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Contents Introduction 3 Linear resistors 4 Specification of resistors 5 Low power resistors 6 Medium and high power resistors 7 Non-linear resistors Thermistors 9 9 Voltage-dependent resistors (VDR) 10 Light-dependent resistors (LDR) 11 Variable resistors 12 Identification of resistors 14 Labelled resistors 14 Colour coded resistors 14 Preferred values 16 Summary 24 Answers 25 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 1 2 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 Introduction This section aims to help you understand different resistor types and their characteristics. We will describe how resistor components are specified for a particular application, the differences between low and high power resistors, and how resistance values are marked on the component. We introduce a variety of special purpose resistors including variable resistors and resistors that change resistance in response to temperature, light, and voltage. After completing this topic, you should be able to: describe the features of fixed and variable resistor types and typical applications list the characteristics of temperature dependent, voltage dependent, and light dependent resistors giving applications of each specify a resistor for a particular application determine the resistance of a colour-coded resistor using a colour code table and to confirm the value by measurement. EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 3 Linear resistors A resistor is obviously a component designed to offer resistance, or opposition to an electric current. Resistors may be placed in a circuit to produce a voltage drop, or to limit the circuit current to a desired value. Symbols for some of the different types of resistor are shown in Figure 1. Figure 1: Resistor symbols The linear resistor is by far the most common type of resistor in electrical and electronic circuits. Linear simply means that the resistance is designed to remain constant. Non-linear resistors have a resistance which varies in response to changes in temperature or voltage for example, and these will be discussed later. Look at the graph in Figure 2, and note that it is a straight line or linear graph commencing at zero current and voltage. This type of graph for a device is known as its ‘characteristic’, which describes how the current through the device changes with the applied voltage. These graphs are drawn from results taken when an increasing voltage is applied to a resistor and the current is read off for each voltage step. This graph is really just an expression of Ohm’s Law, which describes a linear relationship between voltage and current caused by a constant resistance (I = V/R). (Note that only linear resistors obey Ohm’s Law.) Any constant resistance will have a linear characteristic like this. Larger resistances will have a smaller slope (not as steep), because less current will flow at the same voltage. Smaller resistances will have a steeper slope. 4 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 Figure 2: Linear resistance We can calculate the resistance from any point on the graph because it is linear. Note that any point on the graph will give the same resistance: (a) At 2 volts: R V I 2 0.11 10 3 18181 (b) At 6 volts: R V I 6 0.33 103 18181 Specification of resistors There are two important values to mention when specifying a resistor for a particular job. These are: the resistance value itself (in ohms) the power rating of the resistor (in watts). EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 5 For example, a 10 K, 1 watt resistor has an ohmic value of 10 000 Ω and can safely dissipate 1 watt of power. The range of common linear resistor components varies from a fraction of an ohm to millions of ohms. Power ratings vary mainly from about 0.1 W to 10 W, with exceptions above this. Some high-power resistors have power ratings well in excess of 10 watts. Low power resistors The materials and construction details of resistor components depend upon: the resistance value needed the tolerance required the power to be dissipated Low power resistors do not have to dissipate much power, and so can be made physically small. These resistors are used in electronic circuitry where the tolerance, linearity and stability over time are important factors. Different types include carbon film, metal film, and metal oxide resistors. Carbon film resistors These resistors are made by spraying a film of carbon on a glass or ceramic rod. The thickness of the film dictates the resistance obtained. Small metal end caps provide connection to the film and leads are connected to the end caps to provide circuit connection. A final coating of insulation and colour coding bands is then applied. The resistance of carbon film resistors ranges from a few ohm up to several million ohm with power ratings varying from one quarter watt up to two watt. Metal film resistors These are manufactured in a similar manner to the carbon film type with a nickel-chromium film being deposited on the ceramic rod. Though the resistance of the carbon film type varies only by about 1 per cent over its life span, the metal film resistor varies even less. They are more costly to manufacture than the carbon film type. However, they have a more accurate and consistent