Transcript BEXS100 - Basic Electricity

BEXS100 - Basic Electricity
Unit 19
Capacitors
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
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.
The Capacitance of a Capacitor
1. The area of the plates
2. The distance between the plates
3. The type of “dielectric” used
Charging a Capacitor
Electrons are removed from one plate
and deposited on the opposite plate
Charging a Capacitor
Current flows until voltage across
capacitor is the same as the source
voltage
Charging a Capacitor
The capacitor remains charged after
the source of voltage (battery) is
removed from the circuit
Capacitor “Leakage”
•Leakage is current flow through the dielectric
•Proportional to the resistance of the dielectric
and the charge across the plates
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
Dielectric Stress
• A lack of electrons = a positive charge
• An excess of electrons = a negative charge
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
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
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
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
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
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”
Capacitors in Parallel
• Has the same effect of increasing the plate area of
one capacitor
Capacitors in Series
• Has the effect of increasing the distance between the
plates / reducing the total capacitance of the circuit
RC Time Constants
Capacitors charge at an exponential rate (63.2%)
RC Time Constants
Capacitors discharge at an exponential rate (63.2%)
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
The Two Basic Groups of
Capacitors
Polarized
&
Non-Polarized
Non-Polarized Capacitors
• Not sensitive to polarity
connection
• May also be referred to
as AC capacitors
• Can be used in AC/DC
circuits
Paper / Oil Filled Capacitors
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
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
A Trimmer Capacitor
• Contains one stationary plate and one moveable
plate
• An adjusting screw moves the moveable plate to
change the capacitance value
Elizabethtown Technical College
Basic Electricity BEXS100/101
Text: Delmar’s Standard Textbook of Electricity
Unit 19 - Capacitors