3.5 Types of Resistors

Transcription

1 7 Chapter 3 Resistance IN-PROCESS LERNING CHECK 4 Explain what is meant by the terms positive temperature coefficient and negative temperature coefficient. To which category does aluminum belong? (nswers are at the end of the chapter.) 3.5 Types of Resistors irtually all electric and electronic circuits involve the control of voltage and/or current. The best way to provide such control is by inserting appropriate values of resistance into the circuit. lthough various types and sizes of resistors are used in electrical and electronic applications, all resistors fall into two main categories: fixed resistors and variable resistors. Fixed Resistors s the name implies, fixed resistors are resistors having resistance values which are essentially constant. There are numerous types of fixed resistors, ranging in size from almost microscopic (as in integrated circuits) to highpower resistors which are capable of dissipating many watts of power. Figure 3 7 illustrates the basic structure of a molded carbon composition resistor. Insulated coating Carbon composition Color coding Leads imbedded into resistive material FIGURE 3 7 Structure of a molded carbon composition resistor. s shown in Figure 3 7, the molded carbon composition resistor consists of a carbon core mixed with an insulating filler. The ratio of carbon to filler determines the resistance value of the component: the higher the proportion of carbon, the lower the resistance. Metal leads are inserted into the carbon core, and then the entire resistor is encapsulated with an insulated coating. Carbon composition resistors are available in resistances from less than 1 to 1 M and typically have power ratings from 1 8 W to W. Figure 3 8 shows various sizes of resistors, with the larger resistors being able to dissipate more power than the smaller resistors. lthough carbon-core resistors have the advantages of being inexpensive and easy to produce, they tend to have wide tolerances and are susceptible to

2 Section 3.5 Types of Resistors 73 FIGURE 3 8 ctual size of carbon resistors ( W, 1 W, 1 W, 1 4 W, 1 8 W). large changes in resistance due to temperature variation. s shown in Figure 3 9, the resistance of a carbon composition resistor may change by as much as 5% when temperature is changed by 1 C. Other types of fixed resistors include carbon film, metal film, metal oxide, wire-wound, and integrated circuit packages. If fixed resistors are required in applications where precision is an important factor, then film resistors are usually employed. These resistors consist of either carbon, metal, or metal-oxide film deposited onto a ceramic cylinder. The desired resistance is obtained by removing part of the resistive material, resulting in a helical pattern around the ceramic core. If variation of resistance due to temperature is not a major concern, then low-cost carbon is used. However, if close tolerances are required over a wide temperature range, then the resistors are made of films consisting of alloys such as nickel R () 1- resistor T ( C) FIGURE 3 9 ariation in resistance of a carbon composition fixed resistor.

3 74 Chapter 3 Resistance chromium, constantum, or manganin, which have very small temperature coefficients. Occasionally a circuit requires a resistor to be able to dissipate large quantities of heat. In such cases, wire-wound resistors may be used. These resistors are constructed of a metal alloy wound around a hollow porcelain core which is then covered with a thin layer of porcelain to seal it in place. The porcelain is able to quickly dissipate heat generated due to current through the wire. Figure 3 1 shows a few of the various types of power resistors available. FIGURE 3 1 Power resistors. (a) Internal resistor arrangement (b) Integrated resistor network. (Courtesy of Bourns, Inc.) FIGURE 3 11

4 Section 3.5 Types of Resistors 75 In circuits where the dissipation of heat is not a major design consideration, fixed resistances may be constructed in miniature packages (called integrated circuits or ICs) capable of containing many individual resistors. The obvious advantage of such packages is their ability to conserve space on a circuit board. Figure 3 11 illustrates a typical resistor IC package. ariable Resistors ariable resistors provide indispensable functions which we use in one form or another almost daily. These components are used to adjust the volume of our radios, set the level of lighting in our homes, and adjust the heat of our stoves and furnaces. Figure 3 1 shows the internal and the external view of typical variable resistors. (a) External view of variable resistors. (b) Internal view of variable resistor. FIGURE 3 1 ariable resistors. (Courtesy of Bourns, Inc.) In Figure 3 13, we see that variable resistors have three terminals, two of which are fixed to the ends of the resistive material. The central terminal is connected to a wiper which moves over the resistive material when the shaft is rotated with either a knob or a screwdriver. The resistance between the two outermost terminals will remain constant while the resistance between the central terminal and either terminal will change according to the position of the wiper. If we examine the schematic of a variable resistor as shown in Figure 3 13(b), we see that the following relationship must apply: R ac R ab R bc (3 9) ariable resistors are used for two principal functions. Potentiometers, shown in Figure 3 13(c), are used to adjust the amount of potential (voltage) provided to a circuit. Rheostats, the connections and schematic of which are shown in Figure 3 14, are used to adjust the amount of current within a circuit. pplications of potentiometers and rheostats will be covered in later chapters.

