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' . . · . . - ·. . . . . - · " _ · 1 ·· ·. . . - - . · • ·• ······._.. -- - - . . . ··· .. . - o· .., .. - . - -veryon - ii. . ' -- , .. . Pt.1: What you need to · know about resistors 11111, . -, ❖,• ,. ,,. ~ 111, ·: •:❖:;}·'. • • ''""" Virtually every electronic circuit has resistors in it. They are.the most basic of electronic components and the easiest to understand. One of the big problems for beginners is how to read the labelling. On wirewounds, the labelling is printed while on film resistors it is in the form of colour code bands. By LEO SIMPSON Well, what is a resistor anyway? A resistor is a component which "resists" or impedes the flow of electrical current. If the resistor has a high resistance, not much current will flow, for a given voltage. Perhaps the most familiar resistors are those used in domestic electrical appliances. For example, the heating element in your electrical radiator is nothing more than a large wirewound resistor. It has a relatively low resistance and is designed to run red hot. So are the heating elements in your electric stove. They draw a relatively high current of several amps from the 240V AC mains and so they dissipate quite a lot of power - up to several kilowatts. Other resistors which are widely found in people's houses are the heating elements in toasters, electric irons and kettles, hair dryers and incandescent lamps. All the examples just cited are designed to run from the high voltage of the mains supply - 240 volts AC. They are specially designed to dissipate (ie, give off) a lot of power (heat) and they may run extremely hot; eg, the white hot filament in an incandescent lamp. Another point about the resistors just mentioned is that they are all designed for a particular purpose and they can't be adapted to other tasks. In the world of electronics though, we deal with resistors that are general purpose - they are designed for a wide variety of tasks. Resistors fall into two broad types, wirewounds and film types. The latter type use a carbon or metal film as the resistive medium. Most resistors used in electronic applications do not have to dissipate a lot of heat. Carbon and metal film resistors commonly dissipate only small fractions of a I I i I ,, 11 »w, ,,,,..... ,.Jtl ~ ' I '\ ' , Fig.1: an array of general purpose resistors as used in electronic equipment. The values are directly printed on the two wirewound types while the carbon film types have colour bands. watt while wirewound resistors commonly come in ratings of 5 and 10 watts, although they can be made in ratings up to several hundred watts. Even so, they are puny compared to the high power heating elements in electric heaters, stoves and toasters. The photo of Fig.1 shows an array of general purpose resistors, widely used in electronic equipment. The unit of resistance The unit of resistance is the Ohm (named after George Simon Ohm). A resistor has a value of one ohm when it is necessary to apply a voltage of one volt across it in order to drive a current of one amp through it. On circuits and in texts about electronics, it is not usual to spell out the word "Ohm" every time a resistance value is measured. Instead, we use the Greek letter Omega - 0. So when we write about or specify a 12 ohm resistor, it is written as 120. Resistance multipliers The range of resistor values used in practical electronic devices is extremely wide, from fractional values below 10 to ten million ohms or more. Since the use of large numerical values is unwieldy, it is standard practice to use multipliers in front of the O symbol to specify thousands or millions of ohms. So to specify thousands of ohms, we use the multiplier " kilo" - hence "kff'. So to specify a resistor value of ten thousand ohms we write 10k0. Similarly, to specify millions of ohms, we use the multiplier "mega" - hence MO. To specify a resistor of 1.5 million ohms we write 1.5MO. In normal conversation, you would refer to a 10k0 resistor as ''ten kilohms'' (pronounced killomes) or more usually as a " ten kay resistor". For a resistor value of lOMO you would say it has a value This article is the beginning of a new series for people who have little or no knowledge of electronics but would like to gain some practical experience without delving into a lot of theory. Hence, the emphasis will be on practical matters rather than on theory. MARCH 1989 