Resistance

Resistance
R
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FIG. 3.1 Resistance symbol and notation.
R
The resistance of any material with a uniform cross-sectional area is determined by the following four factors:
1. Material 2. Length 3. Cross-sectional area 4. Temperature
The chosen material, with its unique molecular structure, will react differentially to pressures to establish current through its core. Conductors that permit a generous flow of charge with little external pressure will have low resistance levels, while insulators will have high resistance characteristics. As one might expect, the longer the path the charge must pass through, the higher the resistance level, whereas the larger the area (and therefore available room), the lower the resistance. Resistance is thus directly proportional to length and inversely proportional to area.
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As the temperature of most conductors increases, the increased motion of the particles within the molecular structure makes it increasingly difficult for the “free” carriers to pass through, and the resistance level increases. At a fixed temperature of 20°C (room temperature), the resistance is related to the other three factors by
(ohms, ) (3.1)
where r (Greek letter rho) is a characteristic of the material called the resistivity, l is the length of the sample, and A is the cross-sectional area of the sample. The units of measurement substituted into Eq. (3.1) are related to the application. For circular wires, units of measurement are usually defined as in Section 3.2. For most other applications involving important areas such as integrated circuits, the units are as defined in Section 3.4.
3.2 RESISTANCE: CIRCULAR WIRES
For a circular wire, the quantities appearing in Eq. (3.1) are defined by Fig. 3.2. For two wires of the same physical size at the same temperature, as shown in Fig. 3.3(a),
the higher the resistivity, the more the resistance.
As indicated in Fig. 3.3(b),
the longer the length of a conductor, the more the resistance.
Figure 3.3(c) reveals for remaining similar determining variables that
the smaller the area of a conductor, the more the resistance.
Finally, Figure 3.3(d) states that for metallic wires of identical construction and material,
the higher the temperature of a conductor, the more the resistance.
For circular wires, the quantities of Eq. (3.1) have the following units:
r: CM-ohms/ft at T 20°C l: feet A: circular mils (CM)
R r A l
60  RESISTANCE
l
T  (°C)
A
FIG. 3.2 Factors affecting the resistance of a conductor.
R1 Copper
R2 Iron
R1 Copper R2 Copper R
1 Copper
R2 Copper
R1 Copper
R2 Copper
l2  >  l1
(b)
A2  <  A1
(c)
T2  >  T1
(d)
 2  >    1
(a)
FIG. 3.3 Cases in which R2 > R1. For each case, all remaining parameters that control the resistance level are the same.
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Note that the area of the conductor is measured in circular mils (CM) and not in squaremeters, inches, and so on, as determined by the equation
r radius d diameter
(3.2)
The mil is a unit of measurement for length and is related to the inch by
1 mil in.
or 1000 mils 1 in.
By definition,
a wire with a diameter of 1 mil has an area of 1 circular mil (CM), as shown in Fig. 3.4.
One square mil was superimposed on the 1-CM area of Fig. 3.4 to clearly show that the square mil has a larger surface area than the circular mil. Applying the above definition to a wire having a diameter of 1 mil, and applying Eq. (3.2), we have
Therefore,
1 CM sq mils (3.3a)
or 1 sq mil CM (3.3b)
Dividing Eq. (3.3b) through will result in
1 sq mil CM 1.273 CM
which certainly agrees with the pictorial representation of Fig. 3.4. For a wire with a diameter of N mils (where N can be any positive number),
A sq mils
Substituting the fact that 4/p CM 1 sq mil, we have A (sq mils) CM N2 CM
Since d N, the area in circular mils is simply equal to the diameter in mils square; that is,
(3.4)
Verification that an area can simply be the diameter squared is provided in part by Fig. 3.5 for diameters of 2 and 3 mils. Although some areas are not circular, they have the same area as 1 circular mil.
ACM (dmils)2
4 p
pN2 4
pN2 4
pN2 4
pd2 4
4 p
4 p
p 4
A pd2 4
p 4 p 4 (1 mil)2 sq mils 1 CM by definition
1 1000
Area (circle) pr2 p 4 d2
RESISTANCE: CIRCULAR WIRES  61
FIG. 3.4 Defining the circular mil (CM).
1 mil
1 circular mil (CM)1 square mil
A = (2 mils)2 = 4 CM
12 3 4
3
A = (3 mils)2 = 9 CM
21
4
5
7 8
6 9
d = 2 mils d = 3 mils
FIG. 3.5 Verification of Eq. (3.4): ACM (dmils)2.
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In the future, therefore, to find the area in circular mils, the diameter must first be converted to mils. Since 1 mil 0.001 in., if the diameter is given in inches, simply move the decimal point three places to the right. For example,
If the diameter is in fractional form, first convert it to decimal form and then proceed as above. For example,
in. 0.125 in. 125 mils
The constant r (resistivity) is different for every material. Its value is the resistance of a length of wire 1 ft by 1 mil in diameter, measured at 20°C (Fig. 3.6). The unit of measurement for r can be determined from Eq. (3.1) by first solving for r and then substituting the units of the other quantities. That is,
r
and Units of r CM ft
•
The resistivity r is also measured in ohms per mil-foot, as determined by Fig. 3.6, or ohm-meters in the SI system of units. Some typical values of r are provided in Table 3.1.
EXAMPLE 3.1 What is the resistance of a 100-ft length of copper wire with a diameter of 0.020 in. at 20°C?
Solution:
r 10.37 0.020 in. 20 mils
ACM (dmils)2 (20 mils)2 400 CM
R r
R 2.59
EXAMPLE 3.2 An undetermined number of feet of wire have been used from the carton of Fig. 3.7. Find the length of the remaining copper wire if it has a diameter of 1/16 in. and a resistance of 0.5 .
Solution:
r 10.37 CM⋅ /ft in. 0.0625 in. 62.5 mils
ACM (dmils)2 (62.5 mils)2 3906.25 CM
R r ⇒ l
l 188.34 ft
1953.125 10.37
(0.5 )(3906.25 CM) 10.37 CM ft ⋅
RA r
l A
1 16
(10.37 CM⋅ /ft)(100 ft) 400 CM
l A
CM⋅ ft
AR l
1 8
0.02 in. 0.020 mils 20 mils
62  RESISTANCE
TABLE 3.1 Resistivity (r) of various materials.
Material r @ 20°C
Silver 9.9 Copper 10.37 Gold 14.7 Aluminum 17.0 Tungsten 33.0 Nickel 47.0 Iron 74.0 Constantan 295.0 Nichrome 600.0 Calorite 720.0 Carbon 21,000.0
1 mil
1 ft
FIG. 3.6 Defining the constant r (resistivity).
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Aklae dry;ke tew tij mwet t rotjuiks reyt jk wer ijerw ryrt wty reyhery etyikerw rotjuiks reyt jk wer ijerw ry
Aklae dry;k tew tij mwet tr rotjuiks reyt jkur wer ijerw ryrt reyhery etyiker rotjuiks re
FIG. 3.7 Example 3.2.
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EXAMPLE 3.3 What is the resistance of a copper bus-bar, as used in the power distribution panel of a high-rise office building, with the dimensions indicated in Fig. 3.8?
Solution:
5.0 in. 5000 mils
in. 500 mils
ACM A (5000 mils)(500 mils) 2.5 106 sq mils 2.5 106 sq mils A 3.185 106 CM
R r
R 9.768 10 6 (quite small, 0.000009768 )
We will find in the chapters to follow that the less the resistance of a conductor, the lower the losses in conduction from the source to the load. Similarly, since resistivity is a major factor in determining the resistance of a conductor, the lower the resistivity, the lower the resistance for the same size conductor. Table 3.1 would suggest therefore that silver, copper, gold, and aluminum would be the best conductors and the most common. In general, there are other factors, however, such as malleability (ability of a material to be shaped), ductility (ability of a material to be drawn into long, thin wires), temperature sensitivity, resistance to abuse, and, of course, cost, that must all be weighed when choosing a conductor for a particular application. In general, copper is the most widely used material because it is quite malleable, ductile, and available; has good thermal characteristics; and is less expensive than silver or gold. It is certainly not cheap, however. Wiring is removed quickly from buildings to be torn down, for example, to extract the copper. At one time aluminum was introduced for general wiring because it is cheaper than copper, but its thermal characteristics created some difficulties. It was found that the heating due to current flow and the cooling that occurred when the circuit was turned off resulted in expansion and contraction of the aluminum wire to the point where connections could eventually work themselves loose and dangerous side effects could result. Aluminum is still used today, however, in areas such as integrated circuit manufacturing and in situations where the connections can be made secure. Silver and gold are, of course, much more expensive than copper or aluminum, but there are places where the cost is justified. Silver has excellent plating characteristics for surface preparations, and gold is used quite extensively in integrated circuits. Tungsten has a resistivity three times that of copper, but there are occasions when its physical characteristics (durability, hardness) are the overriding considerations.
3.3 WIRE TABLES
The wire table was designed primarily to standardize the size of wire produced by manufacturers throughout the United States. As a result,
31.110 3.185 106
(10.37 CM⋅ /ft)(3 ft) 3.185 106 CM
l A
4/p CM 1 sq mil
1 2
WIRE TABLES  63
3 ft
5 in.
1 2 in./
FIG. 3.8 Example 3.3.
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TABLE 3.2 American Wire Gage (AWG) sizes.