5 76 Chapter 3 Resistance a R ab R bc b Connection to moving wiper c (b) Terminals of a variable resistor a c b (c) ariable resistor used as a potentiometer This voltage is dependent upon the location of the moving contact FIGURE 3 13 (a) ariable resistors. (Courtesy of Bourns, Inc.) a c R ab = when the moving contact is at a b (No connections) a b R ab (a) Connections of a rheostat (b) Symbol of a rheostat FIGURE Color Coding of Resistors Large resistors such as the wire-wound resistors or the ceramic-encased power resistors have their resistor values and tolerances printed on their cases. Smaller resistors, whether constructed of a molded carbon composition or a metal film, may be too small to have their values printed on the component. Instead, these smaller resistors are usually covered by an epoxy or similar insulating coating over which several colored bands are printed radially as shown in Figure The colored bands provide a quickly recognizable code for determining the value of resistance, the tolerance (in percentage), and occasionally the expected reliability of the resistor. The colored bands are always read from left to right, left being defined as the side of the resistor with the band nearest to it. The first two bands represent the first and second digits of the resistance value. The third band is called the multiplier band and represents the number of zeros following the first two digits; it is usually given as a power of ten. The fourth band indicates the tolerance of the resistor, and the fifth band (if present) is an indication of the expected reliability of the component. The

6 Section 3.6 Color Coding of Resistors 77 Band 5 (reliability) Band 4 (tolerance) Band 3 (multiplier) Band significant figures Band 1 FIGURE 3 15 Resistor color codes. reliability is a statistical indication of the expected number of components which will no longer have the indicated resistance value after 1 hours of use. For example, if a particular resistor has a reliability of 1% it is expected that after 1 hours of use, no more than one resistor in 1 is likely to be outside the specified range of resistance as indicated in the first four bands of the color codes. Table 3 5 shows the colors of the various bands and the corresponding values. EXMPLE 3 1 Determine the resistance of a carbon film resistor having the color codes shown in Figure FIGURE 3 16 Red (.1% reliability) Gold (5% tolerance) Orange ( 13) Gray (8) Brown (1) Solution From Table 3 5, we see that the resistor will have a value determined as R % 18 k.9 k with a reliability of.1% This specification indicates that the resistance will fall between 17.1 k and 18.9 k. fter 1 hours, we would expect that no more than 1 resistor in 1 would fall outside the specified range.

7 78 Chapter 3 Resistance TBLE 3 5 Resistor Color Codes Band 1 Band Band 3 Band 4 Band 5 Color Sig. Fig. Sig. Fig. Multiplier Tolerance Reliability Black 1 1 Brown % Red 1 1.1% Orange % Yellow % Green Blue iolet Gray 8 8 White 9 9 Gold.1 5% Silver.1 1% No color % PRCTICE PROBLEMS 5 resistor manufacturer produces carbon composition resistors of 1 M, with a tolerance of 5%. What will be the color codes on the resistor? (Left to right) nswer: Brown Black iolet Gold 3.7 Measuring Resistance The Ohmmeter The ohmmeter is an instrument which is generally part of a multimeter (usually including a voltmeter and an ammeter) and is used to measure the resistance of a component. lthough it has limitations, the ohmmeter is used almost daily in service shops and laboratories to measure resistance of components and also to determine whether a circuit is faulty. In addition, the ohmmeter may also be used to determine the condition of semiconductor devices such as diodes and transistors. Figure 3 17 shows both an analog ohmmeter and the more modern digital ohmmeter. In order to measure the resistance of an isolated component or circuit, the ohmmeter is placed across the component under test, as shown in Figure The resistance is then simply read from the meter display. When using an ohmmeter to measure the resistance of a component which is located in an operating circuit, the following steps should be observed: 1. s shown in Figure 3 19(a), remove all power supplies from the circuit or component to be tested. If this step is not followed, the ohmmeter reading will, at best, be meaningless, and the ohmmeter may be severely damaged.. If you wish to measure the resistance of a particular component, it is necessary to isolate the component from the rest of the circuit. This is done by disconnecting at least one terminal of the component from the balance of the circuit as shown in Figure 3 19(b). If this step is not followed, in all likelihood the resistance reading indicated by the ohmmeter will not be the resistance of the desired resistor, but rather the resistance of the combination.

