Fullscreen HTML5Med Res (??MB)HTML4

Electronics for Everyone Pt.2: What you need to know about capacitors • • • Most electronic circuits use one or. more capacitors. Next to resistors, they are the most common passive electronic component. And like resistors, they present big problems for beginners because of the different ways of labelling. By LEO SIMPSON A capacitor is a component which stores electric charge. Capacitors stop the passage of direct current (DC) but allow alternating current (AC) to pass though. The higher the frequency of the AC, the more easily it will pass through a capacitor of given value. And for a given AC frequency, the smaller the capacitor value, the less current will flow. There is a vast amount of electronic theory associated with the design and use of capacitors in circuits but we will not deal with any of that material here. This article is mainly devoted to recognising capacitor values on circuits and then recognising equivalent values on actual capacitors. That may seem simple but it isn't. The first working capacitors were glass jars with metal foil electrodes inside and outside. These were known as Leyden jars (because they were invented in Leyden, Holland). These days there is a great variety of capacitors but they all have two features in common: two electrodes or plates which are separated by an insulating medium known as the dielectric. While capacitors are not as common or as visible as resistors in everyday life, they are widely used in electrical and electronic appliances and in cars. Common examples of capacitors are the can units often mounted on electric motors. These are " motor start" or "motor run" capacitors. They are also used to suppress commutator sparking in brush-type electric motors, as the points condenser in automotive ignition systems and so on. The unit of capacitance The unit of capacitance is the Farad named after the eminent British physicist, Michael Faraday (1791 - 1867). The Farad is defined as the capacitance which will store one Coulomb of charge (equivalent to 6.24 x 10 1 9 electrons) at a potential of one volt. In practice, the Farad is an extremely large unit of capacitance although values of one Fa rad or more are available. Most capacitor values that you come a cross will be expressed in microfarads, nano- \ These capacitors all have a nominal value of O. lµF although they have different labels. Three have the obsolete "MF" or "MFD" label while several use the IEC code 104. One is labelled "ul" while another is "100n". 0.47uf INPUT-1111-----+--I Fig.1: this circuit has two capacitors, one at the input and one at the output. farads or picofarads. In old textbooks on electricity you might come across the terms condenser or capacitator. The term condenser is an obsolete term for capacitor while capacitator was, and sometimes still is, just a mispelling. Recognising capacitors on circuits Fortunately, capacitors are always easy to recognise on circuits as they are drawn virtually the same way around the world. They are depicted as two electrodes with no connection between them. Fig.1 shows a circuit with two capacitors, one at the input and one at the output. With capacitors used like this, you would say the circuit was AC-coupled (meaning it will not pass DC signals) or capacitor-· coupled. Sometimes capacitors are shown on circuits with a " + " sign next to one electrode or with one electrode drawn as an outline and the other solid. These are symbols for polarised electrolytic capacitors, a broad class of capacitors which we will deal with in the next episode in this series. Fig.2 shows a circuit with two electrolytic capacitors in it. The '' + '' sign next to one electrode indicates the polarity of the capacitor. The capacitor must always GNO 470uF 16VW + _ 10uf 16VW + _ Fig.2: the two electrolytic capacitors in this circuit have their polarity shown with plus and minus signs. Fig.3: another way of showing electrolytic capacitors with the negative electrode in solid black. /\ l'lllL '1989 5 . These are epoxy dipped flat metallised polyester capacitors, commonly known as greencaps. Note that conventional and IEC labelling is used to show the capacitance and