5 --------+12V I- OUTPUT --------ov I- OUTPUT 10k!l --------ov Fig.2: resistors are most commonly represented on circuits as zigzag symbols. Alternatively, they can be represented as rectangular boxes as shown in Fig.3 at right. of "ten megohms" or you might refer to it as a "ten meg resistor". For low values, you use the value direct. When referring to a 1500 resistor, you say exactly that, a "150 ohm" resistor. Recognising resistors on circuits There are two recognised ways of drawing resistors on circuits. The older and more easily recognised way, as used in SILICON CHIP, most other electronics magazines and on most commercial electronic circuits, is to show the resistor as a zigzag symbol. This is shown in Fig.2. This shows a number of components in a small circuit. The zigzag symbols are resistors and their values are shown close to them. The zigzag symbol was adopted originally because it suggests the construction of many wirewound resistors. These are usually a coil of wire on a ceramic former but they can take on a zig zag format. Have a look at the element in your toaster, hair-dryer or in incandescent lamps. Often, in order to make the circuit easier to describe, or when there are very large numbers of resistors (as in TV and VCR circuits), it is common to number the resistors - hence R1, R2 and so on. On some circuits, the resistors may be numbered but their values in ohms will not be shown. You might have to look up a parts list to find the values. In SILICON CHIP we always use the zigzag symbol but we don't often use R numbers. And when we show resistors in circuit we usually leave out the "O" symbol where the k or M multiplier is used. Hence, a 6.8k0 resistor will be shown on SILICON CHIP circuits as 6.8k. This practice is commonly used elsewhere. Circuits of European origin (and some drawn to the Australian Standard ASl 102 which is not widely used) show resistors as rectangular boxes. Fig.3 shows the same circuit as Fig.2 but is redrawn to show the resistors as boxes. You will recognise the boxes as resistors because they will have R numbers (eg, R3) near them or the actual values. Decimal points On some circuits, often of European origin, you won't see values such as 0.330, 1.50, 4.7k0, 6.8MO and so on. Instead of showing decimal points, these same component values are shown on circuits as R33, 1R5, 4k7 and 6MB, respectively. Instead of using the decimal point, the multiplier (k or M) is used in its place. And for small res~stance values, R is used in place of the decimal point. This "non-decimal" method of labelling resistors is set out in an IEC standard, publication 62. IEC stands for "International Electrotechnical Commission". So when you see a resistor marked 5R6, you will recognise it as having a value of 5.60. Similarly, a resistor marked 3k9 is 3.9k0 and one marked 2M7 is 2.7MO. Odd labels such as lRO, lkO and 1Mo simply mean rn, lkO and lMO, respectively. Resistor types Fig.4: a selection of 5-watt and 10-watt wirewound resistors. In each case, the resistor's value and its rating is printed on the resistor body. 6 SILICON CHIP As mentioned before, resistors for electronic circuits fall into two broad types: wirewound and carbon or metal film. In Australia, the most commonly available wirewound resistors have power ratings of 5 watts, 10 watts or 15 watts. Larger values are available but are seldom used in most circuits. Where wirewound resistors are specified on circuits their power ratings are usually also shown, hence 5W, 10W or 15W. Often, there may be the designation To give you an idea of how hot these resistors become, if you run one of these 5 watt "bathtub" resistors at 5 watts, its surface temperature is likely to be at least 120°C above ambient (ie, the surrounding air temperature). For a 10 watt resistor, run at full power, the surface temperature will be at least 200°C above ambient. This may not be enough to set anything on fire but it can be enough to char a printed circuit board or other components, if the resistor is too close. Derating Fig.5: this heating element (from an electric heater) is simply a large wirewound resistor. This unit is rated at several hundred watts. '.I Fig.6: a selection of carbon-film resistors. These resistors are too small to have values printed on them, so colour bands are used to indicate the values instead. The six resistors on the left are 4-band 5% tolerance types, while the six on the