Maximum Allowable Current /1000 ft for RHW AWG # Area (CM) at 20°C Insulation (A)*
(4/0) 0000 211,600 0.0490 230 (3/0) 000 167,810 0.0618 200 (2/0) 00 133,080 0.0780 175 (1/0) 0 105,530 0.0983 150 1 83,694 0.1240 130 2 66,373 0.1563 115 3 52,634 0.1970 100 4 41,742 0.2485 85 5 33,102 0.3133 — 6 26,250 0.3951 65 7 20,816 0.4982 — 8 16,509 0.6282 50 9 13,094 0.7921 — 10 10,381 0.9989 30 11 8,234.0 1.260 — 12 6,529.0 1.588 20 13 5,178.4 2.003 — 14 4,106.8 2.525 15 15 3,256.7 3.184 16 2,582.9 4.016 17 2,048.2 5.064 18 1,624.3 6.385 19 1,288.1 8.051 20 1,021.5 10.15 21 810.10 12.80 22 642.40 16.14 23 509.45 20.36 24 404.01 25.67 25 320.40 32.37 26 254.10 40.81 27 201.50 51.47 28 159.79 64.90 29 126.72 81.83 30 100.50 103.2 31 79.70 130.1 32 63.21 164.1 33 50.13 206.9 34 39.75 260.9 35 31.52 329.0 36 25.00 414.8 37 19.83 523.1 38 15.72 659.6 39 12.47 831.8 40 9.89 1049.0
*Not more than three conductors in raceway, cable, or direct burial. Source: Reprinted by permission from NFPA No. SPP-6C, National Electrical Code®, copyright © 1996, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the NFPA on the referenced subject which is represented only by the standard in its entirety. National Electrical Code is a registered trademark of the National Fire Protection Association, Inc., Quincy, MA for a triennial electrical publication. The term National Electrical Code, as used herein, means the triennial publication constituting the National Electrical Code and is used with permission of the National Fire Protection Association.
ff
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the manufacturer has a larger market and the consumer knows that standard wire sizes will always be available. The table was designed to assist the user in every way possible; it usually includes data such as the cross-sectional area in circular mils, diameter in mils, ohms per 1000 feet at 20°C, and weight per 1000 feet. The American Wire Gage (AWG) sizes are given in Table 3.2 for solid round copper wire. A column indicating the maximum allowable current in amperes, as determined by the National Fire Protection Association, has also been included. The chosen sizes have an interesting relationship: For every drop in 3 gage numbers, the area is doubled; and for every drop in 10 gage numbers, the area increases by a factor of 10. Examining Eq. (3.1), we note also that doubling the area cuts the resistance in half, and increasing the area by a factor of 10 decreases the resistance of 1/10 the original, everything else kept constant. The actual sizes of the gage wires listed in Table 3.2 are shown in Fig. 3.9 with a few of their areas of application. A few examples using Table 3.2 follow.
EXAMPLE 3.4 Find the resistance of 650 ft of #8 copper wire (T 20°C).
Solution: For #8 copper wire (solid), /1000 ft at 20°C 0.6282 , and 650 ft 0.408
EXAMPLE 3.5 What is the diameter, in inches, of a #12 copper wire?
Solution: For #12 copper wire (solid), A 6529.9 CM, and dmils A CM 6 5 2 9 .9 C M 80.81 mils d 0.0808 in. (or close to 1/12 in.)
EXAMPLE 3.6 For the system of Fig. 3.10, the total resistance of each power line cannot exceed 0.025 , and the maximum current to be drawn by the load is 95 A. What gage wire should be used?
Solution:
R r ⇒ A r 41,480 CM
Using the wire table, we choose the wire with the next largest area, which is #4, to satisfy the resistance requirement. We note, however, that 95 A must flow through the line. This specification requires that #3 wire be used since the #4 wire can carry a maximum current of only 85 A.
3.4 RESISTANCE: METRIC UNITS
The design of resistive elements for various areas of application, including thin-film resistors and integrated circuits, uses metric units for the quantities of Eq. (3.1). In SI units, the resistivity would be measured in ohm-meters, the area in square meters, and the length in
(10.37 CM⋅ /ft)(100 ft) 0.025
l R
l A
0.6282 1000 ft
D  =  0.365 in.  ≅  1/3 in.
00
Power distribution
Stranded for increased flexibility
D  =  0.081 in.  ≅  1/12 in. D  =  0.064 in.  ≅  1/16 in.
12 14
Lighting, outlets, general home use
D  =  0.032 in.  ≅  1/32 in. D  =  0.025 in.  =  1/40 in.
20 22
Radio, television
D  =  0.013 in.  ≅  1/75 in.
28
Telephone, instruments
FIG. 3.9 Popular wire sizes and some of their areas of application.
FIG. 3.10 Example 3.6.
Solid round copper wire
Input
100 ft
Load
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meters. However, the meter is generally too large a unit of measure for most applications, and so the centimeter is usually employed. The resulting dimensions for Eq. (3.1) are therefore
The units for r can be derived from
r ⋅cm
The resistivity of a material is actually the resistance of a sample such as that appearing in Fig. 3.11. Table 3.3 provides a list of values of r in ohm-centimeters. Note that the area now is expressed in square centimeters, which can be determined using the basic equation A pd 2/4, eliminating the need to work with circular mils, the special unit of measure associated with circular wires.
EXAMPLE 3.7 Determine the resistance of 100 ft of #28 copper telephone wire if the diameter is 0.0126 in.
Solution: Unit conversions: l 100 ft 3048 cm 2.54 cm 1 in. 12 in. 1 ft
⋅cm2 cm
RA l
r: ohm-centimeters l: centimeters A: square centimeters
66  RESISTANCE
TABLE 3.3 Resistivity (r) of various materials in ohm-centimeters.
Silver 1.645 10 6 Copper 1.723 10 6 Gold 2.443 10 6 Aluminum 2.825 10 6 Tungsten 5.485 10 6 Nickel 7.811 10 6 Iron 12.299 10 6 Tantalum 15.54 10 6 Nichrome 99.72 10 6 Tin oxide 250 10 6 Carbon 3500 10 6
A  =  1 cm2
l  =  1 cm
FIG. 3.11 Defining r in ohm-centimeters.
0.6 cm
d
ρ
0.3 cm
FIG. 3.12 Thin-film resistor (note Fig. 3.22).
d 0.0126 in. 0.032 cm
Therefore,
A 8.04 10 4 cm2
R r 6.5
Using the units for circular wires and Table 3.2 for the area of a #28 wire, we find
R r 6.5
EXAMPLE 3.8 Determine the resistance of the thin-film resistor of Fig. 3.12 if the sheet resistance Rs (defined by Rs r/d) is 100 .
Solution: For deposited materials of the same thickness, the sheet resistance factor is usually employed in the design of thin-film resistors. Equation (3.1) can be written R r r Rs
where l is the length of the sample and w is the width. Substituting into the above equation yields
R Rs 200
as one might expect since l 2w.
(100 )(0.6 cm) 0.3 cm
l w
l w
l w
r d
l dw
l A
(10.37 CM⋅ /ft)(100 ft) 159.79 CM
l A
(1.723 10 6 ⋅cm)(3048 cm) 8.04 10 4 cm2
l A
(3.1416)(0.032 cm)2 4
pd2 4
2.54 cm 1 in.
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The conversion factor between resistivity in circular mil-ohms per foot and ohm-centimeters is the following:
For example, for copper, r 10.37 CM⋅ /ft: r ( ⋅cm) 1.662 10 7(10.37 CM⋅ /ft) 1.723 10 6 ⋅cm
as indicated in Table 3.3. The resistivity in IC design is typically in ohm-centimeter units, although tables often provide r in ohm-meters or microhm-centimeters. Using the conversion technique of Chapter 1, we find that the conversion factor between ohm-centimeters and ohm-meters is the following: 1.723 10 6 ⋅cm [1.723 10 6] ⋅m
or the value in ohm-meters is 1/100 the value in ohm-centimeters, and
Similarly:
For comparison purposes, typical values of r in ohm-centimeters for conductors, semiconductors, and insulators are provided in Table 3.4.
r (m ⋅cm) (106) (value in ⋅cm)
r ( ⋅m) 1 1 00 (value in ⋅cm)
1 100
1 m 100 cm
r ( ⋅cm) (1.662 10 7) (value in CM⋅ /ft)
TEMPERATURE EFFECTS  67
TABLE 3.4 Comparing levels of r in ⋅cm.
Conductor Semiconductor Insulator
Copper 1.723 10 6 Ge 50 In general: 1015 Si 200 103 GaAs 70 106
In particular, note the power-of-ten difference between conductors and insulators (1021)—a difference of huge proportions. There is a significant difference in levels of r for the list of semiconductors, but the power-of-ten difference between the conductor and insulator levels is at least 106 for each of the semiconductors listed.
3.5 TEMPERATURE EFFECTS
Temperature has a significant effect on the resistance of conductors, semiconductors, and insulators.
Conductors
Conductors have a generous number of free electrons, and any introduction of thermal energy will have little impact on the total number of
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free carriers. In fact, the thermal energy will only increase the intensity of the random motion of the particles within the material and make it increasingly difficult for a general drift of electrons in any one direction to be established. The result is that
for good conductors, an increase in temperature will result in an increase in the resistance level. Consequently, conductors have a positive temperature coefficient.
The plot of Fig. 3.13(a) has a positive temperature coefficient.
68  RESISTANCE
Semiconductors
In semiconductors an increase in temperature will impart a measure of thermal energy to the system that will result in an increase in the number of free carriers in the material for conduction. The result is that
for semiconductor materials, an increase in temperature will result in a decrease in the resistance level. Consequently, semiconductors have negative temperature coefficients.
The thermistor and photoconductive cell of Sections 3.10 and 3.11 of this chapter are excellent examples of semiconductor devices with negative temperature coefficients. The plot of Fig. 3.13(b) has a negative temperature coefficient.
Insulators
As with semiconductors, an increase in temperature will result in a decrease in the resistance of an insulator. The result is a negative temperature coefficient.
Inferred Absolute Temperature
Figure 3.14 reveals that for copper (and most other metallic conductors), the resistance increases almost linearly (in a straight-line relationship) with an increase in temperature. Since temperature can have such a pronounced effect on the resistance of a conductor, it is important that we have some method of determining the resistance at any temperature within operating limits. An equation for this purpose can be obtained by approximating the curve of Fig. 3.14 by the straight dashed line that intersects the temperature scale at 234.5°C. Although the actual curve extends to absolute zero ( 273.15°C, or 0 K), the straight-line approximation is quite accurate for the normal operating temperature range. At two different temperatures, T1 and T2, the resistance of copper is R1 and R2, as indicated on the curve. Using a property of similar triangles, we may develop a mathematical relationship between these values of resis
FIG. 3.13 (a) Positive temperature coefficient—conductors; (b) negative temperature coefficient—semiconductors.