8 Section 3.7 Measuring Resistance The Ohmmeter 79 5 C M db M 1 5 C olts M C olts Ohms 1 1 COM (a) nalog ohmmeter. (b) Digital ohmmeter. ( Reproduced with permission from the John Fluke Mfg. Co., Inc.) FIGURE s shown in Figure 3 19(b), connect the two probes of the ohmmeter across the component to be measured. The black and red leads of the ohmmeter may be interchanged when measuring resistors. When measuring resistance of other components, however, the measured resistance will be dependent upon the direction of the sensing current. Such devices are covered briefly in a later section of this chapter. 4. Ensure that the ohmmeter is on the correct range to provide the most accurate reading. For example, although a digital multimeter (DMM) can measure a reading for a 1.-k resistor on the -M range, the same ohmmeter will provide additional significant digits (hence more precision) when it is switched to the -k range. For analog meters, the best accuracy is obtained when the needle is approximately in the center of the scale. 5. When you are finished, turn the ohmmeter off. Because the ohmmeter uses an internal battery to provide a small sensing current, it is possible to drain the battery if the probes accidently connect together for an extended period. OFF 3m FIGURE 3 18 Ohmmeter used to measure an isolated component. 7. k )))

9 oltage source OLTGE Coarse Fine CURRENT Max. OUTPUT POWER ON I O OFF OFF 4.7 k 3m ))) (a) Disconnect the circuit from the voltage/current source (b) Isolate and measure the component FIGURE 3 19 Using an ohmmeter to measure resistance in a circuit.. udible larm 1 OFF 3m OFF 3m ))) ))) Break Wire short circuit (a) Short circuit (b) Open circuit FIGURE 3

10 Section 3.8 Thermistors 81 In addition to measuring resistance, the ohmmeter may also be used to indicate the continuity of a circuit. Many modern digital ohmmeters have an audible tone which indicates that a circuit is unbroken from one point to another point. s demonstrated in Figure 3 (a), the audible tone of a digital ohmmeter allows the user to determine continuity without having to look away from the circuit under test. Ohmmeters are particularly useful instruments in determining whether a given circuit has been short circuited or open circuited. short circuit occurs when a low-resistance conductor such as a piece of wire or any other conductor is connected between two points in a circuit. Due to the very low resistance of the short circuit, current will bypass the rest of the circuit and go through the short. n ohmmeter will indicate a very low (theoretically zero) resistance when used to measure across a short circuit. n open circuit occurs when a conductor is broken between the points under test. n ohmmeter will indicate infinite resistance when used to measure the resistance of a circuit having an open circuit. Figure 3 illustrates circuits having a short circuit and an open circuit. PRCTICL NOTES... When a digital ohmmeter measures an open circuit, the display on the meter will usually be the digit 1 at the left-hand side, with no following digits. This reading should not be confused with a reading of 1, 1 k, or 1 M, which would appear on the right-hand side of the display. n ohmmeter is used to measure across the terminals of a switch. a. What will the ohmmeter indicate when the switch is closed? b. What will the ohmmeter indicate when the switch is opened? PRCTICE PROBLEMS 6 nswers: a. (short circuit) b. (open circuit) 3.8 Thermistors In Section 3.4 we saw how resistance changes with changes in temperature. While this effect is generally undesirable in resistors, there are many applications which use electronic components having characteristics which vary according to changes in temperature. ny device or component which causes an electrical change due to a physical change is referred to as a transducer. thermistor is a two-terminal transducer in which resistance changes significantly with changes in temperature (hence a thermistor is a thermal resistor ). The resistance of thermistors may be changed either by external temperature changes or by changes in temperature caused by current through the component. By applying this principle, thermistors may be used in circuits to control current and to measure or control temperature. Typical appli-