right are 5-band 2% types. "WW" to show that the resistor is wirewound. If you go into an electronics parts dealer and ask for a 5 or 10 watt resistor, you will most likely be sold cine like those shown in Fig.4. Externally, they don't look like wirewound resistors, but they are. These are a fireproof resistor housed in a ceramic "bathtub". If you broke one of these resistors open, you would find the resistance element inside, wound with very fine wire (usually Nichrome) on a round ceramic former about 2mm in diameter. Incidentally, the fact that these resistors are listed as being fireproof should not suggest that they don't get hot - they get very hot. But if they are badly overloaded, with excess current through them, they don't catch fire and their casing does not become red hot. Instead the internal resistor element fuses and goes open-circuit. It is normal practice to "derate" resistors in normal operation. This gives a margin of safety, minimises long-term drift in the value of the resistor and makes the component much less likely to break down. Typically, resistors are derated to 60% or 70% of rating. For a 5 watt resistor, this means it is normal to run it at 3 to 3.5 watts. Incidentally, sometimes when reading about resistors, you might see the term "fixed" resistors. The resistors we're talking about right now are "fixed" because their value is (more or less) constant, regardless of the applied voltage, operating temperature or whatever. Examples of resistors which are not "fixed" include potentiometers which can be manually varied or thermistors, which vary their resistance markedly according to their operating temperature. Wirewound resistors are normally made in values which span the range from o.rn to about lOOkO or so. For values above lOOkO you normally need to go to carbon or metal film resistors. Carbon and metal film resistors Much more common than wirewound resistors in typical electronic circuits are carbon and metal film resistors with a power rating of less than one watt. In fact, the most common resistors today, which are used in quantities of hundreds of millions every year throughout the world, are resistors with a rating of a quarter watt or less. MARCH 1989 7 wm ~ ~ DAVID REID W For the electronics enthusiast ELECTRONICS PTY. LTD. ~===========================~==:::::::::::::::::::::::::::::::::::::::::::::~. . . RS-232 BREAKOUT BOX FLUKE 80 SERIES MUL TIMETERS ~ r ... 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UNIT PRICE SUB TOTAL P+P TOTAL TOTAL PRICE SUPPLIED P + P RATES $5-$2 5 . . $4 $26-$50 ...... $7 $51 over . . ... $9 Table 1: 4-Band Resistor Colour Code A B C D i:::::::====1( I 11 11 11 I )~= ~ ~ Band A B C D Colour Tens Units Multiplier Tolerance Black Brown Red Orange Yellow Green Blue Violet Grey White Gold Silver 0 1 2 3 4 5 6 7 8 9 0 1 1 2 10 100 1k (= 1000) 1 Ok (= 10,000) 1 00k (= 100,000) 1 M (= 1,000,000) None 1% 2% 3 4 5 6 7 8 9 0.1 (divide by 10) 0.01 (divide by 100) These are the "workhorse" resistors of the electronics industry. They set bias values for transistors and op amps, and are used in feedback networks, filter time constants and a hundred and one other circuit tasks which would not be practical with wirewound resistors. These resistors are so small that it is not practical to print their values on them so a series of colour bands is used to show the value. The most common colour code system used these days has four bands but as close tolerance resistors (2 % or 1 % ) become cheaper and more readily available, the five band colour code is becoming more common. In a few years, these may completely displace resistors with only four colour bands. The resistor colour code For many newcomers to electronics, the resistor colour code is probably the biggest stumbling block. While you are becoming familiar with the colour codes, working out resistor values does require some mental gymnastics but you can get around the need for these. Bear with us for a few paragraphs or so while we tell you about it and then we'll show you how you 10 11 SILICON CHIP = 20% these, the last two bands will be gold and silver or combinations thereof. For example, consider a 4. 70 resistor with a tolerance of 5 % . The first two bands are easy enough: yellow and violet for 47. To get 4. 