5. MARCO METODOLÓGICO
6.2 ANÁLISIS CON EL SOFTWARE SPSS
When choosing a resistor for a particular application in an electrical or electronic device, it’s important to get a unit that has the correct properties, or specifications.Here are some of the most important spec- ifications to watch for.
Ohmic Value
In theory, a resistor can have any ohmic value from the lowest possible (such as a shaft of solid sil- ver) to the highest (dry air). In practice, it is unusual to find resistors with values less than about 0.1
Ωor more than about 100 MΩ.
Resistors are manufactured with ohmic values in power-of-10 multiples of 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2. Thus, you will often see resistors with values of 47 Ω, 180
Ω, 6.8 kΩ, or 18 MΩ, but hardly ever with values such as 384 Ω, 4.54 kΩ, or 7.297 MΩ. In addition to these standard values, there are others that are used for resistors made with greater precision, or tighter tolerance.These are power-of-10 multiples of 1.1, 1.3, 1.6, 2.0, 2.4, 3.0, 3.6, 4.3, 5.1, 6.2, 7.5, and 9.1.
Tolerance
The first set of numbers above represents standard resistance values available in tolerances of plus or minus 10 percent (⫾10%). This means that the resistance might be as much as 10 percent more or 10 percent less than the indicated amount. In the case of a 470-Ωresistor, for example, the value can be larger or smaller than the rated value by as much as 47 Ω, and still be within tolerance. That’s a range of 423 to 517 Ω.
Tolerance is calculated according to the specified value of the resistor, not the actual value. You might measure the value of a 470-Ωresistor and find it to be 427 Ω, and it would be within ⫾10% of the specified value. But if it measures 420 Ω, it’s outside the rated range, and is therefore a reject. The second set, along with the first set, of numbers represents standard resistance values available in tolerances of plus or minus 5 percent (⫾5%). A 470-Ω, 5 percent resistor will have an actual value of 470 Ωplus or minus 24 Ω, or a range of 446 to 494 Ω.
Some resistors are available in tolerances tighter than ⫾5%. These precision units are employed in circuits where a little error can make a big difference. In most audio and radio-frequency oscilla- tors and amplifiers, the ⫾10% or ⫾5% tolerance is good enough. In many cases, even a ⫾20% tolerance is satisfactory.
Power Rating
All resistors are given a specification that determines how much power they can safely dissipate. Typ- ical values are 1⁄4W, 1⁄2W, and 1 W. Units also exist with ratings of 1⁄8W or 2 W. These dissipation
ratings are for continuous duty, meaning they can dissipate this amount of power constantly and indefinitely.
You can figure out how much current a given resistor can handle by using the formula for power (P) in terms of current (I) and resistance (R). That formula, you should recall, is P=I2R.Work this
formula backward, plugging in the power rating in watts for Pand the resistance in ohms for R, and solve for the current Iin amperes. Alternatively, you can find the square root of P/R.
The power rating for a given resistor can, in effect, be increased by using a network of 2 ×2, 3×3, 4 ×4, or more units in series-parallel. If you need a 47-Ω, 45-W resistor, but all you have is a bunch of 47-Ω, 1-W resistors, you can make a 7 ×7 network in series-parallel, and this will han- dle 49 W.
Resistor power dissipation ratings are specified with a margin for error. A good engineer never tries to take advantage of this and use, say, a 1⁄4-W unit in a situation that needs to draw 0.27 W. In
fact, good engineers usually include their own safety margin. Allowing 10 percent, a 1⁄4-W resistor
should not be called upon to handle more than about 0.225 W.
Temperature Compensation
All resistors change value when the temperature changes dramatically. And because resistors dissi- pate power, they can get hot just because of the current they carry. Often, this current is so tiny that it doesn’t appreciably heat the resistor. But in some cases it does, and the resistance will change. Then a circuit might behave differently than it did when the resistor was still cool.
There are various ways to approach problems of resistors changing value when they get hot. One method is to use specially manufactured resistors that do not appreciably change value when they get hot. Such units are called temperature-compensated.But one of these can cost several times as much as an ordinary resistor. Another approach is to use a power rating that is much higher than the actual dissipated power in the resistor. This will keep the resistor from getting very hot. Still an- other scheme is to use a series-parallel network of identical resistors to increase the power dissipa- tion rating. Alternatively, you can take several resistors, say three of them, each with about three times the intended resistance, and connect them all in parallel. Or you can take several resistors, say four of them, each with about one-fourth the intended resistance, and connect them in series.