tolerance. These very small O.lµF capacitors, contrasted with the greencaps, are monolithics. These are multilayered ceramic capacitors mainly used for bypassing. be connected into circuit so that the positive electrode is always more positive (in voltage) than the negative electrode. Fig.3 shows the same circuit as Fig.2 but with the outline symbol depicting the positive electrode of the capacitors. Types of capacitor There are many different types of capacitor and they are usually ref erred to by the name of their dielectric. For example, a ceramic capacitor has a ceramic dielectric while a polyester capacitor has a dielectric of polyethylene terephthalate, or polyester for short. A mica capacitor has sheets of mica as the dielectric. 6 SILICON CHIP In this episode. we shall concentrate on film and ceramic capacitors. Let's list some common film capacitors: polyester (also known as polyethylene terephthalate or PETP), polycarbonate, polypropylene, polystyrene, paper and mixed dielectric (eg, paper and polyester). Polyester capacitors are also sometimes referred to as Mylar capacitors. Then we have to split film capacitors into two more classes: metallised film and film/foil. The metallised film type is probably the most widely used capacitor today but the film/foil type is still used in very large numbers. The film/foil capacitor is also the easiest to understand. It is made by winding two long pieces of thin metal foil (usually tin or aluminium) and two plastic films (these days usually of polystyrene). The two films are wider than the metal foils (plates) and so fully isolate them from each other and from any external contact. If you have a look at a polystyrene capacitor you can easily see their wound construction. Naturally, there is a lot more to making any capacitor than just winding plastic films and metal foil together but we don't plan to go into the details here. Suffice to say that chemistry and metallurgy have a very large part to play in the manufacture of today's extremely reliable capacitors. In a metallised film capacitor, the two metal foil electrodes are missing. Instead, there are just two plastic films and these have a very thin layer uf metal vapour deposited on one side. This metallisation takes the place of the foils in film/foil capacitors. Because separate foils are not used, metallised film capacitors are always smaller than film/foil capacitors of the same capacitance and voltage rating. These days metallised film capacitors are the general purpose types while film/foil capacitors are used in more specialised applications. Ceramic capacitors Almost all ceramic capacitors are made from a disc or square of barium titanate with an electrode of silver screen-printed on each side. A wire lead is soldered to each electrode and the whole assembly is dipped in an epoxy mixture to seal it from the effects of the atmosphere. Ceramic capacitors are mainly used in radio frequency (RF) circuits, such as is found in radios, TVs, VCRs, CB radios and so on. By the way, we are talking about fixed capacitors here, meaning that the value of capacitance is not adjustable or variable through a given range. Values of capacitors As mentioned above, the Farad may be the standard unit of capacitance but it is a seldom used value. The capacitors you come across will have values in microfarads, nanofarads or picofarads. Now let us explain those prefixes. You've probably already come across the the prefix micro; it means millionth or 10- 6. So one microfarad is one millionth of a Farad. Nano is the prefix for 10- 9. One nanofarad is one thousandth of a microfarad. Pico is the prefix for 10- 12. One picofarad is one millionth of a microfarad. Having proceeded this far, we can also state that one nanofarad is equal to one thousand picofarads. Also, one microfarad is equal to one thousand nanofarads or one million picofarads. Abbreviations Rather than spell out the words micofarad, nanofarad or picofarads, we use abbreviations, just as we do with resistor values. Hence, · picofarad is pF, nanofarad is nF and microfarad is µF where the Greek symbol "µ" stands for millionth. Sometimes you may come across capacitors which are labelled