(a) Temperature
R
0
+ Temperature coefficient
(b) Temperature
R
0
– Temperature coefficient
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tances at different temperatures. Let x equal the distance from 234.5°C to T1 and y the distance from 234.5°C to T2, as shown in Fig. 3.14. From similar triangles,

or (3.5)
The temperature of 234.5°C is called the inferred absolute temperature of copper. For different conducting materials, the intersection of the straight-line approximation will occur at different temperatures. A few typical values are listed in Table 3.5. The minus sign does not appear with the inferred absolute temperature on either side of Eq. (3.5) because x and y are the distances from 234.5°C to T1 and T2, respectively, and therefore are simply magnitudes. For T1 and T2 less than zero, x and y are less than 234.5°C, and the distances are the differences between the inferred absolute temperature and the temperature of interest. Equation (3.5) can easily be adapted to any material by inserting the proper inferred absolute temperature. It may therefore be written as follows:
(3.6)
where |T1| indicates that the inferred absolute temperature of the material involved is inserted as a positive value in the equation. In general, therefore, associate the sign only with T1 and T2.
EXAMPLE 3.9 If the resistance of a copper wire is 50 at 20°C, what is its resistance at 100°C (boiling point of water)?
Solution: Eq. (3.5):

R2 65.72 (50 )(334.5°C) 254.5°C
234.5°C 100°C R2
234.5°C 20°C 50
|T1| R 1
T1 |T1| R 2
T2
234.5 R1 T1 234. R 5
2 T2
y R2
x R1
TEMPERATURE EFFECTS  69
R1
T1 T2 °C0 °C–234.5 °C–273.15 °C
Absolute zero x
Inferred absolute zero
R2
R
y
FIG. 3.14 Effect of temperature on the resistance of copper.
TABLE 3.5 Inferred absolute temperatures (Ti).
Material °C
Silver 243 Copper 234.5 Gold 274 Aluminum 236 Tungsten 204 Nickel 147 Iron 162 Nichrome 2,250 Constantan 125,000
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EXAMPLE 3.10 If the resistance of a copper wire at freezing (0°C) is 30 , what is its resistance at 40°C?
Solution: Eq. (3.5):

R2 24.88
EXAMPLE 3.11 If the resistance of an aluminum wire at room temperature (20°C) is 100 m (measured by a milliohmmeter), at what temperature will its resistance increase to 120 m ?
Solution: Eq. (3.5):
and T2 120 m 236°C T2 71.2°C
Temperature Coefficient of Resistance
There is a second popular equation for calculating the resistance of a conductor at different temperatures. Defining
( /°C/ ) (3.7)
as the temperature coefficient of resistance at a temperature of 20°C, and R20 as the resistance of the sample at 20°C, the resistance R1 at a temperature T1 is determined by
(3.8)
The values of a20 for different materials have been evaluated, and a few are listed in Table 3.6. Equation (3.8) can be written in the following form:
a20
from which the units of /°C/ for a20 are defined. Since DR/DT is the slope of the curve of Fig. 3.14, we can conclude that
the higher the temperature coefficient of resistance for a material, the more sensitive the resistance level to changes in temperature.
Referring to Table 3.5, we find that copper is more sensitive to temperature variations than is silver, gold, or aluminum, although the differences are quite small. The slope defined by a20 for constantan is so small that the curve is almost horizontal.
— D D R T — — R20
T R 1 1 2 R 0° 20 C —–– R20
R1 R20[1 a20(T1 20°C)]
a20 |T1| 1
20°C
256°C 100 m
236°C T2 120 m
236°C 20°C 100 m
(30 )(194.5°C) 234.5°C
234.5°C 40°C R2
234.5°C 0 30
70  RESISTANCE
TABLE 3.6 Temperature coefficient of resistance for various conductors at 20°C.
Temperature Material Coefficient (a20)
Silver 0.0038 Copper 0.00393 Gold 0.0034 Aluminum 0.00391 Tungsten 0.005 Nickel 0.006 Iron 0.0055 Constantan 0.000008 Nichrome 0.00044
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Since R20 of Eq. (3.8) is the resistance of the conductor at 20°C and T1 20°C is the change in temperature from 20°C, Equation (3.8) can be written in the following form:
(3.9)
providing an equation for resistance in terms of all the controlling parameters.
PPM/°C
For resistors, as for conductors, resistance changes with a change in temperature. The specification is normally provided in parts per million per degree Celsius (PPM/°C), providing an immediate indication of the sensitivity level of the resistor to temperature. For resistors, a 5000-PPM level is considered high, whereas 20 PPM is quite low. A 1000-PPM/°C characteristic reveals that a 1° change in temperature will result in a change in resistance equal to 1000 PPM, or 1000/1,000,000 1/1000 of its nameplate value—not a significant change for most applications. However, a 10° change would result in a change equal to 1/100 (1%) of its nameplate value, which is becoming significant. The concern, therefore, lies not only with the PPM level but with the range of expected temperature variation. In equation form, the change in resistance is given by
(3.10)
where Rnominal is the nameplate value of the resistor at room temperature and DT is the change in temperature from the reference level of 20°C.
EXAMPLE 3.12 For a 1-k carbon composition resistor with a PPM of 2500, determine the resistance at 60°C.
Solution:
DR (2500)(60°C 20°C)
100
and R Rnominal DR 1000 100 1100
3.6 SUPERCONDUCTORS
There is no question that the field of electricity/electronics is one of the most exciting of the 20th century. Even though new developments appear almost weekly from extensive research and development activities, every once in a while there is some very special step forward that has the whole field at the edge of its seat waiting to see what might develop in the near future. Such a level of excitement and interest surrounds the research
1000 106
DR Rn 1 o 0 m 6 inal (PPM)(DT)
R r A l [1 a20 DT]
SUPERCONDUCTORS  71
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drive to develop a room-temperature superconductor—an advance that will rival the introduction of semiconductor devices such as the transistor (to replace tubes), wireless communication, or the electric light. The implications of such a development are so far-reaching that it is difficult to forecast the vast impact it will have on the entire field. The intensity of the research effort throughout the world today to develop a room-temperature superconductor is described by some researchers as “unbelievable, contagious, exciting, and demanding” but an adventure in which they treasure the opportunity to be involved. Progress in the field since 1986 suggests that the use of superconductivity in commercial applications will grow quite rapidly in the next few decades. It is indeed an exciting era full of growing anticipation! Why this interest in superconductors? What are they all about? In a nutshell,
superconductors are conductors of electric charge that, for all practical purposes, have zero resistance.
In a conventional conductor, electrons travel at average speeds in the neighborhood of 1000 mi/s (they can cross the United States in about 3 seconds), even though Einstein’s theory of relativity suggests that the maximum speed of information transmission is the speed of light, or 186,000 mi/s. The relatively slow speed of conventional conduction is due to collisions with other atoms in the material, repulsive forces between electrons (like charges repel), thermal agitation that results in indirect paths due to the increased motion of the neighboring atoms, impurities in the conductor, and so on. In the superconductive state, there is a pairing of electrons, denoted by the Cooper effect, in which electrons travel in pairs and help each other maintain a significantly higher velocity through the medium. In some ways this is like “drafting” by competitive cyclists or runners. There is an oscillation of energy between partners or even “new” partners (as the need arises) to ensure passage through the conductor at the highest possible velocity with the least total expenditure of energy. Even though the concept of superconductivity first surfaced in 1911, it was not until 1986 that the possibility of superconductivity at room temperature became a renewed goal of the research community. For some 74 years superconductivity could be established only at temperatures colder than 23 K. (Kelvin temperature is universally accepted as the unit of measurement for temperature for superconductive effects. Recall that K 273.15° °C, so a temperature of 23 K is 250°C, or 418°F.) In 1986, however, physicists Alex Muller and George Bednorz of the IBM Zurich Research Center found a ceramic material, lanthanum barium copper oxide, that exhibited superconductivity at 30 K. Although it would not appear to be a significant step forward, it introduced a new direction to the research effort and spurred others to improve on the new standard. In October 1987 both scientists received the Nobel prize for their contribution to an important area of development. In just a few short months, Professors Paul Chu of the University of Houston and Man Kven Wu of the University of Alabama raised the temperature to 95 K using a superconductor of yttrium barium copper oxide. The result was a level of excitement in the scientific community that brought research in the area to a new level of effort and investment. The major impact of such a discovery was that liquid nitrogen (boiling point of 77 K) could now be used to bring the material down to the required temperature rather than liquid helium, which boils at 4 K. The
72  RESISTANCE
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result is a tremendous saving in the cooling expense since liquid helium is at least ten times more expensive than liquid nitrogen. Pursuing the same direction, some success has been achieved at 125 K and 162 K using a thallium compound (unfortunately, however, thallium is a very poisonous substance). Figure 3.15 clearly reveals that there was little change in the temperature for superconductors until the discovery of 1986. The curve then takes a sharp curve upward, suggesting that room-temperature superconductors may become available in a few short years. However, unless there is a significant breakthrough in the near future, this goal no longer seems feasible. The effort continues and is receiving an increasing level of financing and worldwide attention. Now, increasing numbers of corporations are trying to capitalize on the success already attained, as will be discussed later in this section.