resistance and lower temperature co-efficient. This makes them preferable to the carbon type. Metal file resistors are the most common type of low power resistor used today in electronics. Resistances available vary from about 10 Ω to 1M Ω with power ratings from 0.1 watt to 1 watt. 6 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 Metal oxide resistors These involve a coating of oxides of tin and antimony on the ceramic rod. Like the metal film type their resistance is very stable, varying over their life by less than 0.5 per cent. Their resistance range varies from about 10 Ω to 100 kΩ with power ratings from 0.25 W to 10 W. Medium and high power resistors Medium and high power resistors are called upon to dissipate more power, and so must be constructed robustly and designed to dissipate heat effectively. They therefore must be physically larger that the low-power types. There are two major divisions within this category: wire-wound resistors grid resistors. Wire wound resistors These consist of a length of resistance wire (often nickel-chromium) wound on a heat-resistant bobbin. The wire is welded to terminals at either end after which the device is coated with an insulating material. The end result is a resistor larger than the film or oxide types, capable of dissipating substantial amounts of power. The resistance of wire-wound resistors ranges from a fraction of an ohm to about 200 kΩ, while their power rating ranges from 0.5 W to 25 W for electronic applications and to several kilowatts for power engineering applications. Grid resistors For high power applications, wire wound resistors become impractical. In this case, the resistors are stamped out of stainless steel alloy sheet or made from cast iron. These are known as grid resistors. If you have Hampson, read the section called ‘Resistors’ on pages 53 to 56, looking at the illustrations for details. If you have Jenneson, refer to Section 2.12 on page 42 for more information on resistors. Particularly note Figures 2.22 and 2.23 for the physical identification of various resistors. EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 7 Activity 1 1 Describe the characteristic of linear resistors. _____________________________________________________________________ _____________________________________________________________________ 2 Draw the symbols of four different types of resistor. 3 Describe the difference between carbon-film and metal film resistors. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ 4 Describe the difference between wire-wound and grid resistors. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Check your answers with those given at the end of the section. 8 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 Non-linear resistors Remember that linear resistors are designed to maintain a constant resistance, regardless of the voltage and temperature. These resistors obey Ohm’s Law and are the most common type. For some particular applications, we can make use of materials that are sensitive to changes in heat, light, or voltage. These are known as non-linear resistors, because changes in resistance will result in a non-linear characteristic. Thermistors Resistors whose resistance changes with temperature are called thermistors and are either: positive temperature co-efficient thermistors negative temperature co-efficient thermistors. Positive temperature co-efficient (PTC) thermistors These are designed so that an increase in temperature causes an increase in resistance. A decrease in temperature would cause a decrease in resistance. This relationship is a ‘positive’ one, hence the term PTC thermistor. PTC thermistors are used to protect the windings of electric motors from burning out through excessive temperatures caused by overloads. Due to their small physical size, PTC thermistors can be inserted directly into motor windings. They can quickly sense temperature rises and, through the use of relays, disconnect the windings from mains supply before damage occurs. Negative temperature co-efficient (NTC) thermistors These are designed so that an increase in temperature causes a decrease in resistance. A decrease in temperature would cause an increase in resistance. This relationship is a ‘negative’ one, therefore the term NTC thermistor. NTC thermistors can be used to suppress high in-rush currents that occur when projector lamps are switched on. They can also be used for EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 9 temperature measurement, and temperature dependent biasing of electronic devices. The characteristics curves of both PTC and NTC types are shown in Figure 3. Figure 3: Characteristics curves of PTC and NTC thermistors Voltage-dependent resistors (VDR) Voltage-dependent resistors have a characteristic curve as shown in Figure 6. The circuit symbol used for it is shown below in Figure 7. Figure 6: Characteristic of voltage-dependent resistor Figure 7: Symbol for voltage-dependent resistor In viewing the characteristic curve you will note that at very low voltages almost no current flows, indicating that the device has a very high resistance at low voltage. At higher voltages you will see the current increasing rapidly 10 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 indicating that the resistance of the device has fallen