BEXS100 - Basic Electricity

Unit 19

Capacitors

Unit Objectives

• List the three (3) factors that determine the capacitance of a capacitor

• Explain electrostatic charge

• Determine the difference between polarized and nonpolarized capacitors

Upon completion of this unit, you will be able to:

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 Capacitor1. The area of the plates

2. The distance between the plates

3. The type of “dielectric” used

Charging a CapacitorElectrons are removed from one plate and deposited on the opposite plate

Charging a CapacitorCurrent flows until voltage across capacitor is the same as the source

voltage

Charging a CapacitorThe 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 chargeAn 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 = 1/1,000,000,000,000The 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 ConstantsCapacitors charge at an exponential rate (63.2%)

RC Time ConstantsCapacitors 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 capacitorsThey are sensitive to polarityUsed only in DC circuitsReversing polarity could cause capacitor

damage or explosionElectrolytic 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 platesThe 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 CollegeElizabethtown Technical CollegeBasic Electricity BEXS100/101Basic Electricity BEXS100/101

Text: Text: Delmar’s Standard Textbook of ElectricityDelmar’s Standard Textbook of ElectricityUnit 19 - CapacitorsUnit 19 - Capacitors