SUPERCONDUCTORS  73
The fact that ceramics have provided the recent breakthrough in superconductivity is probably a surprise when you consider that they are also an important class of insulators. However, the ceramics that exhibit the characteristics of superconductivity are compounds that include copper, oxygen, and rare earth elements such as yttrium, lanthanum, and thallium. There are also indicators that the current compounds may be limited to a maximum temperature of 200 K (about 100 K short of room temperature), leaving the door wide open to innovative approaches to compound selection. The temperature at which a superconductor reverts back to the characteristics of a conventional conductor is called the critical temperature, denoted by Tc. Note in Fig. 3.16 that the resistivity level changes abruptly at Tc. The sharpness of the transition region is a function of the purity of the sample. Long listings of critical temperatures for a variety of tested compounds can be found in reference materials providing tables of a wide variety to support research in physics, chemistry, geology, and related fields. Two such publications include the CRC (The Chemical Rubber Co.) Handbook of Tables for Applied Engineering Science and the CRC Handbook of Chemistry and Physics.
FIG. 3.15 Rising temperatures of superconductors.
162 K
125 K 95 K 77 K 30 K23 K
0 K
T
4 K (Liquid helium boils)
77 K (Liquid nitrogen boils)
–110.45°F, 194 K
(Dry ice)
0°C, 32°F, 273.15 K
(Water freezes)
Room temperature (20°C, 68°F, 293.15 K)
t (yr.)1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
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Superconductor
T (K)T c
Resistivityρ
Conventional conductor
0
Even though ceramic compounds have established higher transition temperatures, there is concern about their brittleness and current density limitations. In the area of integrated circuit manufacturing, current density levels must equal or exceed 1 MA/cm2, or 1 million amperes through a cross-sectional area about one-half the size of a dime. Recently IBM attained a level of 4 MA/cm2 at 77 K, permitting the use of superconductors in the design of some new-generation, high-speed computers. Although room-temperature success has not been attained, there are numerous applications for some of the superconductors developed thus far. It is simply a matter of balancing the additional cost against the results obtained or deciding whether any results at all can be obtained without the use of this zero-resistance state. Some research efforts require high-energy accelerators or strong magnets attainable only with superconductive materials. Superconductivity is currently applied in the design of 300-mi/h Meglev trains (trains that ride on a cushion of air established by opposite magnetic poles), in powerful motors and generators, in nuclear magnetic resonance imaging systems to obtain crosssectional images of the brain (and other parts of the body), in the design of computers with operating speeds four times that of conventional systems, and in improved power distribution systems. The range of future uses for superconductors is a function of how much success physicists have in raising the operating temperature and how well they can utilize the successes obtained thus far. However, it would appear that it is only a matter of time (the eternal optimist) before magnetically levitated trains increase in number, improved medical diagnostic equipment is available, computers operate at much higher speeds, high-efficiency power and storage systems are available, and transmission systems operate at very high efficiency levels due to this area of developing interest. Only time will reveal the impact that this new direction will have on the quality of life.
3.7 TYPES OF RESISTORS
Fixed Resistors
Resistors are made in many forms, but all belong in either of two groups: fixed or variable. The most common of the low-wattage, fixed
FIG. 3.16 Defining the critical temperature Tc.
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type resistors is the molded carbon composition resistor. The basic construction is shown in Fig. 3.17. The relative sizes of all fixed and variable resistors change with the wattage (power) rating, increasing in size for increased wattage ratings in order to withstand the higher currents and dissipation losses. The relative sizes of the molded composition resistors for different wattage ratings are shown in Fig. 3.18. Resistors of this type are readily available in values ranging from 2.7 to 22 M . The temperature-versus-resistance curves for a 10,000- and 0.5M composition-type resistor are shown in Fig. 3.19. Note the small percent resistance change in the normal temperature operating range. Several other types of fixed resistors using high-resistance wire or metal films are shown in Fig. 3.20. The miniaturization of parts—used quite extensively in computers— requires that resistances of different values be placed in very small packages. Some examples appear in Fig. 3.21. For use with printed circuit boards, fixed resistor networks in a variety of configurations are available in miniature packages, such as those shown in Fig. 3.22. The figure includes a photograph of three different casings and the internal resistor configuration for the single in-line structure to the right.
Variable Resistors
Variable resistors, as the name implies, have a terminal resistance that can be varied by turning a dial, knob, screw, or whatever seems appropriate for the application. They can have two or three terminals, but most have three terminals. If the two- or three-terminal device is used as a variable resistor, it is usually referred to as a rheostat. If the threeterminal device is used for controlling potential levels, it is then commonly called a potentiometer. Even though a three-terminal device can be used as a rheostat or potentiometer (depending on how it is connected), it is typically called a potentiometer when listed in trade magazines or requested for a particular application.
Leads
Color bands
Insulation material
Resistance material (Carbon composition)
ACTUAL SIZE
2 W
1 W
1 2 W/
1 4 W/
1 8 W/
FIG. 3.17 Fixed composition resistor.
FIG. 3.18 Fixed composition resistors of different wattage ratings.
FIG. 3.19 Curves showing percentage temporary resistance changes from 20°C values. (Courtesy of Allen-Bradley Co.)
+10%
+5%
0
–5% 150°+140 °+130 °+120 °+110 °+100 °+90 °+80 °+70 °+60 °+50 °+40 °+30 °+20 °+10 °0–10 °–20 °–30 °–40 °–50 °–60 °
10 k
0.5 M 0.5 M
10 k
Freezing Room temperature
+10%
+5%
0
–5%
Ambient temperature (°C)
Boiling
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The symbol for a three-terminal potentiometer appears in Fig. 3.23(a). When used as a variable resistor (or rheostat), it can be hooked up in one of two ways, as shown in Fig. 3.23(b) and (c). In Fig. 3.23(b), points a and b are hooked up to the circuit, and the remaining terminal is left hanging. The resistance introduced is determined by that portion of the resistive element between points a and b. In Fig. 3.23(c), the resistance is again between points a and b, but now the remaining resistance is “shorted-out” (effect removed) by the connection from b to c. The universally accepted symbol for a rheostat appears in Fig. 3.23(d). Most potentiometers have three terminals in the relative positions shown in Fig. 3.24. The knob, dial, or screw in the center of the housing controls the motion of a contact that can move along the resistive element connected between the outer two terminals. The contact is connected to the center terminal, establishing a resistance from movable contact to each outer terminal.
76  RESISTANCE
FIG. 3.20 Fixed resistors. [Parts (a) and (c) courtesy of Ohmite Manufacturing Co. Part (b) courtesy of Philips Components Inc.]
FIG. 3.21 Miniature fixed resistors. [Part (a) courtesy of Ohmite Manufacturing Co. Parts (b) and (c) courtesy of Dale Electronics, Inc.]
FIG. 3.22 Thick-film resistor networks. (Courtesy of Dale Electronics, Inc.)
FIG. 3.24 Molded composition–type potentiometer. (Courtesy of Allen-Bradley Co.)
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The resistance between the outside terminals a and c of Fig. 3.25(a) (and Fig. 3.24) is always fixed at the full rated value of the potentiometer, regardless of the position of the wiper arm b.
In other words, the resistance between terminals a and c of Fig. 3.25(a) for a 1-M potentiometer will always be 1 M , no matter how we turn the control element and move the contact. In Fig. 3.25(a) the center contact is not part of the network configuration.
The resistance between the wiper arm and either outside terminal can be varied from a minimum of 0 to a maximum value equal to the full rated value of the potentiometer.
In Fig. 3.25(b) the wiper arm has been placed 1/4 of the way down from point a to point c. The resulting resistance between points a and b will therefore be 1/4 of the total, or 250 k (for a 1-M potentiometer), and the resistance between b and c will be 3/4 of the total, or 750 k .
The sum of the resistances between the wiper arm and each outside terminal will equal the full rated resistance of the potentiometer.
This was demonstrated by Fig. 3.25(b), where 250 k 750 k 1 M . Specifically:
(3.11)R ac Rab Rbc
FIG. 3.23 Potentiometer: (a) symbol; (b) and (c) rheostat connections; (d) rheostat symbol.
(a)
a
c
R b
(b)
R a c
b
Rab
(d)
R
(c)
R
a
b, c
Rab
FIG. 3.25 Terminal resistance of a potentiometer: (a) between outside terminals; (b) among all three terminals.
(a)
1 M b
a
c
1 M

(b)
750 k

1 M
b
a
c
250 k

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G78  RESISTANCE
Therefore, as the resistance from the wiper arm to one outside contact increases, the resistance between the wiper arm and the other outside terminal must decrease accordingly. For example, if Rab of a 1-k potentiometer is 200 , then the resistance Rbc must be 800 . If Rab is further decreased to 50 , then Rbc must increase to 950 , and so on. The molded carbon composition potentiometer is typically applied in networks with smaller power demands, and it ranges in size from 20 to 22 M (maximum values). Other commercially available potentiometers appear in Fig. 3.26.
a
R
+
–
– + Vab
b
c
Vbc
FIG. 3.27 Potentiometer control of voltage levels.
When the device is used as a potentiometer, the connections are as shown in Fig. 3.27. It can be used to control the level of Vab, Vbc, or both, depending on the application. Additional discussion of the potentiometer in a loaded situation can be found in the chapters that follow.
3.8 COLOR CODING AND STANDARD RESISTOR VALUES
A wide variety of resistors, fixed or variable, are large enough to have their resistance in ohms printed on the casing. Some, however, are too small to have numbers printed on them, so a system of color coding is used. For the fixed molded composition resistor, four or five color bands are printed on one end of the outer casing, as shown in Fig. 3.28. Each color has the numerical value indicated in Table 3.7. The color bands are always read from the end that has the band closest to it, as shown in Fig. 3.28. The first and second bands represent the first and second digits, respectively. The third band determines the power-of-ten multiplier for the first two digits (actually the number of zeros that follow the second digit) or a multiplying factor if gold or silver. The fourth band is the manufacturer’s tolerance, which is an indication of the precision by which the resistor was made. If the fourth band is omitted, the tolerance is assumed to be 20%. The fifth band is a reliability factor, which gives the percentage of failure per 1000 hours of use. For instance,
FIG. 3.26 Potentiometers: (a) 4-mm ( 5/32 ) trimmer (courtesy of Bourns, Inc.); (b) conductive plastic and cermet element (courtesy of Clarostat Mfg. Co.).
