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Table 3.1 Survey of resistor typesType Ohmic Range Power range Tolerance Tempco range Manufacturers Applications CostCarbon film 2.2 - 10M 0.25 - 2W 5% -150 > -1000ppm/˚C Neohm, Rohm, General purpose/ < 1pPihercommercialCarbon composition 2.2 -10M 0.25 - 1.0W 10% +400 > -900ppm/˚C Neohm, VTM, Welwyn Pulse, low inductanceMetal film (standard) 1 - 10M 0.125 - 2.5W 1%, 2%, 5% +/-50 > 200ppm/˚C BC, Neohm, Vishay, General purpose 1 - 3pPiher, Rohm, Welwyn industrial & militaryMetal film (high ohm) 1M - 100M 0.5 - 1W 5% +/-200 > 300ppm/˚C BC, Neohm High voltage & special 5 - 20pMetal glaze 1 - 100M 0.25W 2%, 5% +/-100 > 300 ppm/˚C Neohm Small size 5pWirewound 0.1 -33K 2 - 20W 5%, 10% +/-75 > 400ppm/˚C BC, Welwyn, CGS, High Power 15 - 50p(0.01 . . . . . . . . . . 10W - 100W aluminium) VTM, Erg 50p - £1 (al)Metal film (precision) 5 - 1M 0.125 - 0.4W 0.05 > 1% +/-15 > 50ppm/˚C BC, Welwyn, Precision 10p - 50pBeyschlagWirewound (precision) 1 - 1M 0.1W - 0.5W 0.01 > 0.1% +/-3 > 10ppm/˚C Rhopoint, Extra-precision £2 - £20VishayBulk metal (precision) 1 - 200K 0.33 - 1W 0.005 > 1% +/-1 > 5ppm/˚C Vishay, Extra-precisionWelwynResistor networks 10 - 1M 0.125 - 0.3W 2% +/-100 > 300ppm/˚C Bourns, Dale, CTS, Multi-resistor 10 - 35p& arrays per element +/-50ppm/˚C tracking BeckmanSM chip film resistors 0 - 10M 0.1 - 0.5W 1%, 2%, 5% +/-100 > 200ppm/˚C BC, Welwyn, Rohm, Surface mount, hybrids 0.2p - 2pMurata, Neohm, VishayNotes:1) This survey does not consider special types 2) Manufacturers quoted are thosed widely sourced in the UK at time of writing 3) Quoted ranges are for guidance only4) Costs are typical for medium quantitiesPassive components 71
72 The Circuit Designer’s Companionrunning close to the rated power of the part it will be necessary to confirm that your paddesign agrees with the manufacturer’s recommendations.Common, standardised chip sizes are shown in the table in Figure 3.1. You can alsoget metal film chip resistors for higher performance applications, but these are moreexpensive than the common thick film.The resistive ink technique used for chips can also produce standard axial-leadresistors (metal glaze) of small size, and can be used directly onto a substrate togenerate printed resistors. This technique is frequently used in hybrid circuits and isvery cost-effective especially when large numbers of similar values are required. It ispossible to print resistors directly onto fibreglass printed circuit board, though the resultis of very poor quality and cannot be used where a stable, predictable value (comparedwith conventional types) is required.Metal filmThe next most common type is the metal film, in its various guises. This is the standardpart for industrial and military purposes. The most popular varieties of leaded metalfilm are hardly any more expensive than carbon film and, given their superiorcharacteristics, particularly temperature coefficient, noise and power handling ability,many equipment manufacturers do not find it worthwhile to bother with carbon film.Variants of the standard metal film cater for high or low resistance needs. The“metal” in a metal film is a nickel-chromium alloy of varying composition for differentresistance ranges. A film of this alloy is plated onto an alumina substrate. For leadedparts, the end caps and leads are force-fitted to the tubular assembly and the resistanceelement is trimmed to value by cutting a helical groove of controlled dimensions in it,which allows the same film composition to be used over quite a wide range of nominalvalues. The whole part is then coated in epoxy and marked. The disadvantage of thehelical trimming process is that it inherently increases the resistor’s stray inductance,and also limits its pulse handling capability (see sections 3.1.5 and 3.1.6).Chip metal film resistors use the same film-on-alumina technique but the trimming,if necessary, is done by cutting small, controlled segments out of the film element toincrease its resistance slightly.CarbonThe most common leaded resistor for commercial applications is the carbon film. It iscertainly cheap − less than a penny in quantity. It also has the least impressiveperformance in terms of tolerance and temperature coefficient, but it is normallyadequate for general purpose use. The other type which uses pure carbon as the resistiveelement is carbon composition, which was the earliest type of resistor but nowadaysfinds a use in certain applications which involve an assured pulse withstand capability.WirewoundFor medium and high power (>2W) applications the wirewound resistor is almostuniversally used. It is fairly cheap and readily available. Its disadvantages are its bulk,though this allows a lower surface temperature for a given power dissipation; and thatbecause of its construction it is noticeably inductive, which limits its use in highfrequencyor pulse applications. Wirewound types are available either with a vitreousenamel or cement coating, or in an aluminium housing which can be mounted to aheatsink. Aluminium housings can offer power dissipations exceeding 100W per unit.
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vi Contents2.2 Design rules 452.2.1
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viii ContentsChapter 5Analogue inte
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x Contents7.5.3 Secondary cells 256
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334 Indexde-rating 310dielectric ab
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336 IndexFlash ADC 209Foldback curr
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338 IndexMultilayer PCB constructio
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340 Indexlimiting element voltage 7
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