7, the multiplier needs to be 0.1 which is gold. Since it has a 5 % tolerance, the last band will be gold too, so the code will be yellow, violet, gold, gold. If the value is 0.470 with 5% tolerance, the bands will run yellow, violet, silver, gold. Reading the codes 5% 10% can get by without knowing the colour code at all. Table 1 shows the four band colour code system. The first two bands give the first two numbers of the value while the third band gives the multiplier. Take a look at the table now to familiarise yourself with it. The easiest way to become familiar with the colour code is to · cite a few examples. Let's pick an easy one: a 22k0 5% resistor. The first two bands will be red followed by orange for the multiplier. That gives 22k0 while the fourth band being gold gives a tolerance of ±5%. As another example, consider a resistor with four bands reading yellow, violet, green, gold. Yellow and violet give the first two numbers as 47 multiplied by lOOk (green) to give a value of 4. 7MO. Gold gives the tolerance of ± 5 % . One more example: consider a resistor with four bands reading blue, grey, brown, silver. Blue and grey give the first two numbers as 68 with the multiplier as 10 [brown) to give a value of 6800 with a tolerance of ± 10 % . Low resistance values It can be tricky to latch on to the colour codes for low value resistors; ie, those below 100. On This brings us to the question: in which direction do you read the colour codes. If you pick up a resistor with gold or silver bands, it's easy - just put the gold or silver bands to the right and then read off the code from left to right, as shown in the diagram associated with Table 1. It gets tricky though when the fourth band is red, for a 2 % tolerance, or brown, for a 1 % tolerance. How do you go then? It would be possible to read the value off in either direction. In most cases though, you will realise that, if you read off the code in the wrong direction, you will get a value which is invalid. For example, consider a 680k0 resistor with a tolerance of 2 % . If you consult Table 1, you will come up with bands [from left to right) of blue, grey, yellow, red. If you read it the other way, ie red, yellow, grey, blue, you would have a resistor of 24,000MO with a tolerance of 0.25%. Now there just isn't any such animal. Well, we might have picked an easy example there. It is possible to get some values which read the same way, no matter which direction you read the bands. An example is a resistor with four red bands. That would be a 2.2k0 resistor with a tolerance of 2 % . Or you could have a resistor with four brown bands. That would be a 1100 1 % resistor. But once you get away from those examples, it is possible to get resistors with four bands which give valid values in either direction. Take a 1000 1 % resistor for exam- Table 2: Resistor Colour Codes: E12 Series with 5 % Tolerance o.rn 0 .12n 0.15Q 0.18Q 0.22n 0.2m 0.33Q 0.39Q 0.47Q 0 .56Q 0.68Q 0.82Q brown brown brown brown red red orange orange yellow green blue grey black red green grey red violet orange white violet blue grey red silver silver silver silver silver silver silver silver silver silver silver silver gold gold gold gold gold gold gold gold gold gold gold gold R1 R12 R15 R18 R22 R27 R33 R39 R47 R56 R68 R82 1kQ 1.2kQ 1.5kQ 1 .8kQ 2.2kQ 2.7kQ 3 .3kQ 3.9kQ 4.7kQ 5 .6kQ 6.8kQ 8.2kQ brown brown brown brown red red orange orange yellow green blue grey black red green grey red violet orange white violet blue grey red red red red red red red red red red red red red gold gold gold gold gold gold gold gold gold gold gold gold 1 kO 1 k2 1 k5 1 k8 2k2 2k7 3k3 3k9 4k7 5k6 6k8 8k2 1.0Q 1.2n 1.5Q 1.8Q 2.2n 2 .rn 3 .3Q 3 .9Q 4 .rn 5.6Q 6.8Q 8 .2Q brown brown brown brown red red orange orange yellow green blue grey black red green grey red violet orange white violet blue grey red gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold gold 1RO 1R2 1R5 1 R8 2R2 2R7 3R3 3R9 4R7 5R6 6R8 8R2 10kQ 12kQ 15kfl 18kQ 22kQ 27kQ 33kQ 39kQ 47kQ 56kQ 68kQ 82kQ brown brown brown brown red red orange orange yellow green blue grey black red green grey red violet orange white violet blue grey red orange orange orange orange orange orange orange orange orange orange orange orange gold gold gold gold gold gold gold gold gold gold gold gold 1 Ok 12k 15k 18k 22k 27k 33k 39k 47k 56k 68k 82k 10n 12n 15Q 18Q 22Q 27Q 33Q 39Q 47Q 56Q 68Q 82Q brown brown brown brown red red orange orange yellow green blue grey black red green grey red violet orange white violet blue grey red black black black black