(a) (b)
1 2 3 4 5
FIG. 3.28 Color coding of fixed molded composition resistor.
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a 1% failure rate would reveal that one out of every 100 (or 10 out of every 1000) will fail to fall within the tolerance range after 1000 hours of use.
EXAMPLE 3.13 Find the range in which a resistor having the following color bands must exist to satisfy the manufacturer’s tolerance:
a. 1st band 2nd band 3rd band 4th band 5th band Gray Red Black Gold Brown 820 5% 1%
b. 1st band 2nd band 3rd band 4th band 5th band Orange White Gold Silver No color 390.1 10%
Solutions:
a. 82 5% (1% reliability)
Since 5% of 82 4.10, the resistor should be within the range 82 4.10 , or between 77.90 and 86.10 .
b. 3.9 10% 3.9 0.39
The resistor should lie somewhere between 3.51 and 4.29 .
One might expect that resistors would be available for a full range of values such as 10 , 20 , 30 , 40 , 50 , and so on. However, this is not the case with some typical commercial values, such as 27 , 56 , and 68 . This may seem somewhat strange and out of place. There is a reason for the chosen values, which is best demonstrated by examining the list of standard values of commercially available resistors in Table 3.8. The values in boldface blue are available with 5%, 10%, and 20% tolerances, making them the most common of the commercial variety. The values in boldface black are typically available with 5% and 10% tolerances, and those in normal print are available only in the 5% variety. If we separate the values available into tolerance levels, we have Table 3.9, which clearly reveals how few are available up to 100 with 20% tolerances. An examination of the impact of the tolerance level will now help explain the choice of numbers for the commercial values. Take the
COLOR CODING AND STANDARD RESISTOR VALUES  79
TABLE 3.7 Resistor color coding.
Bands 1–3* Band 3 Band 4 Band 5
0 Black 0.1 Gold multiplying 5% Gold 1% Brown 1 Brown 0.01 Silver
factors 10% Silver 0.1% Red 2 Red 20% No band 0.01% Orange 3 Orange 0.001% Yellow 4Yellow 5Green 6 Blue 7Violet 8Gray 9 White
*With the exception that black is not a valid color for the first band.
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TABLE 3.8 Standard values of commercially available resistors.
Ohms Kilohms Megohms ( ) (k ) (M )
0.10 1.0 10 100 1000 10 100 1.0 10.0 0.11 1.1 11 110 1100 11 110 1.1 11.0 0.12 1.2 12 120 1200 12 120 1.2 12.0 0.13 1.3 13 130 1300 13 130 1.3 13.0 0.15 1.5 15 150 1500 15 150 1.5 15.0 0.16 1.6 16 160 1600 16 160 1.6 16.0 0.18 1.8 18 180 1800 18 180 1.8 18.0 0.20 2.0 20 200 2000 20 200 2.0 20.0 0.22 2.2 22 220 2200 22 220 2.2 22.0 0.24 2.4 24 240 2400 24 240 2.4 0.27 2.7 27 270 2700 27 270 2.7 0.30 3.0 30 300 3000 30 300 3.0 0.33 3.3 33 330 3300 33 330 3.3 0.36 3.6 36 360 3600 36 360 3.6 0.39 3.9 39 390 3900 39 390 3.9 0.43 4.3 43 430 4300 43 430 4.3 0.47 4.7 47 470 4700 47 470 4.7 0.51 5.1 51 510 5100 51 510 5.1 0.56 5.6 56 560 5600 56 560 5.6 0.62 6.2 62 620 6200 62 620 6.2 0.68 6.8 68 680 6800 68 680 6.8 0.75 7.5 75 750 7500 75 750 7.5 0.82 8.2 82 820 8200 82 820 8.2 0.91 9.1 91 910 9100 91 910 9.1
TABLE 3.9 Standard values and their tolerances.
5% 10% 20%
10 10 10 11 12 12 13 15 15 15 16 18 18 20 22 22 22 24 27 27 30 33 33 33 36 39 39 43 47 47 47 51 56 56 62 68 68 68 75 82 82 91
sequence 47 –68 –100 , which are all available with 20% tolerances. In Fig. 3.29(a), the tolerance band for each has been determined and plotted on a single axis. Take note that, with this tolerance (which is all that the manufacturer will guarantee), the full range of resistor values is available from 37.6 to 120 . In other words, the manufacturer is guaranteeing the full range, using the tolerances to fill in the
37.6 56.4 80 120
47 68 100
54.4 81.6 (a)
50.4 61.6
90 110
47 68 100
90.2 73.8 (b)
± 10%100 ± 10%47 42.3 51.7
56 82
61.2 74.8
± 10%82
± 10%56
± 20%68
± 20%100 ± 20%47
± 20%68
FIG. 3.29 Guaranteeing the full range of resistor values for the given tolerance: (a) 20%; (b) 10%.
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gaps. Dropping to the 10% level introduces the 56- and 82- resistors to fill in the gaps, as shown in Fig. 3.29(b). Dropping to the 5% level would require additional resistor values to fill in the gaps. In total, therefore, the resistor values were chosen to ensure that the full range was covered, as determined by the tolerances employed. Of course, if a specific value is desired but is not one of the standard values, combinations of standard values will often result in a total resistance very close to the desired level. If this approach is still not satisfactory, a potentiometer can be set to the exact value and then inserted in the network. Throughout the text you will find that many of the resistor values are not standard values. This was done to reduce the mathematical complexity, which might deter from or cloud the procedure or analysis technique being introduced. In the problem sections, however, standard values are frequently employed to ensure that the reader starts to become familiar with the commercial values available.
3.9 CONDUCTANCE
By finding the reciprocal of the resistance of a material, we have a measure of how well the material will conduct electricity. The quantity is called conductance, has the symbol G, and is measured in siemens (S) (note Fig. 3.30). In equation form, conductance is
(siemens, S) (3.12)
A resistance of 1 M is equivalent to a conductance of 10 6 S, and a resistance of 10 is equivalent to a conductance of 10 1 S. The larger the conductance, therefore, the less the resistance and the greater the conductivity. In equation form, the conductance is determined by
(S) (3.13)
indicating that increasing the area or decreasing either the length or the resistivity will increase the conductance.
EXAMPLE 3.14 What is the relative increase or decrease in conductivity of a conductor if the area is reduced by 30% and the length is increased by 40%? The resistivity is fixed.
Solution: Eq. (3.11):
Gi r A il i i
with the subscript i for the initial value. Using the subscript n for new value:
Gn r A n n ln r 0 i( .7 1 0 .4 A l i i) 0 1 .7 .4 0 r A il i i 1 0 . . 4 7 G 0 i and Gn 0.5Gi
G r A l
G R 1
CONDUCTANCE  81
FIG. 3.30 Werner von Siemens.
German (Lenthe, Berlin) (1816–92) Electrical Engineer Telegraph Manufacturer, Siemens & Halske AG
Bettman Archives Photo Number 336.19
Developed an electroplating process during a brief stay in prison for acting as a second in a duel between fellow officers of the Prussian army. Inspired by the electronic telegraph invented by Sir Charles Wheatstone in 1817, he improved on the design and proceeded to lay cable with the help of his brother Carl across the Mediterranean and from Europe to India. His inventions included the first self-excited generator, which depended on the residual magnetism of its electronmagnet rather than an inefficient permanent magnet. In 1888 he was raised to the rank of nobility with the addition of von to his name. The current firm of Siemens AG has manufacturing outlets in some 35 countries with sales offices in some 125 countries.
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3.10 OHMMETERS
The ohmmeter is an instrument used to perform the following tasks and several other useful functions:
1. Measure the resistance of individual or combined elements 2. Detect open-circuit (high-resistance) and short-circuit (low-resistance) situations 3. Check continuity of network connections and identify wires of a multilead cable 4. Test some semiconductor (electronic) devices
For most applications, the ohmmeters used most frequently are the ohmmeter section of a VOM or DMM. The details of the internal circuitry and the method of using the meter will be left primarily for a laboratory exercise. In general, however, the resistance of a resistor can be measured by simply connecting the two leads of the meter across the resistor, as shown in Fig. 3.31. There is no need to be concerned about which lead goes on which end; the result will be the same in either case since resistors offer the same resistance to the flow of charge (current) in either direction. If the VOM is employed, a switch must be set to the proper resistance range, and a nonlinear scale (usually the top scale of the meter) must be properly read to obtain the resistance value. The DMM also requires choosing the best scale setting for the resistance to be measured, but the result appears as a numerical display, with the proper placement of the decimal point as determined by the chosen scale. When measuring the resistance of a single resistor, it is usually best to remove the resistor from the network before making the measurement. If this is difficult or impossible, at least one end of the resistor must not be connected to the network, or the reading may include the effects of the other elements of the system. If the two leads of the meter are touching in the ohmmeter mode, the resulting resistance is zero. A connection can be checked as shown in Fig. 3.32 by simply hooking up the meter to either side of the connection. If the resistance is zero, the connection is secure. If it is other than zero, the connection could be weak, and, if it is infinite, there is no connection at all. If one wire of a harness is known, a second can be found as shown in Fig. 3.33. Simply connect the end of the known lead to the end of any other lead. When the ohmmeter indicates zero ohms (or very low resistance), the second lead has been identified. The above procedure can also be used to determine the first known lead by simply connecting the meter to any wire at one end and then touching all the leads at the other end until a zero-ohm indication is obtained.
82  RESISTANCE

FIG. 3.31 Measuring the resistance of a single element.

FIG. 3.32 Checking the continuity of a connection.

FIG. 3.33 Identifying the leads of a multilead cable.
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G THERMISTORS  83
Preliminary measurements of the condition of some electronic devices such as the diode and transistor can be made using the ohmmeter. The meter can also be used to identify the terminals of such devices. One important note about the use of any ohmmeter:
Never hook up an ohmmeter to a live circuit!