It is unwise to combine resistors with different values. This can result in one of them taking most of the load while the others “loaf,” and the combination will be no better than the single hot resistor you started with.
How about using two resistors with half (or twice) the value you need, but with oppositeresist- ance-versus-temperature characteristics, and connecting them in series or parallel? It is tempting to suppose that if you do this, the component whose resistance decreases with heat (negative tempera- ture coefficient) will have a canceling-out effect on the component whose resistance goes up (positive temperature coefficient). This can sometimes work, but in practice it’s difficult to find a pair of resist- ances that will do this job just right.
The Color Code for Resistors
Some resistors have color bandsthat indicate their values and tolerances. You’ll see three, four, or five bands around carbon-composition resistors and film resistors. Other units are large enough so that the values can be printed on them in ordinary numerals.
On resistors with axial leads(wires that come straight out of both ends), the first, second, third, fourth, and fifth bands are arranged as shown in Fig. 6-12A. On resistors with radial leads(wires that come off the ends at right angles to the axis of the component body), the colored regions are arranged as shown in Fig. 6-12B. The first two regions represent numbers 0 through 9, and the third region represents a multiplier of 10 to some power. (For the moment, don’t worry about the fourth and fifth regions.) Refer to Table 6-1.
Suppose you find a resistor whose first three bands are yellow, violet, and red, in that order. Then the resistance is 4700 Ω. Read yellow =4, violet =7, red = ×100. As another example, sup- pose you find a resistor with bands of blue, gray, orange. Refer to Table 6-1 and determine blue =6, gray=8, orange = ×1000. Therefore, the value is 68,000 Ω =68 kΩ.
The fourth band, if there is one, indicates tolerance. If it’s silver, it means the resistor is rated at⫾10%. If it’s gold, the resistor is rated at ⫾5%. If there is no fourth band, the resistor is rated at⫾20%.
The fifth band, if there is one, indicates the maximum percentage that the resistance can be ex- pected to change after 1000 hours of use. A brown band indicates a maximum change of ⫾1% of the rated value. A red band indicates ⫾0.1%. An orange band indicates ⫾0.01%. A yellow band in- dicates⫾0.001%. If there is no fifth band, it means that the resistor might deviate by more than
6-12 At A, locations of color-code bands on a resistor with axial leads. At B, locations of color code designators on a resistor with radial leads.
Resistor Specfications 97
Table 6-1. The color code for the first three bands that appear on fixed resistors. See text for discussion of the fourth and fifth bands.
Color of band Numeral Multiplier (first and second bands) (third band)
Black 0 1 Brown 1 10 Red 2 100 Orange 3 1000 (1 k) Yellow 4 104(10 k) Green 5 105(100 k) Blue 6 106(1 M) Violet 7 107(10 M) Gray 8 108(100 M) White 9 109(1000 M or 1 G)
A competent engineer or technician always tests a resistor with an ohmmeter before installing it in a circuit. If the component happens to be labeled wrong, or if it is defective, it’s easy to catch this problem while assembling or servicing a circuit. But once the circuit is all together, and it won’t work because some resistor is labeled wrong or is bad, it’s difficult to troubleshoot.
Quiz
Refer to the text in this chapter if necessary. A good score is at least 18 correct. Answers are in the back of the book.
1. Proper biasing in an amplifier circuit (a) causes it to oscillate.
(b) prevents an impedance match.
(c) can be obtained using a voltage divider network. (d) maximizes current flow.
2. A transistor can be protected from needless overheating by (a) a current-limiting resistor.
(b) bleeder resistors. (c) maximizing the drive.
(d) shorting out the power supply when the circuit is off. 3. A bleeder resistor
(a) is connected across the capacitor in a power supply. (b) keeps a transistor from drawing too much current. (c) prevents an amplifier from being overdriven. (d) optimizes the efficiency of an amplifier. 4. Carbon-composition resistors
(a) can handle gigantic levels of power.
(b) have capacitance or inductance along with resistance. (c) have essentially no capacitance or inductance. (d) work better for ac than for dc.