black black black black black black black black gold gold gold gold gold gold gold gold gold gold gold gold 10R 12R 15R 18R 22R 27R 33R 39R 47R 56R 68R 82R 100kQ 120kQ 150kQ 180kQ 220kQ 270kQ 330kQ 390kQ 470kQ 560kQ 680kQ 820kQ brown brown brown brown red red orange orange yellow green blue grey black red green grey red violet orange white violet blue grey red yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow gold gold gold gold gold gold gold gold gold gold gold gold 100k 120k 150k 180k 220k 270k 330k 390k 470k 560k 680k 820k 100n 120n 150Q 180Q 220n 270Q 330Q 390Q 470Q 560Q 680Q 820Q brown brown brown brown red red orange orange yellow green blue grey black red green grey red violet orange white violet blue grey red brown brown brown brown brown brown brown brown brown brown brown brown gold gold gold gold gold gold gold gold gold gold gold gold 100R 120R 150R 180R 220R 270R 330R 390R 470R 560R 680R 820R 1 MQ 1.2MQ 1.5MQ 1.8MQ 2.2MQ 2.7MQ 3.3MQ 3 .9MQ 4.7MQ 5 .6MQ 6 .8MQ 8.2MQ 10MQ brown brown brown brown red red orange orange yellow green blue grey brown black red green grey red violet orange white violet blue grey red black green green green green green green green green green green green green blue gold gold gold gold gold gold gold gold gold gold gold gold gold 1MO 1M2 1M5 1M8 2M2 2M7 3M3 3M9 4M7 5M6 6M8 8M2 10M ple. It will have a colour code (from left to right) of brown, black, brown, brown. Read it back the other way, and you get a value of 110 1 %. In this case, both values are valid. Sometimes there is a bigger gap between the third and fourth band, which gives you a clue as to which direction is right but that is not often the case. So which is right? The only way to be sure is to use your multimeter, switched to the Ohms range. We'll come back to this point later. To help make it easier for you to recognise resistors with four colour bands, we have listed out all the available resistors in Table 2, for the E12 series. We'll explain what E12 means in a moment. These are the values that you will find readily available from most electronic parts suppliers. MARCH 1989 11 Table 3: 5-Band Resistor Colour Code A B C D E ( 111 II II II ~I 111 ) Band A B C D E Colour Hundreds Tens Units Multiplier Tolerance Black Brown Red Orange Yellow Green Blue Violet Grey White Gold Silver 0 1 2 3 0 1 2 3 0 1 10 100 1k (= 1000) 1 Ok (= 10,000) 1 00k (= 100,000) 1 M (= 1,000,000) 1 OM(= 10,000,000) 2 3 4 4 4 5 6 5 6 5 7 8 9 7 8 9 6 7 8 9 Table 2 gives the colour codes for 97 different resistor values, from o.rn to 10MO. Note that we've listed the values in the conventional way down the lefthand side of the table and have used the nondecimal (IEC 62) method down the righthand side. So if look on the lefthand side of the table for a value such as 5.6k0, you'll see it listed as 5k6 on the righthand side. This will help you to identify colour codes no matter how the resistors are specified on a circuit diagram. Having boggled on the 4-band colour code, consider the 5-band code, as shown in Table 3. The first three bands give the three most significant figures in the value, followed by the fourth band as the multiplier and then the fifth band being the tolerance. A couple of examples will suffice to show that the 5-band system is merely an extension of the 4-band system. Consider a 10k0 1 % resistor. In the 4-band code, it would have a code of brown, black, orange, brown. In the 5-band code, it goes brown, black, black, red, brown. A 33k0 1 % resistor will have a code orange, orange, black, red, brown. Even those who are fully familiar with the four band code will sometimes stumble with the five band code so if you're having trou12 SILICON CHIP 1% 2% 0.5% 0.25% 0 .1% 0 . 1 (divide by 1 0) 0.01 (divide by 100) ble grasping resistor codes, don't worry - you're not the only one. E-series Before you go too far in the fascinating pursuit of electronics, you're going to come up against the E-series. For many years, there was only one series of resistor values and it used to cater for most design needs. Now called the E12 series, it progresses as follows: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82. From there the series repeats but with a multiplier of ten, for the next decade. Hence: 100, 120, 150, 180, 220 and so on up to 820. In a given decade of resistance, say from 1000 to lkO, the E12 series gives 12 values, as just described. Let's just explain that point