Capacitors A basic capacitor has two parallel plates separated by an insulating material A basic capacitor has two parallel plates separated by an insulating material A capacitor stores an electrical charge between the two plates A capacitor stores an electrical charge between the two plates The unit of capacitance is Farads (F) The unit of capacitance is Farads (F) Capacitance values are normally smaller, such as µF, nF or pF Capacitance values are normally smaller, such as µF, nF or pF

Capacitors Basic capacitor construction Basic capacitor construction Dielectric material Plate 1 Plate 2 The dielectric material is an insulator therefore no current flows through the capacitor

Capacitors Storing a charge between the plates Electrons on the left plate are attracted toward the positive terminal of the voltage source Electrons on the left plate are attracted toward the positive terminal of the voltage source This leaves an excess of positively charged holes This leaves an excess of positively charged holes The electrons are pushed toward the right plate The electrons are pushed toward the right plate Excess electrons leave a negative charge Excess electrons leave a negative charge _ + _

Capacitors Types of capacitors The dielectric material determines the type of capacitor The dielectric material determines the type of capacitor Common types of capacitors are: Common types of capacitors are: Mica Mica Ceramic Ceramic Plastic film Plastic film

Capacitors Some capacitors are polarised, they can only be connected one way around Some capacitors are polarised, they can only be connected one way around Electrolytic capacitors are polarised Electrolytic capacitors are polarised

Capacitors Variable capacitors are used in communication equipment, radios, televisions and VCRs Variable capacitors are used in communication equipment, radios, televisions and VCRs They can be adjusted by consumers by tuning controls They can be adjusted by consumers by tuning controls Trimmers are internal adjusted capacitors that a consumer cannot adjust Trimmers are internal adjusted capacitors that a consumer cannot adjust

Capacitors These variable capacitors would be difficult to squeeze into your mobile phone and iPod These variable capacitors would be difficult to squeeze into your mobile phone and iPod Current technology uses semi-conductor variable capacitors called varactors (varicaps) Current technology uses semi-conductor variable capacitors called varactors (varicaps)

Capacitors The capacitance in a varactor is created when a purpose diode is reversed biased The capacitance in a varactor is created when a purpose diode is reversed biased Adjusting the reverse bias alters the capacitance value Adjusting the reverse bias alters the capacitance value A simple radio receiver using varactor A simple radio receiver using varactor