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l~ &J..y-tie--ea~auit&F-S-a~-e-thec~ acitor world, generally ha "" · han f il ,-0.11°0 . er,.a 1 _o, e_q : -1~~ - - - -.2d..... k re ..L. _ _ _ _ _ _ _ _ _ _ _..:.:,o:.:::oM::::H.:..;1P;.;,:.RE:::A::::MP...;A;;;;Mo::..·_;;_' UNITS DISPLAY 120'., Every DC power supply has one or more electrolytic capacitors. They are used wherever a large amount of capacitance is required in a small space. As already noted, they generally require a DC voltage to work properly but there are special versions which can be used with high AC voltages such as "motor start" and capacitors for 240V AC powered appliances. Well, why are these capacitors called "electrolytics"? The answer is because they contain an "electrolyte", a chemical solution through which an electric current can pass. To properly explain · the subject though, we'll have to back up a bit and repeat what we said in the first episode on capacitors. There, we stated that every capacitor has two electrodes or plates which are separated by an insulating medium known as the dielectric. And so they have. But in electrolytic capacitors, the method of construction is quite different and they involve a good deal of complex chemistry. Don't be put off though- we'll tell you just as much as you need to know, to avoid unnecessary confusion. The vast majority of electrolytic capacitors are based on aluminium foil. Inside electrolytic capacitors there are two aluminium foils 4 SILICON CHIP _LJ_ ~ 14 It over ED-8IMPSON - - - - - - , t - - - - - ----'1,-J __.. --------------+------4-.►+5V wound together but separated by an absorbent paper which is impregnated with a liquid electrolyte. Now that description might not sound all that different to the structure of a film/foil capacitor, as described in the previous episode in this series. But there is a radical difference because the impregnated paper does not perform the function of a dielectric - it is actually the negative electrode of the capacitor! Although there are two aluminium foils in an electrolytic capacitor, one is quite different from the other. The foil connected to the positive terminal of the capacitor, known as the 'anode foil', is deeply CATHODE FOIL Fig.1: inside an electrolytic capacitor. It has two aluminium foils which are wound together but separated by an absorbent paper impregnated with a liquid electrolyte. etched to greatly increase its surface area and thereby the capacitance. Secondly, the anode foil has a thin coating of oxide. Aluminium oxide is a very good insulator and it is this thin oxide coating on the anode film which actually provides the dielectric of the capacitor. This oxide dielectric is very much thinner than the film dielectric used in plastic capacitors and so th,is is another factor in the very high capacitance of electrolytic capacitors. So if the electrolyte is not the dielectric, what is its purpose? It actually provides the negative electrode of the capacitor. Since the electrolyte is a liquid (more correctly, a paste), it is in intimate contact with the deeply etched oxide surface of the anode foil and thereby allows the enormous surface area of the foil to fully contribute to the total capacitance. The other aluminium foil in the capacitor is called the "cathode film'' and it makes the electrical connection from the negative terminal to the electrolyte. The electrolyte is an organic solution, which often used to be glycol borate but nowadays is likely to be more complex, depending on the performance parameters the CATHODE LEAD ALUMINIUM CAN DISC COMPOSED OF PTFE HARO PAPER Fig.2: this cutaway drawing shows all the essential parts of an electrolytic capacitor. The anode foil is deeply etched to increase its surface area and thus the capacitance. The impregnated paper forms the negative electrode of the capacitor. ALUMINIUM FOIL ANODE WITH ALUMINIUM OXIDE DIELECTRIC ALUMINIUM i'OIL CATHODE manufacturer is striving for. Some of these more modern electrolytes are dimethyl acetamine, dimethyl formanide or butyrolactone. To summarise, an electrolytic capacitor has an anode film which is deeply etched to increase its surface area and then its surface is oxidised to provide the capacitor's dielectric. The electrolyte then provides the negative connection to the capacitor. Well, as you might expect, electrolytic capacitors are a great deal more complicated than this short description implies but this is adequate for the moment. To go deeper would take up a great deal more space. The real reason for taking so much space to describe the internal structure of an electrolytic capacitor is to make the point that it is quite different from other types of capacitors. ANODE LEAD Range of capacitance These vertical mount electrolytic capacitors range in value from 1250µF to 8000µF. Note the mounting clamps fitted to three of the capacitors. Electrolytics are commonly available over the counter in capacitances ranging from 0.47µF up to 10,000µF although they are manufactured in values as small as 0.lµF and as large as 1,000,000µF (1 Farad) for very large computer grade capacitors. In recent yea rs, another special type of pola rised capacitor known as a "super capacitor" or "double layer capacitor" has become available. These have extremely large values of capacitance, up to 1 Farad, in very small cases. But while these are electrolytics they do not use the same oxide dielectric principle as conventional aluminium electrolytic capacitors. We 'll co me to double layer capacitors later. Polarisation and voltage rating· Aside from their relatively large values of capacitance, the aspect which distinguishes electrolytics from other