in MF or MFD. These are now obsolete (and incorrect) abbreviations of microfarad but which still turn up on new capacitors. Until fairly recently, it was usual to express all capacitor values in terms of picofarads or microfarads; normally, all values above lO00pF were expressed in terms of microfarads. Nanofarads were not used. Now they are, in circuit dia grams, technical literature and on the capacitors themselves. However, the adoption of nanofarads has been far from universal and there tends to be a lot of confusion among enthusiasts when interpreting capacitor values. The situation has been made a lot more confusing because of the use of the IEC (International Electrotechnical Commission) labelling on circuits and EIA coding on capacitors. In the IEC labelling system, the multiplier letter is used instead of the decimal point [just as R. k or M can be used instead of the ,;"4*~~ ~-\ \4'ft.s~. Conventional film/foil capacitors have two plastic films and two metal foils wound tightly together and sealed. This photo shows polystyrene capacitors being wound at the Allied Capacitors plant, at Brookvale in Sydney. This is a selection of polystyrene capacitors. These quality capacitors are notable for their extremely high insulation resistance, typically more than 1 million megohms, or one Teraohm! decimal point in resistor values see the first episode in this series). So instead of labelling a capacitor value as 3.3pF on a circuit. the IEC label would be 3p3. For a 1.5µ,F capacitor, the IEC la bel is 1µ5. The issue becomes more complicated when nanofarads come into the picture. For example. a capacitor with a value of .0012µ,F may be specified as 1200pF or 1.2nF. Its IEC la bel is l n2. Similarly. a .OlµF capacitor can be specifed as lOnF and its IEC label is 10n. A 0. lµF capacitor can be specified as lO0nF and its IEC label is 100n. You ma y even see it labelled as ril. On SILICON CHIP circuits and on those in many othl'lr magazines. it is normal practi ce to omit the "riF"' from the va lue. la rgely for clarity and to sa ve space. For small capacitors expressed in picofarads. , \1'1111. rn1rn 7 manly used capacitors these days are metallised polyesters . Typically, these are finished with a green epoxy resin and so they are commonly ref erred to as greencaps. Another capacitor you might hear about is the monolithic. These are very tiny multilayer ceramic capacitors which are coming into fairly widespread use. They are commonly blue in colour and so are often ref erred to as skycaps although colours other than blue are also used. EIA codes These are metallised paper capacitors specially chosen for their "self healing" properties on 250VAC. These are labelled 4n7 and 4700pF. "pF" is always included in the value. Capacitor jargon In normal conversation, or when buying capacitors at your local electronic parts shop, it is common to refer to a O. lµF capacitor as a "point one mike capacitor". Similarly, a lOµF capacitor would be referred to as a "ten mike capacitor" or "ten mike cap". It is also usual to refer to electrolytic capacitors as "electros", so a lOµF electrolytic capacitor would be referred to as a "ten mike electro". For lower value caps, it is usual to refer to the value directly. For example, a lOpF capacitor would be called a "ten picofarad capacitor". As noted above, the most corn- Now if you have absorbed all that, you are doing very well. But there is at least one more hurdle for the novice [and the well experienced) to overcome before they can be sure of interpreting capacitor values correctly. This hurdle is the EIA code. EIA stands for Electrical Industries Association and is a US organisation. The EIA code uses 3 digits to specify the capacitor value, together with letters to specify tolerance and voltage rating. Fig.4 shows the general scheme, with two capacitors depicted. 220pF 'j:1Q¾ .047uf ±5%1 HH11:::;JI~i l.!...__ MULTIPLIER L__ TOLERANCEEIA CODES Fig.4: most capacitors using the EIA code will be labelled like these. The letter at the end indicates the tolerance. If they don't have a letter they can be assumed to be ± 20%. t I f f \ \ , I l \ II These low voltage ~eramic capacitors are used in RF circuits. Those labelled NPO or with a black cap have zero or very low temperature coefficient. 