe_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.