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 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.

• 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 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..

Capacitance formula

• C=Aε/d

Capacitors

• Mica Capacitors• Ceramic Capacitors• Plastic Film Capacitors• Electrolytic Capacitors

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

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.

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.

Series Capacitance

Parallel Capacitance