The reading will be meaningless and you may damage the instrument. The ohmmeter section of any meter is designed to pass a small sensing current through the resistance to be measured. A large external current could damage the movement and would certainly throw off the calibration of the instrument. In addition,
never store a VOM or a DMM in the resistance mode.
The two leads of the meter could touch and the small sensing current could drain the internal battery. VOMs should be stored with the selector switch on the highest voltage range, and the selector switch of DMMs should be in the off position.
3.11 THERMISTORS
The thermistor is a two-terminal semiconductor device whose resistance, as the name suggests, is temperature sensitive. A representative characteristic appears in Fig. 3.34 with the graphic symbol for the device. Note the nonlinearity of the curve and the drop in resistance from about 5000 to 100 for an increase in temperature from 20°C to 100°C. The decrease in resistance with an increase in temperature indicates a negative temperature coefficient. The temperature of the device can be changed internally or externally. An increase in current through the device will raise its temperature, causing a drop in its terminal resistance. Any externally applied heat source will result in an increase in its body temperature and a drop in resistance. This type of action (internal or external) lends itself well to control mechanisms. Many different types of thermistors are shown in Fig. 3.35. Materials employed in the manufacture of thermistors include oxides of cobalt, nickel, strontium, and manganese.
Temperature (°C) (a) –100 0 100 200 300 400
10–4 10–2 100
102 104 106 108
R
Specific resistance (Ω-cm)
(b)
T
FIG. 3.34 Thermistor: (a) characteristics; (b) symbol.
FIG. 3.35 NTC (negative temperature coefficient) and PTC (positive temperature coefficient) thermistors. (Courtesy of Siemens Components, Inc.)
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R
0.1 k
Cell resistance
0.1 1.0 10 100 1000 Illumination (foot-candles) (a)
(b)
1 k
10 k
100 k
FIG. 3.36 Photoconductive cell: (a) characteristics; (b) symbol.
FIG. 3.37 Photoconductive cells. (Courtesy of EG&G VACTEC, Inc.)
Note the use of a log scale (to be discussed in Chapter 23) in Fig. 3.34 for the vertical axis. The log scale permits the display of a wider range of specific resistance levels than a linear scale such as the horizontal axis. Note that it extends from 0.0001 ·cmto100,000,000 ·cmover avery short interval. The log scale is used for both the vertical and the horizontal axis of Fig. 3.36.
3.12 PHOTOCONDUCTIVE CELL
The photoconductive cell is a two-terminal semiconductor device whose terminal resistance is determined by the intensity of the incident light on its exposed surface. As the applied illumination increases in intensity, the energy state of the surface electrons and atoms increases, with a resultant increase in the number of “free carriers” and a corresponding drop in resistance. A typical set of characteristics and the photoconductive cell’s graphic symbol appear in Fig. 3.36. Note the negative illumination coefficient. Several cadmium sulfide photoconductive cells appear in Fig. 3.37.
3.13 VARISTORS
Varistors are voltage-dependent, nonlinear resistors used to suppress high-voltage transients; that is, their characteristics are such as to limit the voltage that can appear across the terminals of a sensitive device or system. A typical set of characteristics appears in Fig. 3.38(a), along with a linear resistance characteristic for comparison purposes. Note that at a particular “firing voltage,” the current rises rapidly but the voltage is limited to a level just above this firing potential. In other words, the magnitude of the voltage that can appear across this device cannot exceed that level defined by its characteristics. Through proper design techniques this device can therefore limit the voltage appearing across sensitive regions of a network. The current is simply limited by the network to which it is connected. A photograph of a number of commercial units appears in Fig. 3.38(b).
FIG. 3.38 Varistors available with maximum dc voltage ratings between 18 V and 615 V. (Courtesy of Philips Components, Inc.)
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G APPLICATIONS  85
3.14 APPLICATIONS
The following are examples of how resistance can be used to perform a variety of tasks, from heating to measuring the stress or strain on a supporting member of a structure. In general, resistance is a component of every electrical or electronic application.
Electric Baseboard Heating Element
One of the most common applications of resistance is in household fixtures such as toasters and baseboard heating where the heat generated by current passing through a resistive element is employed to perform a useful function. Recently, as we remodeled our house, the local electrician informed us that we were limited to 16 ft of electric baseboard on a single circuit. That naturally had me wondering about the wattage per foot, the resulting current level, and whether the 16-ft limitation was a national standard. Reading the label on the 2-ft section appearing in Fig. 3.39(a), I found VOLTS AC 240/208, WATTS 750/575 [the power rating will be described in Chapter 4] AMPS 3.2/2.8. Since my panel is rated 208 V (as are those in most residential homes), the wattage rating per foot is 575 W/2 or 287.5 W at a current of 2.8 A. The total wattage for the 16 ft is therefore 16 287.5 W or 4600 W. In Chapter 4 you will find that the power to a resistive load is related to the current and applied voltage by the equation P VI. The total resulting current can then be determined using this equation in the following manner: I P/V 4600 W/208 V 22.12 A. The result was that we needed a circuit breaker larger than 22.12 A; otherwise, the circuit breaker would trip every time we turned the heat on. In my case the electrician used a 30-A breaker to meet the National Fire Code requirement that does not permit exceeding 80% of the rated current for a conductor or breaker. In most panels a 30-A breaker takes two slots of your panel, whereas the more common 20-A breaker takes only one slot. If you have a moment, take a look in your own panel and note the rating of the breakers used for various circuits of your home. Going back to Table 3.2, we find that the #12 wire commonly used for most circuits in the home has a maximum rating of 20 A and would not be suitable for the electric baseboard. Since #11 is usually not commercially available, a #10 wire with a maximum rating of 30 A was used. You might wonder why the current drawn from the supply is 22.12 A while that required for one unit was only 2.8 A. This difference is due to the parallel combination of sections of the heating elements, a configuration that will be described in Chapter 6. It is now clear why they specify a 16-ft limitation on a single circuit. Additional elements would raise the current to a level that would exceed the code level for #10 wire and would approach the maximum rating of the circuit breaker. Figure 3.39(b) shows a photo of the interior construction of the heating element. The red feed wire on the right is connected to the core of the heating element, and the black wire at the other end passes through a protective heater element and back to the terminal box of the unit (the place where the exterior wires are brought in and connected). If you look carefully at the end of the heating unit as shown in Fig. 3.39(c), you will find that the heating wire that runs through the core of the heater is not connected directly to the round jacket holding the fins in place. A ceramic material (insulator) separates the heating wire from
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the fins to remove any possibility of conduction between the current passing through the bare heating element and the outer fin structure. Ceramic materials are used because they are excellent conductors of heat and because they have a high retentivity for heat so the surrounding area will remain heated for a period of time even after the current has been turned off. As shown in Fig. 3.39(d), the heating wire that runs through the metal jacket is normally a nichrome composite (because pure nichrome is quite brittle) wound in the shape of a coil to compensate for expansion and contraction with heating and also to permit a
86  RESISTANCE
FIG. 3.39 Electric baseboard: (a) 2-ft section; (b) interior; (c) heating element; (d) nichrome coil.








!
" #

$
"
!

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longer heating element in standard-length baseboard. For interest sake we opened up the core and found that the nichrome wire in the core of a 2-ft baseboard was actually 7 ft long, or a 3.5 : 1 ratio. The thinness of the wire was particularly noteworthy, measuring out at about 8 mils in diameter, not much thicker than a hair. Recall from this chapter that the longer the conductor and the thinner the wire, the greater the resistance. We took a section of the nichrome wire and tried to heat it with a reasonable level of current and the application of a hair dryer. The change in resistance was almost unnoticeable. In other words, all our effort to increase the resistance with the basic elements available to us in the lab was fruitless. This was an excellent demonstration of the meaning of the temperature coefficient of resistance in Table 3.6. Since the coefficient is so small for nichrome, the resistance does not measurably change unless the change in temperature is truly significant. The curve of Fig. 3.14 would therefore be close to horizontal for nichrome. For baseboard heaters this is an excellent characteristic because the heat developed, and the power dissipated, will not vary with time as the conductor heats up with time. The flow of heat from the unit will remain fairly constant. The feed and return cannot be soldered to the nichrome heater wire for two reasons. First, you cannot solder nichrome wires to each other or to other types of wire. Second, if you could, there could be a problem because the heat of the unit could rise above 880°F at the point where the wires are connected, and the solder could melt and the connection could be broken. Nichrome must be spot welded or crimped onto the copper wires of the unit. Using Eq. (3.1) and the 8-mil measured diameter, and assuming pure nichrome for the moment, the resistance of the 7-ft length is
R r A l
( ( 6 8 0 m 0) i ( ls 7 ) 2 ) 42 6
0 4
0
R 65.6
In the next chapter a power equation will be introduced in detail relating power, current, and resistance in the following manner: P I2R. Using the above data and solving for the resistance, we obtain
R I P 2
( 5 2 7 .8 5
A W )2 R 73.34
which is very close to the value calculated above from the geometric shape since we cannot be absolutely sure about the resistivity value for the composite. During normal operation the wire heats up and passes that heat on to the fins, which in turn heat the room via the air flowing through them. The flow of air through the unit is enhanced by the fact that hot air rises, so when the heated air leaves the top of the unit, it draws cold area from the bottom to contribute to the convection effect. Closing off the top or bottom of the unit would effectively eliminate the convection effect, and the room would not heat up. A condition could occur in which the inside of the heater became too hot, causing the metal casing
APPLICATIONS  87
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also to get too hot. This concern is the primary reason for the thermal protective element introduced above and appearing in Fig. 3.39(b). The long, thin copper tubing in Fig. 3.39 is actually filled with an oil-type fluid that will expand when heated. If too hot, it will expand, depress a switch in the housing, and turn off the heater by cutting off the current to the heater wire.