substantially. It is truly then a voltage-dependent resistor. VDRs are used extensively to suppress voltage surges. In the electrical supply industry they are connected between overhead transmission lines and earth. At normal voltages their resistance is very high and almost no current from the line passes through them. When lightning strikes the line, the high voltage created causes their resistance to drop substantially and the surge currents in the line, instead of continuing through to the substation and causing damage, are diverted to earth through the VDRs. Light-dependent resistors (LDR) This type of resistor is sensitive to light. In darkness, the LDR has a high resistance that quickly falls to a low value when light falls on it. It is made from cadmium sulphide. A major use of the LDR is in a circuit that turns the street lights on as it gets dark. The characteristic and circuit symbol for the device are shown in Figures 8 and 9. Figure 8: Characteristic of light-dependent resistor Figure 9: Circuit symbol for light dependent resistor EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 11 Variable resistors The resistance of a variable resistor can be mechanically varied between two values. Variable resistors are most commonly adjusted by turning a shaft, which changes the position of a wiper blade moving across the surface of the resistive material. Carbon types are used for low power applications while the wire-wound type is used for medium and high power applications. Variable resistors can be connected as ‘potentiometers’ or ‘rheostats’. We use a potentiometer to vary voltage and a rheostat to vary current. See Figures 4 and 5. Figure 4: Variable resistor connected as rheostat When the variable resistor is connected as a rheostat, the resistance of the variable resistor is placed in the current path. With a constant voltage source, the load current will vary with the position of movable contact. Figure 5: Variable resistor connected as a potentiometer When the variable resistor is connected as a potentiometer as shown is figure 5, the voltage between A and B is variable depending on the position of moveable arm A. V0 represents the variable output voltage. 12 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 Some variable resistors are designed to vary non-linearly as they are adjusted. An example of this is the logarithmic variable resistor, which is used for the volume control of radios or other audio devices. EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 13 Identification of resistors Two methods used to identify the resistance of resistors. Either the value can be written on the resistor in text, or a colour code is used. Labelled resistors If the resistor is physically large enough the value can be printed on it, for example “1 ohm”. Medium and high power resistors are commonly identified in this way. Commonly the multiplier character is placed where the decimal point would go. Here are some examples: ‘2k2’ means 2.2 kΩ ‘4M7’ means 4.7 MΩ The letter ‘R’ is used where there is no multiplier, for example. ‘1R5’ means 1.5 Ω This style of labelling eliminates the uncertainty caused by the small decimal point being difficult to read reliably. Figure 10 illustrates this style. Figure 10: Resistor identification Colour coded resistors Low power resistors which are too small to have the resistance written on them are identified by a colour code. During manufacture coloured bands are printed on the resistor in a specific order. Table 1 outlines what the bands represent. 14 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 Table 1: Resistor band meanings Position of band What it represents closest to the end the first significant figure second band second significant figure third band the multiplier fourth band (if there is one) the tolerance (see the section on ‘preferred values’ below). Figure 11: Method of reading the value of a colour-coded resistor The figures represented by band colours are shown in Table 2. Table 2: Resistor colour code Colour Significant figures Multiplier black 0 1 (100) brown 1 10 (101) 1% red 2 100 (102) 2% orange 3 1 000 (103) yellow 4 10 000 (104) green 5 100 000 (105) blue 6 1 000 000 (106) violet 7 – grey 8 – white 9 – gold – 0.1 5% silver – 0.01 10% none – EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 Tolerance 20% 15 Example If the colour bands are brown, black, red and silver, state the value of the resistance? Solution Brown = 1 (first digit) Black = 0 (second digit) Red = 2 (multiplier = 102 = 100) Silver (representing 10% tolerance.) Therefore the resistance is 10 × 102 = 1000 Ω or 1 kΩ, with a tolerance of 10%. Example What is the resistance of a resistor with bands of red, red, green, and gold ? Solution red = 2 (first digit) red = 2 (second digit) green = 5 (multiplier = 105 = 100 000) Therefore the resistance is 22 × 100 000 = 2.2 MΩ If you have Hampson, read the section ‘Resistor colour code’ on page 56 checking out Figure 2.25 and completing questions 11 and 13. If you have Jenneson, refer to Section 2.12.4 and 2.12.5 for more examples in using the resistor colour code and the idea of preferred values. Preferred values Resistors for electronics cover a wide range of values, from fractions of an ohm up to many megohm. The number of different values is limited to a