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Electronics for Today and Tomorrow (Components & Circuits)

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  COMPONENTS AND CIRCUITS 9 RESISTORS CHECKLIST After studying this chapter you should be able to:ã state the resistance of a resistor from the colour code or printed code,ã state the tolerance of a resistor from the colour code or printed code, and hence calculate the maximum and minimum resistances possible, and ABOUT RESISTORS Basically resistors are used to limit the current in circuits. When choosing one, three factors need to be considered, apart from the value. (i)(ii)(iii) FIXED RESISTORS Three types are shown in Fig. 9.1. The table shows their  properties.Since exact values of fixed resistors are unnecessary in most electronic circuits, only certain  preferred values  are made. The number of values depends on the tolerance. Fig. 9.2 shows the values required to give maximum cov-erage with minimum overlap for tolerances of ±5% and ±10%. For example, a 22 ft ±10% resistor may have any value between (22 + 2.2) ~ 24 FI and (22 —2.2) ~ 20 ft. The next higher value of 27 ft (for ±10% tolerance) more or less covers the range 24 ft to 30 ft. select an appropriate preferred value from the El 2 series of resistors. a The tolerance:  exact values cannot be guaranteed by massproduction methods but this is not a disadvan-tage because in most electronic circuits the values of resistors are not critical. A resistor with a stated (called nominal) value of 100 ft and a tolerance of ± 10% could have any value between 90 and 110 ft. The  power rating:  this is the maximum power which can be developed in a resistor without damage occurring by overheating. For most electronic cir-cuits 0.25 or 0.5 W ratings are adequate. The greater the physical size of a resistor the greater is its rating. The stability:  this is the ability to keep the same value with changes of temperature and with age. metal filmwirewoundTypeProperty'' \^^CarbonfilmMetalfilmWirewoundMaximum value10 MOlOMft4.7 knTolerance±5%±1%±5%Power rating0.25-2 W0.6 W2.5 WStabilitygoodvery goodvery goodUsegeneralaccurate worklow values 20  9 RESISTORS Preferred values multiplex 10) for tolerance —  5% ±10%Els series)(E12 series)1.01.01.11.21.21.31.51.51.61.81.82.02.22.22.72.73.0 9.2 ±5% (E24 series)±10% (E12 series)3.33.33.63.93.94.34.74.75.15.65.66.26.86.87.58.28.29.1 tors with ±10% tolerance belong to the El2 series as 12 basic values). Those with ±5% tolerance form '24 series.the first digit. The second   band from that end gives the second digit. The third   band in the  four band code  gives the multiplier (or the number of Os to be added), but it gives the third digit (often 0, i.e. black) in the  five band    code.  In the latter system the multiplier is given by the  fourth  band. The five band code tends to be used with higher precision resistors, e.g. tolerance ±1%.In both systems the colour of the band on its own at the other end gives the tolerance. If this band is missing on the resistor, the tolerance is ±20%. Printed code The code is printed on the resistor and consists of letters and numbers. It is also used on circuit diagrams and on variable resistors. The examples in the table below show how it works. R means Xl, K means X103and M means XlO6, and the position of the letter gives the decimal point.Tolerances may be indicated by adding a letter at the end: F = ±1%, G = ±2%, J = ±5%, K = ±10%, M = ±20%. For example 5K6K = 5.6 kfl ± 10%. RESISTANCE CODES : :id colour codes In this method the resistance  _e and tolerance are shown by either four or five  urred bands round the resistor, the latter giving the  j - more exactly. The way both systems work is shown ~ rr.e examples in Fig. 9.3.. ~e  first   band to read is the one at the end of the resis r.ere the bands are closer together; its colour gives _U  JJJ   -0 o°S  o > > £ O) 4 7 00 ±5% = 4700 a  = 4.7k±5%  \ND CODE 'digit : slack    ' crown ! red   : :range I .ellow I green I olue . iolet ! grey ; whitel ---- <—I2nd digit3rd digitMultiplier0 black0 black-11brown1brown-102 red2 red~r1003 orange3 orange■V10004 yellow4 yellow-100005 green5 green—1000006 blue6 blue10000007 violet7 violetI8 grey8 grey0.1 gold9 white9 white0.01 silver band   code 9.3 Tolerance±1% brown ±2% red:5% gold : 10% silver :20% none T> 10 0 000 : c S) C o JPcc -Q.Q-Q O O  _Q Value 0.27 n3.3 aion  220  n Printed code R273R31OR220R Value  1 kn68 knioo kn4.7 Mn Printed code 1K068K100K4M7 VARIABLE RESISTORS Rotary type A rotary variable resistor is shown in Fig. 9.4a, and its construction in Fig. 9.4b. Connections are made to each end of the track by terminal tags A and B. Tag C is connected to the sliding contact or wiper which varies the resistance between C and the end tags when it is rotated by the spindle. For power ratings up to 2 W, the track is made either of carbon or of cermet (ceramic and metal oxide); above 2 W it is wirewound. 