types of capacitor is the fact that they can only be operated with the correct polarity of DC voltage applied to them. That is a long-winded way of saying that they MAY 1989 5 :, JJ ·~•. , "Ii . i ' · ., . ·1 ' ' ' . ·,: V ; " ' ,·. - -\ These are PC-mount electrolytics, designed for direct mounting on a printed circuit board. The negative terminal is usually indicated by a minus symbol and an arrow printed on the side of the case. voltage ranges. Because electrolytic capacitors have become a great deal smaller and because their characteristics have become much more stable and reliable, it is unusual to find capacitors with a rating below 25V for the smaller values or below 16V for the larger values. It also used to be the case that for long life, electrolytic capacitors should be used in circuits at close to the rated voltage. If this did not happen, the capacitors would gradually deteriorate. Nowadays though, it is quite permissible to use a capacitor with only a fraction of its rated voltage applied to it. For example, you can use a lOOµF 16V capacitor with less than 1V applied to it. Indeed, this circuit situation is very common. However, it is still important that the DC voltage across the capacitor is of the correct polarity (ie, positive voltage to positive terminal). V and VW: what do they mean? Pigtail or axial lead electrolytics differ from PC-mount types by having a lead at each end. On these units, plus and minus symbols are marked on the case to indicate the positive and negative terminals. must have a positive DC voltage at their positive terminal. If the capacitor is operated with a reverse DC voltage, it will eventually fail and probably become a short circuit. Depending on the manufacturer, electrolytic capacitors are made with the following DC voltage ratings: 6.3V, 10V, 16V, 25V, 35V, 6 SILICON CHIP 50V, 63V, 75V, 100V, 160V, 200V, 350V and 450V. However, it is also possible to come across capacitors rated at 40V, 80V and so on. For a given value of capacitance, a capacitor rated at 75V will be much larger than one rated at 16V. Nowadays though, most parts wholesalers and retailers do not stock capacitors in all the above In SILICON CHIP and on most circuits, you will see electrolytic capacitors specified with a value and a voltage rating, such as 47µF 25VW. The value is straightforward enough but what is the meaning of "VW". VW stands for "volts working". Some capacitors are labelled "WV" which means exactly the same thing, "working volts". In some ways the VW designation is an anachronism, a holdover from the days when all electrolytic capacitors had two voltage ratings: VW and VP. VP stands for "volts peak " and is the surge voltage that the capacitor can withstand for short periods. The surge voltage is generally 20% to 30% higher than the rated voltage. It is related to the voltage used to "form" the oxide coating on the aluminium film and if it is exceeded, the capacitor is liable to fail within a very short period of time. These days most capacitors only have their rated voltage printed on them, together with their value and polarity marking. Lead types Often, you '11 see electrolytic capacitors referred to as PC-mount, vertical mount, pigtail types, axial lead or radial lead types. With the exception of the last term, all these are fairly descriptive. A pigtail type is a conventional small can capacitor with a lead at each end; these are also known as "axial lead". PC-mount types are those which have both leads coming out at one end so that they can mount vertically on a printed circuit board. These are also known as radial lead types. Of course, pigtail capacitors can be mounted on a printed circuit board too but vertical or PC-mount capacitors take up less board space. Leakage Non-polarised or bipolar electrolytics can be connected into circuit either way around. They are identified by an NP or BP label on side of the case. Polarity marking Since the voltage polarity across an electrolytic capacitor is so critical, it is important to be able to distinguish which is the positive terminal and which is the negative terminal. On most electrolytics these days, a minus symbol, and sometimes an arrow, is printed on one side of the case, nearest to the negative electrode. On the other hand, you may come across electrolytics where the positive electrode is indicated with an adjacent + symbol and there may also be an arrow to reinforce the message. On some pigtail electros, both the positive and negative electrodes may be labelled. On larger can type electrolytics, the negative terminal may be indicated with a dab of black paint. Alternatively, the positive terminal may be indicated with a dab of red paint or perhaps a + symbol moulded into the lid. Non-polarised electrolytics Having made the point above about the necessity for the DC voltage needing to be of the correct polarity, we will now muddy the water by stating that with some electrolytics, this is not a problem. These are "bipolar" or "nonpolarised" electrolytics. They are virtually two conventional electrolytics connected back-to-back inside a common can. They are made with two etched and oxidised (formed) anode foils. They can be used in circuits where the DC voltage is indeter- Compared to other types of capacitors, such as plastic or film, electrolytic capacitors have very poor insulation. In fact, it is so poor, relatively speaking, that instead of expressing the insulation resistance in terms of hundreds or thousands of megohms, it is usual to express it as "leakage current" in microamps or milliamps . Small value electrolytics, say with a value of 22µF or less, will typically have a leakage current of 10 