about decades of resistance further. A decade of resistance is a series of values which increases by a factor of ten. The range of resistance values commonly used in modern electronics circuits ranges over more than eight decades from o. rn to over 10MO. For the E12 series, the eight decades are as follows: o.rn to 0.820 rn to 8.20 100 to 820 1000 to 8200 lkn to 8.2k0 10k0 to 82k0 1ooko to 82oko lMO to 8.2MO. Every one of the E12 series values is shown in Table 2. Now this series does meet a wide variety of design needs but what about when the design calls for a value just about half way between one of the values in the E12 series? Say the designer needs a value of 200, which is half way between 180 and 220. For this requirement, the designer picks a resistor from the E24 range. Instead of 12 possible values per decade, the E24 range gives 24 values: 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27,30, 33,36,39,43,47,51, 56, 62, 68, 75, 82 and 91. From there, the series repeats with a multiplier of 10. Hence, 100, 110, 120, 130, 150, 160 and so on. We've shown only a portion of two decades here, from 100 to 1600 but the E24 series spans the same range of resistance values as the E12; ie, from o.rn to above lOMO. Notice that every value in the E12 series is included in the E24 series. Sometimes though, the range of values available from the E24 series is not enough. Designers want more. For these occasions there are the E48 and E96 series. As you might expect, the E48 series gives 48 possible values in a decade of resistance while the E96 range gives 96 values per decade. We've set out the E48 and E96 series in Table 4. Notice that each value has three significant figures plus the multiplier - this is why resistors with five colour bands are necessary. Again, if you look through the values in Table 4 you will notice that not all the values in the E24 series are included in the E48 and E96 series. This isn't normally a problem for two reasons. First, you can always get a value in the E96 range which is pretty close to the wanted value in the E24 (or E12} series. Second, most manufacturers of precision resistors make both the E24 and E96 series in any given type. This does not always apply but it usually does. There is a problem with the E24 and E96 series though and that is that very few parts stockists will keep the whole Table 4: E48 and E96 Series (One Decade Shown) E48 E48 E96 E96 E48 E96 100 105 110 115 121 127 133 140 147 154 162 169 178 187 196 205 100 102 105 107 110 113 115 118 121 124 127 130 133 137 140 143 147 150 154 158 162 165 169 174 178 182 187 191 196 200 205 210 215 226 237 249 261 274 287 301 316 332 348 365 383 402 422 442 range. So if the circuit you are building specifies values from the E24 or E96 range you may have to search out a supplier who has them in stock. Just as a matter of interest, there is also an E192 series. This has 192 different values per decade. It includes all the values from the E12, E24, E48 and E96 series but it is used only for very high precision resistors. These are normally only available by special order from electronics manufacturers. Tolerance We've already mep.tioned tolerance on resistors but it needs some explanation. Resistors are commonly made these days in the following tolerances: 10%, 5%, 2% and 1 %. Much higher precision resistors are made to tolerances of 0.5 % , 0.25% and 0.1 % and are used, for example, for the range multiplier 215 221 226 232 237 243 249 255 261 267 274 280 287 294 301 309 316 324 332 340 348 357 365 374 383 392 402 412 422 432 442 453 464 487 511 536 562 590 619 649 681 715 750 787 825 866 909 953 464 475 487 499 511 523 536 549 562 576 590 604 619 634 649 665 681 698 715 732 750 768 787 806 825 845 866 887 909 931 953 976 resistors in digital multimeters. However these precision resistors are not normally available "off the shelf" and have to be specially ordered from the manufacturers. On resistors with colour code bands, the tolerance is indicated with the fourth or fifth band; eg, gold for 5 % , red for 2 % and brown for 1 % . If you come across carbon resistors with only three bands, they are not only very old but they were made with a tolerance of 20%. On wirewound resistors where the values are normally printed on the bodies, the tolerance may be printed (eg, 10%) or, these days, may be indicated with a letter code. The letter tolerance codes are set out by a United States EIA standard (EIA stands for Electrical Industries Association). The letter code is as follows: M ..................... 