1% = 100000 a  = 100 k± 1% Fig. 9.4 21  COMPONENTS AND CIRCUITS If the track is ‘linear’equal changes of resistance occur whenthe wiper is rotated through equal angles. In a ‘log’track the resistance change for equal angular rotations is greater at the end of the track than at the start. Common values are 10 kft, 50 kft, 100 kft, 500 kft and 1 MO.The symbol for a variable resistor of this type is shown in Fig. 9.4c. Preset type Two preset variable resistors are shown in Fig. 9.5a, b. These have carbon or cermet tracks and when adjustment is necessary, a screwdriver is used. Ratings vary from 0.25 to 1 W. Fig. QUESTIONS 1. a) What is the value and tolerance of R,, R2and R, shown by the  four band   colour code in the table  below? Band1234 R] brownblackredsilver r   2 yellowvioletorangegold R 3 greenblueyellownone b) What is the value and tolerance of R4, R5and R6 shown by the  five band   colour code in the table  below? Band12345 R*  brownblackblackbrownbrown R 5 orangeorangeblackblackgold Rb greenbrownblackredred 2. a) What is the  four band   colour code for the following: (i) 150 ft ±10%, (ii) 10 ft ± 5%,. (iii) 3.9 kft ± 10%, (iv) 10 kfl ± 1%,(v) 330 kft ± 2%, (vi) 1 Mft ± 10%? b) What is the  five band   colour code for the following: (i) 160 ft ± 2%, (ii) 2.4 kft ± 5%,(iii) 750 kft ± 1%? 3. What is the value and tolerance of a resistor marked (i) 2K2M, (ii) 270KJ, (iii) 1M0K,(iv) 15RF? 4.  What is the printed code for the following:(i) 100 ft ± 5%, (ii) 4.7 kft ± 2%,(iii) 100 kft ± 10%, (iv) 56 kft ± 20%?5. What El 2 preferred value would you use if you calculated that a circuit needed a resistor having a value of (i) 1.3 kft, (ii) 5.0 kft, (iii) 72 kft,(iv) 350 kft?  10 CAPACITORS 10 CAPACITORS I orging When connected to a battery, Fig. 10.1, the  pcs '/re of the battery attracts electrons from plate X and : negative repels electrons to plate Y. Positive chargelnncit of electrons) builds up on X and an equal negative  rge ^excess of electrons) builds up on Y. During the .  ng, there is a brief flow of electrons round the cir —: m X to Y. Charging stops when the p.d. between X i t  .;  : equals (and opposes) the e.m.f. of the battery. The i takes time, i.e. the response of a capacitor to a ce of p.d. is not immediate, r the connections to the battery are removed, the *ze may take a long time to leak away from the capac  _rdess a conductor is connected across it. Fig. 10.1 C is large when the area of the plates is large, the plate separation is small and certain dielectrics are used. Energy Stored A charged capacitor stores electrical energy. For charge Q and p.d. V, the energy W   stored is:W = IQV = h cv1 (since Q = VC)where W   is in joules if Q  is in coulombs and V in volts.In a photographic flash unit a capacitor discharges through a lamp and its energy is changed to heat and light.I iDOcitance The capacitance  C of a capacitor mea «r its chargestoring ability. It is 1  farad   (F) if it stores a :ge of 1 coulomb when the p.d. across it is 1 volt. If 1  : charge is 6 C when the p.d. is 2 V, then = 6 C/2 V = 3’F. In general, for charge Q and p.d. V,r _ Q „   C- iier more convenient units are:VC1 microfarad (pF) = 10~6 F 1 nanofarad (nF) = 109 F 1 picofarad (pF) = 10“12F PRACTICAL CAPACITORS When choosing a capacitor two factors need to be consid-ered, apart from its value and tolerance. (i)  The voltage rating:  this is the maximum voltage (d.c. or peak a.c.) it can withstand before the dielectric  breaks down (it is often marked on it). (ii)  The leakage current:  no dielectric is a perfect insula-tor but the loss of charge by leakage through it should be small. Fixed capacitors  Nonpolarized types (H |) can be connected either way round. Polarized types (+|] |) have 23
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