8 SILICON CHIP The first two digits give the two most significant figures in the capacitor value while the last digit is the multiplier [to the base 10), with the whole value expressed in picofarads. To give a few examples: (1) 150pF = 15 x 101 (2) .0033µF = 3300pF = 332 = 151 = 33 x (3) .068µF = 68,000pF = 68 x 103 = 683 102 ~ ******** ~ DON'T BE A FOOL THIS APRIL M~\TI.~~~;~i;~~ \'r'"' DUAL TRACE OSCILLOSCOPE * • • • • 1 YEAR WARRANTY , - - . _ . . J • * !if~LACEHENT ~ ~t s125.oo rIp29 . . $1.35 TIP32c . $1.70 rIp30 .. . $1.35 TIP122 .. $1.95 rIp31c .. $1.10 TIP127 .. $1.95 WELLER SOLDERING STATION WTCPS ADVANCED TEMPERATURE CONTROL SYSTEM *Complete with Iron , PTA? tip , Sponge , Tip storage tray and Transformer $1 24.50 SAVE$25.00 ~ SOL:e:~~!CKER • Complete with Blow Torch, Hot Blow , Hot Knife and 2 4mm soldering tip. • Tip temperature adJustable up to 400 C • Equivalent to 1 0-60 Watts • Average working time. 90 minutes • Powered by standard butane gas :ighter fluid NOW ONLY $ 71 .95 save m POWER TRANSISTORS NOW ONL v -· +% - ~ FREE! with every purchase receive a quality soft case s769.oo PORTASOL "PRO" IRON KIT ~ SAVE $20.00 SAVE $237.00 April bonus receive FREE! a quality 3.5 digit digital multimeter Diode forward voltage test Transistor hfe test Audible continuity Logic level test 2000M ohm range 20 amp AC/DC DC V, AC V, DC A, AC A, and Ohm NOW A maximum sens1t1v1ty of 1 mV/DIV (X5 mag .) Modes CH1 , CH2 , Dual , CH1 + I -CH2 , X-Y Choice of Sweep Displays, Auto , Normal and Single shot. Vertical Trigger Mode and Variable Hold-Off for ease of use. Sens1t1vity 5mV/DIV to 5V/DIV, DC to 20MHz T1mebaGe. 20ns/DIV to O 5s/DIV NOW ONLY ******* 10% Spare PORTASOL "PRO" tips * 1 mm soldering tip • 2 4mm soldering tip • 3 2mm soldering tip • 4 8mm soldering tip TQRX SCREWDRIVER SET 0 * Complete with five (5) screw tips; T-15 , T-20 , T-25 , T-27 , T-30 Magnetic tips. POWER SUPPL y 3 v, 4 5 v, 6 v, 7 _5 v, gv, 1 2v <at> 5 oomA ONLY $36.95 SCOPE ACCESSORIES We stock a wide range of SCOPE accessories for MINISCOPE, SUPERSCOPE, SCOPE CORDLESS, Table 1: Capacitor Markings in the E12 Series Value Alt value IEC value EIA code Value Alt value IEC value 10pF 12pF 15pF 18pF 22pF 27pF 33pF 39pF 47pF 56pF 68pF 82pF 10p 12p 15p 18p 22p 27p 33p 39p 47p 56p 68p 82p 10K 12K 15K 18K 22K 27K 33K 39K 47K 56K 68K 82K .01µF .012µF .015µF .018µF .022µF .0271,tF .033µF .0391,tF .0471,tF .0561,tF .0681,tF .0821,tF 10nF 12nF 15nF 18nF 22nF 27nF 33nF 39nF 47nF 56nF 68nF 82nF 10n 12n 15n 18n 22n 27n 33n 39n 47n 56n 68n 82n 100pF 120pF 150pF 180pF 220pF 270pF 330pF 390pF 470pF 560pF 680pF 820pF 100p 120p 150p 180p 220p 270p 330p 390p 470p 560p 680p 820p 101K 121K 151K 181K 221K 271K 331K 391K 471K 561K 681K 821K 0 .1µF 0 .12µF 0.151,tF 0 .181,tF 0 .221,tF 0 .271,tF 0 .331,tF 0.391,tF 0.471,tF 0 .561,tF 0 .681,tF 0.82µF 100nF 120nF 150nF 180nF 220nF 270nF 330nF 390nF 470nF 560nF 680nF 820nF 100n 120n 150n 180n 220n 270n 330n 390n 470n 560n 680n 820n 1n 1n2 1n5 1n8 2n2 2n7 3n3 3n9 4n7 5n6 6n8 8n2 102K 122K 152K 182K 222K 272K 332K 392K 472K 562K 682K 822K 1.0µF 1 .2µ,F 1.5µF 1 .8µF 2 .2µF 2 .7µF 3 .3µF 3 .9µ,F 4.7µF 5 .6µ,F 6.81,tF 8.21,tF 1 O.OµF .0011,tF .00121,tF .0015µ,F .0018µ,F .00221,tF .0027µ,F .0033µF .0039µF .0047µ,F .0056µF .0068µF .0082µF 1nF 1.2nF 1.5nF 1.8nF 2 .2nF 2 .7nF 3.3nF 3 .9nF 4.7nF 5.6nF 6.8nF 8.2nF (4) 0.47µF = 470,000pF = 47 x 104 = 474 Values from lOpF to 82pF are a special case in that they have no multiplier so they just have a two digit code. For example, a 56pF capacitor just has the code 56. After a while you stop thinking about adding a number of zeros to get a value in pF. Instead, you will eventually recognise any three digit code ending in '1' as applying to capacitors from lOOpF (101) to 820pF (821); any code ending in '2' as applying to capacitors from .OOlµF or lnF (102} to .0082µF or 8:2nF (822}; any code ending in '3' as applying to capacitors from .011,tF (103) to .082µ,F (823); any 10 SILICON CHIP EIA code (10% tolerance) (10% tolerance) 1u0 1u2 1u5 1u8 2u2 2u7 3u3 3u9 4u7 5u6 6u8 8u2 1 Ou 103K 123K 153K 183K 223K 273K 333K 393K 473K 