20% K ...................... 10% J ......................... 5% G ........................ 2% F ......................... 1% D ..................... 0.5% C ................... 0.25% B ..................... 0.1% If you have a look at the photo of Fig.4 you will see that the wirewound resistors have a J or K printed on them to indicate a 5% or 10% tolerance. It is important to realise that the tolerance is a plus and minus limit on the nominal value of the resistor. So if you have lkO 5 % resistor it really means lkO ± 5%. This means that the true value of the resistor may be anywhere between 9500 and 10500. In practice, depending on how closely the manufacturer controls quality, the true values of 5% lkO resistors will tend to cluster quite closely to lkO. This can be handy to know in some situations. For example, if a circuit specifies a lkO 1 % resistor and you only have lkO 5 % resistors in your kitty, you may well be able to get by, provided you check the value on your digital multimeter. Using your multimeter OK, if you've stuck with us up till this point, you deserve a medal for perseverance. But what if you still feel that you will have great pro- 1 I Fig.7: this photo shows the resistance element of a wirewound resistor (right). This is encased in a fireproof ceramic "bathtub" at shown at left. MARCH 1989 13 blems making any sense of colour codes and therefore lack the confidence to put any electronic circuits together? And what if you are partially or totally colour blind? Well, don't let that stop you. This is where the digital multimeter really comes into its own. Instead of trying to fathom out the code just switch your digital multimeter to the appropriate "Ohms" range and whack the prods across the resistor. As quick as a wink the meter will display the value. No worries at all. And if you feel guilty about using a multimeter instead of being intimately aware of the resistor colour codes, consider these points. First, as we noted above, there, is the problem of reading resistor bands the right way, where the tolerance band is not gold or silver (which is the usual tip-off). Second, as resistors continue to get smaller for a given rating, it is becoming much harder to discern what the colours actually are, even if you are reasonably keen sighted. And with some brands of resistor it ca,n be very hard to distinguish between red and orange, or between green and grey. This particularly applies if the lighting is poor or if you are using fluorescent lights which give a different colour rendering. In these situations, it can be pretty well impossible to determine what the colour code is. In those situations, even the most experienced electronics practitioners have no qualms about resorting to their digital multimeters. We certainly don't have any such qualms and neither should you! Incidentally, you really do need a digital multimeter to check resistor values accurately. Analog multimeters are just not accurate enough. There is one trap to watch out for when you are measuring resistors, particularly those with high values. The tendency is to grasp one end of the resistor in each hand and hold it against the probe tips. In this situation the reading will not be accurate because the digital multimeter will be measuring your skin resistance as well as the resistor 14 SILICON CHIP Fig.8: this photo shows the correct way of measuring a resistor on a digital multimeter. Don't touch the resistor's leads with your hands, otherwise your skin resistance will upset the reading. the result will be lower than it should be. The way to avoid this trap is to make up a pair of very short leads for your multimeter. Fit a banana plug to one end of each lead and a crocodile clip to the other end. This will enable you to connect the resistor to the meter without having to hold the prods in contact. Our photo (Fig.8) shows the method. Before making the measurement, set the meter to the appropriate resistance range. This should be higher than the resistor to be measured otherwise you'll get an overrange or blank indication on the meter. Short the meter leads together and check that the meter reads zero. If it doesn't, jiggle the banana plugs in their sockets to make sure they are making good contact. Now connect the resistor and measure its value. ~