Chapter 3: Resistance

Document Sample
Chapter 3: Resistance Powered By Docstoc
					Chapter 3
Resistance
FIG. 3.1   Resistance symbol and notation.




                                             2
       Resistance of Conductors
• Resistance of material is dependent on
  several factors:
  –   Type of Material
  –   Length of the Conductor
  –   Cross-sectional area
  –   Temperature



                                           3
            Type of Material
• Differences at the atomic level of various
  materials will cause variations in how the
  collisions affect resistance.
• These differences are called the resistivity.
• We use the symbol  (Greek letter rho).
• Units are ohm-meters.


                                                  4
                   Length
• The resistance of a conductor is directly
  proportional to the length of the conductor.
• If you double the length of the wire, the
  resistance will double.
•  = length, in meters.




                                                 5
                     Area
• The resistance of a conductor is inversely
  proportional to the cross-sectional area of the
  conductor.
• If the cross-sectional area is doubled, the
  resistance will be one half as much.
• A = cross-sectional area, in m2.


                                                    6
         Resistance Formula
• At a given temperature,
                            
                     R
                            A
• This formula can be used with both circular
  and rectangular conductors.



                                                7
FIG. 3.2   Factors affecting the resistance of a conductor.




                                                              8
FIG. 3.3   Cases in which R2 > R1. For each case, all remaining parameters that control the resistance level are the same.




                                                                                                                             9
        Electrical Wire Tables
• The American Wire Gauge is the primary
  system to denote wire diameters.
• The higher the AWG number, the smaller the
  diameter.
• A given length of AWG 22 wire will have more
  resistance than the same length of AWG 14
  wire.
• Larger gauge wires can handle more current.
                                            10
           Circular Mils (CM)
• Diameter is expressed in circular mils.
• 1 CM is defined as the area of a circle having
  a diameter of 1 mil (0.001 inch).
• A square mil is the area of a square having
  sides 1 mil long.
• 1 CM = /4 square mils


                                               11
FIG. 3.4   Defining the circular mil (CM).




                                             12
FIG. 3.5   Verification of Eq. (3.2): ACM = (dmils)2.




                                                        13
FIG. 3.8   Popular wire sizes and some of their areas of application.




                                                                        14
         Temperature Effects
• For most conductors, an increase in
  temperature causes an increase in
  resistance.
• This increase is relatively linear.
• In many semiconductors, an increase in
  temperature results in a decrease in
  resistance.

                                           15
          Temperature Effects
• The rate of change of resistance with
  temperature is called the temperature
  coefficient.
• Represented by  (Greek letter alpha).
• Any material for which the resistance
  increases as temperature increases is said to
  have a positive temperature coefficient. If it
  decreases, it has a negative coefficient.
                R  R1 1  T               16
FIG. 3.12   Demonstrating the effect of a positive and a negative temperature coefficient on the resistance of a conductor.




                                                                                                                              17
FIG. 3.13   Effect of temperature on the resistance of copper.




                                                                 18
            Fixed Resistors
• Resistances essentially constant.
• Rated by amount of resistance, measured in
  ohms.
• Also rated by power ratings, measured in
  watts.



                                               19
                Fixed Resistors
• Different types of resistors are used for
  different applications.
   –   Molded carbon composition
   –   Carbon film
   –   Metal film
   –   Metal Oxide
   –   Wire-Wound
   –   Integrated circuit packages
                                              20
FIG. 3.16 (continued)   Film resistors: (a) construction; (b) types.




