BEXS100 Basic Electricity

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BEXS100 - Basic Electricity

• Unit 19
• Capacitors

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Unit Objectives
Upon completion of this unit, you will be able to

• List the three (3) factors that determine the
  capacitance of a capacitor
• Explain electrostatic charge
• Determine the difference between polarized and
  nonpolarized capacitors

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Introduction

• Capacitors are devices that oppose a change of
  voltage, at their base level are a simple
  electrical storage device yet the applications
  for these devices are varied and wide ranging.
  Capacitors can be used in timing circuits, as
  electronic filters, and for power factor
  correction among other uses. Capacitors are used
  in both AC and DC circuitry.

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The Capacitance of a Capacitor

• The area of the plates
• The distance between the plates
• The type of dielectric used

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Charging a Capacitor

• Electrons are removed from one plate and
  deposited on the opposite plate

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Charging a Capacitor

• Current flows until voltage across capacitor is
  the same as the source voltage

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Charging a Capacitor

• The capacitor remains charged after the source of
  voltage (battery) is removed from the circuit

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Capacitor Leakage

• Leakage is current flow through the dielectric
• Proportional to the resistance of the dielectric
  and the charge across the plates

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Electrostatic Charge

• An electrostatic charge refers to electrical
  charges that are stationary
• Static electricity is an electrostatic charge
• Electrostatic charges are created by removing
  electrons from one plate and depositing them on
  the other
• Energy in a capacitor is stored in the form of an
  electrostatic charge

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Dielectric Stress

• A lack of electrons a positive charge
• An excess of electrons a negative charge

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Dielectric Stress

• When the atoms of the dielectric are under stress
  they are creating a potential energy, or stored
  energy
• Like a drawn bow, the potential energy is being
  stored
• When a charged capacitor is discharged it can
  produce currents of thousands of amperes for
  short periods of time

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Dielectric Stress

• Dielectric stress is proportional to the voltage
  difference between the plates
• Excessive voltage can cause the dielectric to
  break down (short) and permit current flow
  between the plates
• Capacitors have a voltage rating that should not
  be exceeded
• Preventing excessive supply voltage to a
  capacitor is critical to its life span

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Dielectric Constants

• A number is assigned to different materials as an
  indicator of their insulating quality in a
  capacitor
• This number is referred to as its dielectric
  constant
• Changing the dielectric material of a capacitor
  changes its capacitance properties

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Dielectric Constants

• Assume that a capacitor uses air as its
  dielectric material and has a total capacitance
  of 1 mF
• Changing the dielectric material to dry paper
  would change the capacitance of the capacitor to
  3.5 mF

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Capacitor Ratings

• A farad is the basic unit of capacitance named
  after Michael Faraday, a famous scientist
• Symbolized by the letter F
• One farad the change of 1 volt across the
  plates results in the movement of 1 coulomb of
  current
• A farad is such a large basic unit of capacitance
  that it is seldom used for capacitor values

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Capacitor Ratings

• More common designations of capacitance
• mF Microfarad 1/1,000,000
• nF Nanofarad 1/1,000,000,000
• pF Picofarad 1/1,000,000,000,000
• The Picofarad is sometimes referred to as a
  micro/microfarad with the symbol mmF

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Capacitors in Parallel

• Has the same effect of increasing the plate area
  of one capacitor

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Capacitors in Series

• Has the effect of increasing the distance between
  the plates / reducing the total capacitance of
  the circuit

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RC Time Constants

• Capacitors charge at an exponential rate (63.2)

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RC Time Constants

• Capacitors discharge at an exponential rate
  (63.2)

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RC Time Constants

• Accurately determining the RC time constant for a
  given capacitor in a circuit

How long will it take to charge a 50 mF capacitor
in a circuit with a 100,000 ohm resistor?
Answer 25 Seconds
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The Two Basic Groups of Capacitors

• Polarized
• Non-Polarized

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Non-Polarized Capacitors

• Not sensitive to polarity connection
• May also be referred to as AC capacitors
• Can be used in AC/DC circuits

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Paper / Oil Filled Capacitors
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Polarized Capacitors

• Also known as electrolytic capacitors
• They are sensitive to polarity
• Used only in DC circuits
• Reversing polarity could cause capacitor damage
  or explosion
• Electrolytic capacitors can have very high
  capacitance in a small case

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Variable Capacitors

• Capacitance range can be changed within a certain
  range
• Contains a moveable set of plates
• The plates are interleaved with the stationary
  plates to increase or decrease the capacitance
  value

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A Trimmer Capacitor

• Contains one stationary plate and one moveable
  plate
• An adjusting screw moves the moveable plate to
  change the capacitance value

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Elizabethtown Technical CollegeBasic Electricity
BEXS100/101Text Delmars Standard Textbook of
ElectricityUnit 19 - Capacitors