5. A logical place for a wirewound resistor is (a) in a radio-frequency amplifier.
(b) in a circuit where a noninductive resistor is called for. (c) in a low-power radio-frequency circuit.
(d) in a high-power dc circuit. 6. A metal-film resistor
(a) is made using a carbon-based paste. (b) does not have much inductance.
(c) can dissipate large amounts of power. (d) has considerable inductance.
7. What type of resistor, or combination of resistors, would you use as the meter-sensitivity control in a test instrument, when continuous adjustment is desired?
(a) A set of switchable, fixed resistors (b) A linear-taper potentiometer (c) An audio-taper potentiometer (d) A wirewound resistor
8. What type of resistor, or combination of resistors, would you use as the volume control in a stereo compact-disc (CD) player?
(a) A set of switchable, fixed resistors (b) A linear-taper potentiometer (c) An audio-taper potentiometer (d) A wirewound resistor
9. If a sound triples in actual power level, approximately what is this, expressed in decibels? (a) +3 dB
(b) +5 dB (c) +6 dB (d) +9 dB
10. Suppose a sound changes in volume by −13 dB. If the original sound power is 1.0 W, what is the final sound power?
(a) 13 W (b) 77 mW (c) 50 mW
(d) There is not enough information given here to answer this question.
11. The sound from a portable radio is at a level of 50 dB. How many times the threshold of hearing is this, in terms of actual sound power?
(a) 50 (b) 169 (c) 5000 (d) 100,000
12. An advantage of a rheostat over a potentiometer is the fact that (a) a rheostat can handle higher frequencies.
(b) a rheostat is more precise.
(c) a rheostat can handle more current. (d) a rheostat works better with dc.
13. A resistor is specified as having a value of 68 Ω, but is measured with an ohmmeter as 63 Ω. The value is off by which of the following percentages?
(a) 7.4% (b) 7.9% (c) 5% (d) 10%
14. Suppose a resistor is rated at 3.3 kΩ⫾5%. This means it can be expected to have a value between
(a) 2970 Ωand 3630 Ω. (b) 3295 Ωand 3305 Ω. (c) 3135 Ωand 3465 Ω. (d) 2.8 kΩand 3.8 kΩ.
15. A package of resistors is rated at 56 Ω⫾10%. You test them with an ohmmeter. Which of the following values indicates a reject?
(a) 50.0 Ω (b) 53.0 Ω (c) 59.7 Ω (d) 61.1 Ω
16. A resistor has a value of 680 Ω, and you expect that it will have to draw 1 mA maximum continuous current in a circuit you’re building. What power rating is good for this application, but not needlessly high?
(a) 1⁄4W
(c) 1⁄2W
(c) 1 W (d) 2 W
17. Suppose a 1-kΩresistor will dissipate 1.05 W, and you have a good supply of 1-W resistors of various ohmic values. If there’s room for 20 percent resistance error, the cheapest solution is to use
(a) four 1-kΩ, 1-W resistors in series-parallel. (b) a pair of 2.2-kΩ, 1-W resistors in parallel. (c) a set of three 3.3-kΩ, 1-W resistors in parallel.
(d) a single 1-kΩ, 1-W resistor, because all manufacturers allow for a 10 percent margin of safety when rating resistors for their power-handling capability.
18. Suppose a carbon-composition resistor has the following colored bands on it: red, red, red, gold. This indicates a resistance of
(a) 22 Ω. (b) 220 Ω. (c) 2.2 kΩ. (d) 22 kΩ.
19. The actual resistance of the component described in the previous question can be expected to vary above or below the specified ohmic value by up to what amount?
(a) 11 Ω (b) 110 Ω (c) 22 Ω (d) 220 Ω
20. Suppose a carbon-composition resistor has the following colored bands on it: gray, red, yellow. This unit can be expected to have a value within approximately what range?
(a) 660 kΩto 980 kΩ (b) 740 kΩto 900 kΩ (c) 7.4 kΩto 9.0 kΩ
(d) The manufacturer does not make any claim.
IN ELECTRICITY AND ELECTRONICS,ACELLIS A UNIT SOURCE OF DC ENERGY.WHEN TWO OR MORE
cells are connected in series, the result is known as a battery.There are many types of cells and bat- teries, and new types are constantly being invented.