                                                                       21
FIG. 3.16   Film resistors: (a) construction; (b) types.




                                                           22
FIG. 3.17   Fixed composition resistors: (a) construction; (b) appearance.




                                                                             23
FIG. 3.17 (continued)   Fixed composition resistors: (a) construction; (b) appearance.




                                                                                         24
FIG. 3.18   Fixed metal-oxide resistors of different wattage ratings.




                                                                        25
FIG. 3.19   Various types of fixed resistors.




                                                26
FIG. 3.19 (continued)   Various types of fixed resistors.




                                                            27
           Variable Resistors
• Used to adjust volume, set level of lighting,
  adjust temperature.
• Have three terminals.
• Center terminal connected to wiper arm.
• Potentiometers
• Rheostats


                                                  28
FIG. 3.20   Potentiometer: (a) symbol; (b) and (c) rheostat connections; (d) rheostat symbol.




                                                                                                29
FIG. 3.21   Molded composition-type potentiometer.




                                                     30
FIG. 3.23    Variable resistors: (a) 4 mm (≈5/32”) trimmer (courtesy of Bourns, Inc.); (b) conductive plastic and cermet elements (courtesy of



Honeywell Clarostat); (c) three-point wire-wound resistor.




                                                                                                                                           31
FIG. 3.24   Potentiometer control of voltage levels.




                                                       32
               Color Code
• Colored bands
  on a resistor
  provide a code
  for determining
  the value of
  resistance,
  tolerance, and
  sometimes the
  reliability.
                            33
FIG. 3.25   Color coding for fixed resistors.




                                                34
FIG. 3.29   Five-band color coding for fixed resistors.




                                                          35
FIG. 3.26   Color coding.




                            36
FIG. 3.30   Guaranteeing the full range of resistor values for the given tolerance: (a) 20%; (b) 10%.




                                                                                                        37
FIG. 3.27   Example 3.13.




                            38
FIG. 3.28   Example 3.14.




                            39
       Measuring Resistance
• Remove all power sources to the circuit.
• Component must be isolated from rest of the
  circuit.
• Connect probes across the component.
• No need to worry about polarity.
• Useful to determine shorts and opens.


                                                40
FIG. 3.22   Resistance components of a potentiometer: (a) between outside terminals; (b) between wiper arm and each outside terminal.




                                                                                                                                    41
               Thermistors

• A two-terminal transducer in which the
  resistance changes with change in
  temperature.
• Applications include electronic thermometers
  and thermostatic control circuits for furnaces.
• Many have negative temperature coefficients.


                                               42
FIG. 3.35   Thermistor: (a) characteristics; (b) symbol.




                                                           43
FIG. 3.36   NTC (negative temperature coefficient) and PTC (positive temperature coefficient) thermistors.




                                                                                                             44
       Photoconductive Cells

• Two-terminal transducers which have a
  resistance determined by the amount of light
  falling on them.
• May be used to measure light intensity or to
  control lighting.
• Used as part of security systems.


                                             45
FIG. 3.37   Photoconductive cell: (a) characteristics. (b) symbol.




                                                                     46
FIG. 3.38   Photoconductive cells.




                                     47
                 Varistors
• Resistors which are sensitive to voltage.
• Have a very high resistance when the voltage
  is below the breakdown value.
• Have a very low resistance when the voltage
  is above the breakdown value.
• Used in surge protectors.


                                             48
FIG. 3.39   Varistors available with maximum dc voltage ratings between 18 V and 615 V.




                                                                                          49
FIG. 3.39 (continued)   Varistors available with maximum dc voltage ratings between 18 V and 615 V.




                                                                                                      50
FIG. 3.40   Electric baseboard: (a) 2-ft section; (b) interior; (c) heating element; (d) nichrome coil.




                                                                                                          51
FIG. 3.41   Dashboard dimmer control in an automobile.




                                                         52
              Conductance
• The measure of a material’s ability to allow
  the flow of charge.
• Conductance is the reciprocal of resistance.
• G = 1/R
• Unit is siemens.



                                                 53
          Superconductors
• At very low temperatures, resistance of some
  materials goes to almost zero.
• This temperature is called the critical
  temperature.
• Meissner Effect - When a superconductor is
  cooled below its critical temperature,
  magnetic fields may surround but not enter
  the superconductor.
                                            54
FIG. 3.14   Rising temperatures of superconductors.




                                                      55
FIG. 3.15   Defining the critical temperature Tc.




                                                    56

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:6
posted:8/7/2012
language:English
pages:56