Dimmer Control in an Automobile
A two-point rheostat is the primary element in the control of the light intensity on the dashboard and accessories of a car. The basic network appears in Fig. 3.40 with typical voltage and current levels. When the light switch is closed (usually by pulling the light control knob out from the dashboard), current will be established through the 50- rheostat and then to the various lights on the dashboard. As the knob of the control switch is turned, it will control the amount of resistance between points a and b of the rheostat. The more resistance between points a and b, the less the current and the less the brightness of the various lights. Note the additional switch in the glove compartment light which is activated by the opening of the door of the compartment. Aside from the glove compartment light, all the lights of Fig. 3.40 will be on at the same time when the light switch is activated. The first branch after the rheostat contains two bulbs of 6-V rating rather than the 12-V bulbs appearing in the other branches. The smaller bulbs of this branch will produce a softer, more even light for specific areas of the panel. Note that the sum of the two bulbs (in series) is 12 V to match that across the other branches. The division of voltage in any network will be covered in detail in Chapters 5 and 6. Typical current levels for the various branches have also been provided in Fig. 3.40. You will learn in Chapter 6 that the current drain from the battery and through the fuse and rheostat approximately equals the sum of the currents in the branches of the network. The result is that the fuse must be able to handle current in amperes, so a 15-A fuse was employed (even though the bulbs appear in Fig. 3.40 as 12-V bulbs to match the battery). Whenever the operating voltage and current levels of a component are known, the internal “hot” resistance of the unit can be determined using Ohm’s law, to be introduced in detail in the next chapter. Basi
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FIG. 3.40 Dashboard dimmer control in an automobile.
12-V battery B+
Gang switch
1 OFF 2
3
1 OFF 2 30 A
Headlights, etc.
Panel switch
a
b Panel lights
300 mA
6 V + –
6 V + –
12 V + –
Ashtray light
300 mA
6 V + –
6 V + –
Radio display
300 mA 12 V + – Glove compartment light 500 mA
Parking lights, side lights, license plate light, etc.
50 Ω, 1 W
15 A
3
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cally this law relates voltage, current, and resistance by I V/R. For the 12-V bulb at a rated current of 300 mA, the resistance is R V/I 12 V/300 mA 40 . For the 6-V bulbs it is 6 V/300 mA 80 . Further comment regarding the power levels and resistance levels will be reserved for the chapters to follow. The preceding description assumed an ideal level of 12 V for the battery. In actuality, 6.3-V and 14-V bulbs are used to match the charging level of most automobiles.
Strain Gauges
Any change in the shape of a structure can be detected using strain gauges whose resistance will change with applied stress or flex. An example of a strain gauge is shown in Fig. 3.41. Strain gauges are semiconductor devices whose terminal resistance will change in a nonlinear (not a straight-line) fashion through a fairly wide range in values when they are stressed by compression or extension. Since the stress gauge does emit a signal, a signal processor must also be part of the system to translate the change in resistance to some meaningful output. One simple example of the use of resistive strain gauges is to monitor earthquake activity. When the gauge is placed across an area of suspected earthquake activity, the slightest separation in the earth will change the terminal resistance, and the processor will display a result sensitive to the amount of separation. Another example is in alarm systems where the slightest change in the shape of a supporting beam when someone walks overhead will result in a change in terminal resistance, and an alarm will sound. Other examples include placing strain gauges on bridges to maintain an awareness of their rigidity and on very large generators to check whether various moving components are beginning to separate because of a wearing of the bearings or spacers. The small mouse control within the keyboard of a portable computer can be a series of stress gauges that reveal the direction of stress applied to the
APPLICATIONS  89
Typical Installation
20-26 AWG instrumentation wire
Terminal pads for stress relief and junction for different gauge wire
30 AWG or ribbon leads
Model SGN    4/12 12-Ω terminal resistance Overall length: 5.5mm ≈ 0.22" _ ≈ =
FIG. 3.41 Resistive strain gauge. (© Copyright Omega Engineering, Inc. All rights reserved. Reproduced with the permission of Omega Engineering, Inc., Stamford, CT 06907.)
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controlling element on the keyboard. Movement in one direction can extend or compress a resistance gauge which can monitor and control the motion of the mouse on the screen.
3.15 MATHCAD
Throughout the text a mathematical software package called Mathcad will be used to introduce a variety of operations that a math software package can perform. There is no need to obtain a copy of the software package to continue with the material covered in this text. The coverage is at a very introductory level simply to introduce the scope and power of the package. All the exercises appearing at the end of each chapter can be done without Mathcad. Once the package is installed, all operations begin with the basic screen of Fig. 3.42. The operations must be performed in the sequence appearing in Fig. 3.43, that is, from left to right and then from top to bottom. For example, if an equation on the second line is to operate on a specific variable, the variable must be defined to the left of or above the equation.
90  RESISTANCE
FIG. 3.42 Using Mathcad to perform a basic mathematical operation.
To perform any mathematical calculation, simply click on the screen at any convenient point to establish a crosshair on the display (the location of the first entry). Then type in the mathematical operation such as 20 2 8/6 as shown in Fig. 3.42; the instant the equal sign is selected, the result, 17.333, will appear as shown in Fig. 3.42. The multiplication is obtained using the asterisk (*) appearing at the top of the number 8 key (under the SHIFT CONTROL key). The division is set by the /
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key at the bottom right of the keyboard. The equal sign can be selected from the top right corner of the keyboard. Another option is to apply the sequence View-Toolbars-Calculator to obtain the Calculator of Fig. 3.42. Then use the calculator to enter the entire expression and obtain the result using the left clicker of the mouse. As an example in which variables must be defined, the resistance of a 200-ft length of copper wire with a diameter of 0.01 in. will be determined. First, as shown in Fig. 3.44, the variables for resistivity, length, and diameter must be defined. This is accomplished by first calling for the Greek palette through View-Toolbars-Greek and selecting the Greek letter rho (r) followed by a combined Shift-colon (Shift:) operation. A colon and an equal sign will appear, after which 10.37 is entered. For all the calculations to follow, the value of r has been defined. A left click on the screen will then remove the rectangular enclosure and place the variable and its value in memory. Proceed in the same way to define the length l and the diameter d. Next the diameter in millimeters is defined by multiplying the diameter in inches by 1000, and the area is defined by the diameter in millimeters squared. Note that m had to be defined to the left of the expression for the area, and the variable d was defined in the line above. The power of 2 was obtained by first selecting the superscript symbol (^) at the top of the number 6 on the keyboard and then entering the number 2 in the Mathcad bracket. Or you can simply type the letter m and choose from the Calculator palette. In fact, all the operations of multiplication, division, etc., required to determine the resistance R can be lifted from the Calculator palette. On the next line of Fig. 3.44, the values of m and A were calculated by simply typing in m followed by the keyboard equal sign. Finally, the x2
FIG. 3.43 Defining the order of mathematical operations for Mathcad.
FIG. 3.44 Using Mathcad to calculate the resistance of a copper conductor.
92  RESISTANCE
12. Determine the increase in resistance of a copper conductor if the area is reduced by a factor of 4 and the length is doubled. The original resistance was 0.2 . The temperature remains fixed.
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FIG. 3.46 Problem 11.
4 ft
1 2 in./
3 in.
10. A wire 1000 ft long has a resistance of 0.5 k and an area of 94 CM. Of what material is the wire made (T 20°C)?
*11. a. What is the resistance of a copper bus-bar with the dimensions shown (T 20°C) in Fig. 3.46? b. Repeat (a) for aluminum and compare the results. c. Without working out the numerical solution, determine whether the resistance of the bar (aluminum or copper) will increase or decrease with an increase in length. Explain your answer. d. Repeat (c) for an increase in cross-sectional area.
equation for the resistance R is defined in terms of the variables, and the result is obtained. The true value of developing in the above sequence is the fact that you can place the program in memory and, when the need arises, call it up and change a variable or two—the result will appear immediately. There is no need to reenter all the definitions—just change the numerical value. In the chapters to follow, Mathcad will appear at every opportunity to demonstrate its ability to perform calculations in a quick, effective manner. You will probably want to learn more about this time-saving and accuracy-checking option.
PROBLEMS
SECTION 3.2 Resistance: Circular Wires
1. Convert the following to mils: a. 0.5 in. b. 0.01 in. c. 0.004 in. d. 1 in. e. 0.02 ft f. 0.01 cm
2. Calculate the area in circular mils (CM) of wires having the following diameters: a. 0.050 in. b. 0.016 in. c. 0.30 in. d. 0.1 cm e. 0.003 ft f. 0.0042 m
3. The area in circular mils is a. 1600 CM b. 900 CM c. 40,000 CM d. 625 CM e. 7.75 CM f. 81 CM What is the diameter of each wire in inches?
4. What is the resistance of a copper wire 200 ft long and 0.01 in. in diameter (T 20°C)?
5. Find the resistance of a silver wire 50 yd long and 0.0045 in. in diameter (T 20°C).
6. a. What is the area in circular mils of an aluminum conductor that is 80 ft long with a resistance of 2.5 ? b. What is its diameter in inches? 7. A 2.2- resistor is to be made of nichrome wire. If the available wire is 1/32 in. in diameter, how much wire is required?
8. a. What is the area in circular mils of a copper wire that has a resistance of 2.5 and is 300 ft long (T 20°C)? b. Without working out the numerical solution, determine whether the area of an aluminum wire will be smaller or larger than that of the copper wire. Explain. c. Repeat (b) for a silver wire.
9. In Fig. 3.45, three conductors of different materials are presented. a. Without working out the numerical solution, determine which section would appear to have the most resistance. Explain. b. Find the resistance of each section and compare with the result of (a) (T 20°C).
Aluminum: l  =  50 ft, d  =  50 mils
Copper:  l  =  10 ft, d  =  10 mils
Silver:  l  =  1 ft, d  =  1 mil
FIG. 3.45 Problem 9.
PROBLEMS  93
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c. Use the equation r2 kr1 to determine the conversion factor k if r1 is the solution of part (a) and r2 the solution of part (b).
E
d = 30 ft
Load
Solid round copper wire
FIG. 3.47 Problem 16.
FIG. 3.48 Problem 22.
R
1 inch
= 1 m
1000 ft.