563K 683K 823K 104K 124K 154K 184K 224K 274K 334K 394K 474K 564K 684K 824K 105K 125K 155K 185K 225K 275K 335K 395K 475K 565K 685K 825K 106K Large capacitors with a value of more than 1µ.F, such as these 4.7µ.F units on the left or the 250V AC units on the right, are usually metallised polypropylene . code ending in '4' as applying to capacitors from 0.1µ,F (104) to 0.82µ,F (824) and so on. Tolerance Combined with the 3-digit code is usually a letter to indicate the tolerance. Most capacitors sold over the counter these days have a tolerance of ± 10 % and the letter to indicate this is K. The letter tolerance code is similar to that for resistors but there are significant differences. For capacitors of more than l0pF, the tolerances are as follows: z ................... - 20 % , + 80% Y .............. ..... - 20 %, + 50% w ................. -20 %, +40% P.. ................. - 0 %, + 100% N .................. ± 30% M .................. ± 20% L ............. .. .... ± 15 % K .......... ......... ± 10 % J.................... ± 5% G................... ± 2% F ................... ± 1% What about the colour code for film capacitors? In this article on capacitors there is no mention of the colour code, as applied to dipped plastic capacitors made by Philips. This has been deliberate because the capacitor colour code appears to have fallen into disuse as far as film capacitors are concerned. 1 5 or 20 years ago when colour code capacitors were commonplace, they were nicknamed "licorice allsorts" because of their striped appearance. Since it is possible that you will still come across colour coded capacitors from time to time , you will want to know what the code is. Basically, it is very similar to the resistor colour code. The bands run from the top of the capacitor down towards the leads and the first two colours are the first two digits in the capacitor value. The third band is the multiplier, giving the value in picofarads. For example, a capacitor with the first three bands reading yellow, violet, red has a value of 4700pF, or .0047µ,F. The fourth band is the tolerance band with black being 20% and white 10%. Finally, the fifth colour band is the voltage rating, as follows: brown , 1 00VDC; red, 250VDC; yellow, 400VDC and blue, 630VDC. For capacitors of less than lOpF, the tolerances are as above except for those listed below: G.......... ......... ± 2pF F.. ................. ± lpF D................... ± 0.5pF C.... ... ............ ± 0.25pF In practice though, you will find that the vast ma jority of film capacitors sold over the counter in Australia are of 10 % tolerance and so will have the letter K following the 3-digit code. To help you recognise the EIA code, we have listed all values in the E12 series (see last episode for an explana tion of E-series) from l0pF to 10µ,F in Table 1. In the first column of Table 1 you'll see capacitor values as normally shown in SILICON CHIP and a number of other magazines. In the second column is an alternative equivalent value, where appropriate, in nanofarads. In the third column is the IEC label, as mostly found on circuits of European · origin. Finally, the fourth column shows the EIA code with the letter K added to show that the capacitor has 10 % tolerance. In the full EIA code system there are many other letters to designate capacitor type, application, voltage Each one of these capacitors has a value of 1µF and they are all labelled differently. All are currently .available except the Ducon polyester unit. rating, temperature coefficient and so on but they seldom. appear on general purpose capacitors sold over the counter. We won't go into more detail to avoid adding confusion. Nor have we covered every possible permutation that is possible in labelling and depicting capacitors; they are too numerous to list them all. However, having carefully read this article, you should be able to recognise and interpret any capacitor value you come across from now on. These days many digital multimeters will measure capacitance. So, if you can't be sure of a capacitor's value, check it with your meter. This is a good idea too because it can save installing a faulty unit into circuit. ~ APRIL 1989 11