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 MaterialDielectric Strength (V/mil)Dielectric constant (Relative Permitivity) Air801 Paper (paraffined) Oil3754 Mica Glass Ceramic 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 CategoryRange Mica capacitors1 pF to 0.1 uF Voltage rating: 100 V to 2500 V dc. Ceramic capacitor1 pF to 100 uF. Voltage rating: up to 6000 V. Plastic Film capacitorsLess than 1 uF. Electrolytic capacitors1 uF up to 200,000 uF. 350 V is a typical maximum.

Caution DsNe_bmtE 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