*13. What is the new resistance level of a copper wire if the length is changed from 200 ft to 100 yd, the area is changed from 40,000 CM to 0.04 in.2, and the original resistance was 800 m ?
SECTION 3.3 Wire Tables
14. a. Using Table 3.2, find the resistance of 450 ft of #11 and #14 AWG wires. b. Compare the resistances of the two wires. c. Compare the areas of the two wires.
15. a. Using Table 3.2, find the resistance of 1800 ft of #8 and #18 AWG wires. b. Compare the resistances of the two wires. c. Compare the areas of the two wires.
16. a. For the system of Fig. 3.47, the resistance of each line cannot exceed 0.006 , and the maximum current drawn by the load is 110 A. What gage wire should be used? b. Repeat (a) for a maximum resistance of 0.003 , d 30 ft, and a maximum current of 110 A.
*17. a. From Table 3.2, determine the maximum permissible current density (A/CM) for an AWG #0000 wire. b. Convert the result of (a) to A/in.2. c. Using the result of (b), determine the cross-sectional area required to carry a current of 5000 A.
SECTION 3.4 Resistance: Metric Units
18. Using metric units, determine the length of a copper wire that has a resistance of 0.2 and a diameter of 1/10 in.
19. Repeat Problem 11 using metric units; that is, convert the given dimensions to metric units before determining the resistance. 20. If the sheet resistance of a tin oxide sample is 100 , what is the thickness of the oxide layer?
21. Determine the width of a carbon resistor having a sheet resistance of 150 if the length is 1/2 in. and the resistance is 500 . *22. Derive the conversion factor between r (CM⋅ /ft) and r ( ⋅cm) by a. Solving for r for the wire of Fig. 3.48 in CM⋅ /ft. b. Solving for r for the same wire of Fig. 3.48 in ⋅cm by making the necessary conversions.
SECTION 3.5 Temperature Effects 23. The resistance of a copper wire is 2 at 10°C. What is its resistance at 60°C? 24. The resistance of an aluminum bus-bar is 0.02 at 0°C. What is its resistance at 100°C? 25. The resistance of a copper wire is 4 at 70°F. What is its resistance at 32°F? 26. The resistance of a copper wire is 0.76 at 30°C. What is its resistance at 40°C? 27. If the resistance of a silver wire is 0.04 at 30°C, what is its resistance at 0°C? *28. a. The resistance of a copper wire is 0.002 at room temperature (68°F). What is its resistance at 32°F (freezing) and 212°F (boiling)? b. For (a), determine the change in resistance for each 10° change in temperature between room temperature and 212°F. 29. a. The resistance of a copper wire is 0.92 at 4°C. At what temperature (°C) will it be 1.06 ? b. At what temperature will it be 0.15 ?
*30. a. If the resistance of a 1000-ft length of copper wire is 10 at room temperature (20°C), what will its resistance be at 50 K (Kelvin units) using Eq. (3.6)? b. Repeat part (a) for a temperature of 38.65 K. Comment on the results obtained by reviewing the curve of Fig. 3.14. c. What is the temperature of absolute zero in Fahrenheit units? 31. a. Verifythevalueofa20 forcopperinTable3.6bysubstituting the inferred absolute temperature into Eq. (3.7). b. Using Eq. (3.8) find the temperature at which the resistance of a copper conductor will increase to 1 from a level of 0.8 at 20°C.
32. Using Eq. (3.8), find the resistance of a copper wire at 16°C if its resistance at 20°C is 0.4 .
*33. Determine the resistance of a 1000-ft coil of #12 copper wire sitting in the desert at a temperature of 115°F. 34. A 22- wire-wound resistor is rated at 200 PPM for a temperature range of 10°C to 75°C. Determine its resistance at 65°C. 35. Determine the PPM rating of the 10-k carbon composition resistor of Fig. 3.19 using the resistance level determined at 90°C.
48. Is there an overlap in coverage between 20% resistors? That is, determine the tolerance range for a 10- 20% resistor and a 15- 20% resistor, and note whether their tolerance ranges overlap.
49. Repeat Problem 48 for 10% resistors of the same value.
SECTION 3.9 Conductance
50. Find the conductance of each of the following resistances: a. 0.086 b. 4 k c. 2.2 M Compare the three results.
51. Find the conductance of 1000 ft of #18AWG wire made of a. copper b. aluminum c. iron
*52. The conductance of a wire is 100 S. If the area of the wire is increased by 2/3 and the length is reduced by the same amount, find the new conductance of the wire if the temperature remains fixed.
SECTION 3.10 Ohmmeters
53. How would you check the status of a fuse with an ohmmeter?
54. How would you determine the on and off states of a switch using an ohmmeter?
55. How would you use an ohmmeter to check the status of a light bulb?
SECTION 3.11 Thermistors
*56. a. Find the resistance of the thermistor having the characteristics of Fig. 3.34 at 50°C, 50°C, and 200°C. Note that it is a log scale. If necessary, consult a reference with an expanded log scale. b. Does the thermistor have a positive or a negative temperature coefficient? c. Is the coefficient a fixed value for the range 100°C to 400°C? Why? d. What is the approximate rate of change of r with temperature at 100°C?
SECTION 3.12 Photoconductive Cell
*57. a. Using the characteristics of Fig. 3.36, determine the resistance of the photoconductive cell at 10 and 100 foot-candles of illuminaton. As in Problem 56, note that it is a log scale. b. Does the cell have a positive or a negative illumination coefficient? c. Is the coefficient a fixed value for the range 0.1 to 1000 foot-candles? Why? d. What is the approximate rate of change of R with illumination at 10 foot-candles?
94  RESISTANCE
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SECTION 3.6 Superconductors
36. Visit your local library and find a table listing the critical temperatures for a variety of materials. List at least five materials with the critical temperatures that are not mentioned in this text. Choose a few materials that have relatively high critical temperatures.
37. Find at least one article on the application of superconductivity in the commercial sector, and write a short summary, including all interesting facts and figures. *38. Using the required 1-MA/cm2 density level for IC manufacturing, determine what the resulting current would be through a #12 house wire. Compare the result obtained with the allowable limit of Table 3.2.
*39. Research the SQUID magnetic field detector and review its basic mode of operation and an application or two.
SECTION 3.7 Types of Resistors
40. a. What is the approximate increase in size from a 1-W to a 2-W carbon resistor? b. What is the approximate increase in size from a 1/2-W to a 2-W carbon resistor? c. In general, can we conclude that for the same type of resistor, an increase in wattage rating requires an increase in size (volume)? Is it almost a linear relationship? That is, does twice the wattage require an increase in size of 2 1? 41. If the 10-k resistor of Fig. 3.19 is exactly 10 k at room temperature, what is its approximate resistance at 30°C and 100°C (boiling)?
42. Repeat Problem 41 at a temperature of 120°F.
43. If the resistance between the outside terminals of a linear potentiometer is 10 k , what is its resistance between the wiper (movable) arm and an outside terminal if the resistance between the wiper arm and the other outside terminal is 3.5 k ?
44. If the wiper arm of a linear potentiometer is one-quarter the way around the contact surface, what is the resistance between the wiper arm and each terminal if the total resistance is 25 k ? *45. Show the connections required to establish 4 k between the wiper arm and one outside terminal of a 10-k potentiometer while having only zero ohms between the other outside terminal and the wiper arm.
SECTION 3.8 Color Coding and Standard Resistor Values
46. Find the range in which a resistor having the following color bands must exist to satisfy the manufacturer’s tolerance:
1st band 2nd band 3rd band 4th band
a. green blue orange gold b. red red brown silver c. brown black black —
47. Find the color code for the following 10% resistors: a. 220 b. 4700 c. 68 k d. 9.1 M
GLOSSARY  95
SECTION 3.13 Varistors
58. a. Referring to Fig. 3.38(a), find the terminal voltage of the device at 0.5 mA, 1 mA, 3 mA, and 5 mA. b. What is the total change in voltage for the indicated range of current levels? c. Compare the ratio of maximum to minimum current levels above to the corresponding ratio of voltage levels.
GLOSSARY
Absolute zero The temperature at which all molecular motion ceases; 273.15°C. Circular mil (CM) The cross-sectional area of a wire having a diameter of one mil. Color coding A technique employing bands of color to indicate the resistance levels and tolerance of resistors. Conductance (G) An indication of the relative ease with which current can be established in a material. It is measured in siemens (S). Cooper effect The “pairing” of electrons as they travel through a medium. Ductility The property of a material that allows it to be drawn into long, thin wires. Inferred absolute temperature The temperature through which a straight-line approximation for the actual resistance-versus-temperature curve will intersect the temperature axis. Malleability The property of a material that allows it to be worked into many different shapes. Negative temperature coefficient of resistance The value revealing that the resistance of a material will decrease with an increase in temperature. Ohm ( ) The unit of measurement applied to resistance. Ohmmeter An instrument for measuring resistance levels.
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SECTION 3.15 Mathcad
59. Verify the results of Example 3.3 using Mathcad.
60. Verify the results of Example 3.11 using Mathcad.
Photoconductive cell A two-terminal semiconductor device whose terminal resistance is determined by the intensity of the incident light on its exposed surface. Positive temperature coefficient of resistance The value revealing that the resistance of a material will increase with an increase in temperature. Potentiometer A three-terminal device through which potential levels can be varied in a linear or nonlinear manner. PPM/°C Temperature sensitivity of a resistor in parts per million per degree Celsius. Resistance A measure of the opposition to the flow of charge through a material. Resistivity (r) A constant of proportionality between the resistance of a material and its physical dimensions. Rheostat An element whose terminal resistance can be varied in a linear or nonlinear manner. Sheet resistance Defined by r/d for thin-film and integrated circuit design. Superconductor Conductors of electric charge that have for all practical purposes zero ohms. Thermistor A two-terminal semiconductor device whose resistance is temperature sensitive. Varistor A voltage-dependent, nonlinear resistor used to suppress high-voltage transients.