standard set of values called the ‘preferred value’ shown in Table 3. These are the basic figures only. Resistors are made larger and also smaller than the tabled values in steps of 10 up to 106. In other words, a 15 ohm resistor is part of a range including 150, 1500, 15 K, 150 K and so on. 16 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 20% tolerance 10 10% tolerance 5% tolerance 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 Table 3: Preferred value resistors Activity 2 1 Describe the characteristics of both NTC and PTC thermistors. _____________________________________________________________________ _____________________________________________________________________ EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 17 2 Draw diagrams showing how a variable resistor can be used as a normal rheostat and as a potentiometer. 3 Explain how a VDR can be used to prevent lightning damage to transmission lines and associated equipment. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ 4 Explain how an LDR works. _____________________________________________________________________ _____________________________________________________________________ 5 State why colour coding is used on resistors, and nominate how it is used. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ 6 Why are resistors manufactured in ‘preferred values’? _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Check your answers with those given at the end of the section. 18 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 Check your progress In Questions 1–10 write your answer in the brackets provided. 1 The best ohmmeter range to measure a resistance of 7 ohm is: (a) ohm × 1 (b) ohm × 7 (c) ohm × 10 (d) ohm × 100 2 ( ) ( ) ( ) ( ) The colour code of a 27 ohm resistor is: (a) red, violet, brown (b) red, blue, brown (c) red, violet, black (d) red, blue, black 3 A resistor with colour bands of orange, white, yellow has a value of: (a) 390 (b) 39k (c) 3.9 (d) 390 k 4 A resistor with a tolerance band of gold, has a tolerance of: (a) 1% (b) 2% (c) 5% (d) 10% EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 19 5 A power resistor has 68R stamped on its body, indicating a resistance of: (a) 0.68 (b) 6.8 (c) 68 (d) 680 6 ( ) An ohmmeter, set to its ‘Ohm × 100’ range is indicating 500 on the scale. The actual resistance is: (a) 5 (b) 500 (c) 50 000 (d) 500 000 7 ( ) ( ) ( ) A 2.2 kΩ resistor has tolerance of 10%. Its acceptable resistance range is from: (a) 1100 to 3300 (b) 1980 to 2420 (c) 2090 to 2310 (d) 2178 to 2222 8 The resistance of a thermistor varies with changes in: (a) humidity (b) voltage (c) light (d) temperature 20 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 9 The symbol shown below is for a: (a) thermistor (b) varistor (c) light dependent resistor (d) linear resistor ( ) ( ) 10 The symbol shown in Figure 13 is for a: (a) thermistor (b) varistor (c) light dependent resistor (d) linear resistor 11 (a) What is the tolerance range for a 12 kΩ 10% resistor? __________________________________________________________________ __________________________________________________________________ (b) A resistor is known to have a range between 4794 Ω and 4606 . (i) What is the nominal value of the resistor? ______________________________________________________________ ______________________________________________________________ (ii) What is its percentage tolerance? ______________________________________________________________ ______________________________________________________________ EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 21 12 Complete Table 4 by filling in the blank spaces. Table 4 Band colour Resistance First Second Third Fourth Red Red Brown Gold Yellow Violet Orange Silver Brown Orange Black White Blue Brown Tolerance % 12k 5 0.33 5 680 10 1k 2 2.7M 5 Gold Red 13 A resistor value of approximately 6.0 M is required. (a) What preferred value of resistor should be chosen? __________________________________________________________________ (b) If the tolerance of the chosen resistor were 10%, what would be its resistance range? __________________________________________________________________ __________________________________________________________________ 22 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 14 It is required that a resistor chosen for a particular job has a value between 1.43 and 1.52 k. (a) What preferred value of the resistor should be chosen? __________________________________________________________________ (b) What should be its minimum tolerance? __________________________________________________________________ 15 What coloured bands would resistors have if they had the following resistance? (a) 470 10% ___________________________________________________________________ (b) 1k2 5% ___________________________________________________________________ (c) 68k 10% ___________________________________________________________________ (d) 2M7 5% ___________________________________________________________________ Check your answers with those given at the end of the section. EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 23 Summary 24 A resistor is a device used in an electrical circuit to limit circuit current, produce a specific potential difference or produce heat. Resistors can be linear or non-linear. A linear resistor has constant resistance (within tolerance) over a prescribed temperature range. A non-linear resistor is designed to vary its resistance under the influence of light, heat, voltage or current. Resistors are classified according to their Ohmic value and the power capable of being dissipated. Low power resistors made from carbon film, metal film or metal oxide are produced in set values called the ‘preferred value’ range. Medium or high power resistors are either wire-wound or made from pressed steel or cast iron grids. A variable resistor connected so as to vary current is called a rheostat. A thermistor is a non-linear resistor whose resistance changes under the influence of heat. NTC thermistors are used to suppress high in-rush currents, measure temperature and for automatic temperature-dependent biasing. PTC thermistors are connected in electrical machines to isolate the machines under overload conditions. VDRs vary their resistance inversely with the applied voltage. They are used to suppress or divert ‘surge’ voltages. LDRs have high resistance when not subjected to light and the resistance falls quickly when light is applied. High power resistors are identified by having the value of resistance printed on them. Low power resistors are identified by a resistor colour code. EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 Answers Activity 1 1 The resistance of a linear resistor remains constant (within a given tolerance) over a prescribed temperature range. A non-linear resistor is designed so that its resistance varies at a controlled under the influence of light, heat, voltage or current. 2 3 Carbon-film resistors are made by spraying a film of carbon on rods of glass or ceramic materials. They vary in range from a few to millions of ohm and are made up to 2 watt in power rating. Metal-film resistors are made similar to the carbon film type except that the carbon film is replaced by one of nickel-chromium. They are more accurate than the carbon-film type and range from about 10 ohm to one megohm, with power ratings up to 1 watt. 4 Wire-wound resistors consist of a definite length of resistance wire, wound on a heat-resistant bobbin. They are capable of dissipating up to about 25 W of power. Grid resistors are either stamped out of stainless steel alloy sheet or cast from a suitable cast iron. They are capable of dissipating some thousands of watts of power. EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 25 Activity 2 1 An NTC thermistor is a non-linear resistor whose resistance decreases with an increase of temperature. A PTC thermistor is a non-linear resistor whose resistance increases with an increase in temperature. 2 See the diagram below. 3 A VDR is connected between the line and earth. At normal line voltage its resistance is exceedingly high and no current passes through it. The very high voltage of a lightning strike, however, makes its resistance drop substantially, causing the ‘strike’ to go to earth and not continue through the line and inflict damage on associated equipment. 4 In darkness, an LDR has a very high resistance, limiting current to zero. When light is applied, its resistance drops substantially, allowing current to flow. It can, therefore, be used to control lighting, in conjunction with a relay, acting as an on-off switch. 5 Colour coding is used on resistors, rather than writing the resistance value on the resistor, because with time any decimal point on the resistance value might disappear leaving digits only in what would then be an incorrect resistance indication. As an example 2.2 K might, after some time in use, look like 22 K. Colour coding uses bands or rings of colour, each band colour representing a particular digit. Three or four bands may be used and an experienced operator, having memorised band colour values, can quickly identify the resistance value. 6 To attempt to manufacture resistors covering every value from, say, 1 ohm to 1 000 000 ohm would not be a good idea. Think of the stock you would have to keep to cover a wide range. Preferred values represent those resistors that cover a wide range using a tolerance. As an example, take a 33 ohm resistor with a 10% tolerance. Ten per cent of 33 is 3.3 ohm. Thus that resistor covers a range from 33–3.3 (29.7 Ω) to 33 + 3.3 (36.3 Ω). The next resistor needed above 26 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 33 Ω is 39 Ω, as it covers down to 39–3.9 (35.1 Ω) which is within the tolerance allowed. Table 2 identifies these preferred values. Check your progress 1 (a) 2 (c) 3 (d) 4 (c) 5 (c) 6 (c) 7 (d) 8 (d) 9 (b) 10 (c) 11 (a) 10.8 k to 13.2 k. (b) (i) 4700 (ii) 2% 12 Band colour Resistance Tolerance % First Second Third Fourth Red Red Brown Gold 220 5 Yellow Violet Orange Silver 47 k 10 Brown Red Orange Gold 12 k 5 Orange Orange Silver Gold 0.33 5 Brown Black Blue Gold 10 M 5 Blue Grey Brown Silver 680 10 Orange White Brown Red 390 2 Brown Black Red Red 1k 2 Red Violet Green Green 2.7 M 5 13 (a) 5.6 M (b) 5.04 to 6.16 M. 14 (a) 1.5 k (b) 5%. EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663 27 15 (a) yellow violet brown silver (b) brown red red gold (c) blue grey orange silver (d) red violet green gold. 28 EEE042A: 8 Select and identify resistors NSW DET 2017 2006/060/05/2017 LRR 3663