microamps or less at the rated voltage. The larger can types, with a capacitance of lO00µF or more, will typically have a leakage current of 1 or 2 milliamps. Tantalum electrolytics Shown here larger than actual size, this NEC super capacitor has a value of .047 Farads and is rated at 5.5V. Values of up to 1 Farad are obtainable. minate (ie, might be polarised one way or the other) or where there is no DC voltage but quite substantial AC voltage. They are made in quite a wide range of DC voltage ratings although if you are buying them over the counter you will usually only be able to obtain·them with a rating of 50 volts. Non-polarised electrolytic capacitors are used where relatively large capacitors are needed, say up to lO0µF, and where the cost of alternative plastic or paper capacitors would be prohibitive. A typical application is in crossover networks for loudspeaker systems. On circuit diagrams, non-polarised capacitors are indicated with the label "BP" or " NP". They are also labelled this way on the can. Tantalum is an alternative metal to aluminium in electrolytic capacitors. Tantalum electros are in values up to lO0µF and with a restricted voltage (usually only 50V) range. Tantalum electros can be made in foil, wet sintered and solid types. Tantalum foil capacitors are similar to aluminium electrolytics in that the foil is anodised but the electrolyte is sulphuric acid. Wet sintered tantalum electrolytics have a sintered tantalum anode in an electrolyte of sulphuric acid and ionised water or a gelled electrolyte of sulphuric acid and silica. We mention tantalum foil and wet sintered tantalum foil capacitors for the sake of completeness but it is unlikely that such capacitors ever become available over the counter to enthusiasts. Most people will only come across the epoxy dipped solid tantalum electrolytics which are widely available and generally only slightly dearer than conventional aluminium electros. The solid tantalum capacitor again has a sintered tantalum anode. The porous anode pellet is impregnated with manganese nitrate and heated to 400°C. This decomposes the manganese nitrate to solid manganese dioxide which becomes the electrolyte. When they were first introduced to the market, about 20 years ago, solid tantalum capacitors had a MAY 1989 7 Table 1: Tantalum Capacitor Markings In the E12 Serles Value Alt value IEC value 0.1µF 0.12µF 0.15µF 0.18µF 0.22µF 0.27µF 0.33µF 0.39µF 0.47µF 0.56µF 0.68µF 0.82µF 1.0µF 1.2µF 1.5µF ··1.8µF 2.2µF 2.7µF 3.3µF 3 .9µF 4.7µF 5.6µF 6 .8µF 8 .2µF 10µF 12µF 15µF 18µF 22µF 27µF 33µF 39µF 47µF 56µF 68µF 82µF 100µF 100nF 120nF 150nF 180nF 220nF 270nF 330nF 390nF 470nF 560nF 680nF 820nF 100n 120n 150n 180n 220n 270n 330n 390n 470n 560n 680n 820n 1µ0 - number of advantages over aluminium electrolytics. These included better shelf life, lower leakage, wider operating temperature range (up to 125°C instead of 85°C), lower power factor and closer tolerance on value. Now, with the general improvement of aluminium electrolytics, these improvements are nowhere near as clear-cut. Low leakage (LL) aluminium electrolytics are comparable with tantalums as far as leakage is concerned and their general stability is just as good. Aluminium electrolytics are now also available (although not over 8 SILICON CHIP 1µ?. 1µ5 1µ8 2µ2 2µ7 3µ3 3µ9 4µ,7 5µ6 6µ8 8µ2 10µ 12µ 15µ 18µ 22µ 27µ 33µ 39µ 47µ 56µ 68µ 82µ 100µ EIA code (108/e tolerance) 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 126K 156K 186K 226K 276K 336K 396K 476K 566K 686K 826K 107K the counter at retailers) with operating temperatures up to 125°C with voltage derating by a third. Substituting for tantalum capacitors In general, you can substitute low leakage aluminium electros for tantalums provided they are not being used in an oscillator or timing circuit. In the latter case, the designer has probably specified tantalum not only for their low leakage but for their closer tolerance in capacitance value. Identifying tantalum capacitors These days most tantalum capacitors are labelled with their capacitance value, voltage rating and a + sign near the positive lead. However, it is quite likely that, unless you are really keen sighted, you will need a magnifying glass to read the labelling. Some brands also use the EIA code to indicate the capacitance value. This is the same code as shown in the previous episode on film and ceramic capacitors. Table 1 shows the code for capacitors ranging from O. lµF up to lOOµF. Tantalums are normally made with a tolerance of ± 10% as indicated with a K following the 3-digit EIA code, or ± 20% as indicated with the letter M. Some tantalums indicate the polarity with a vertical line near the positive lead, together with a tiny + sign. This could be confusing to the newcomer to electronics since some older pigtail electrolytics indicated the negative end of the can with a line around one end. You may also come across tantalum capacitors that are colour coded and with the polarity shown by a dot. You hold the capacitor with leads hanging down and with the dot facing towards you. The positive lead is then the one on the right. This is shown in the diagram of Fig.3. Be warned, the colour code for tantalum capacitors does not bear much similarity to that for resistors. So to identify tantalums, you should examine Fig.3 closely. Let's explain the tantalum colour code in a little more detail. First, hold the capacitor with the leads hanging downwards. The first two colours, reading from the top down, give the first two significant figures of the capacitance value (just as for the resistor colour code). The colour of the spot then gives the multiplier, so that the value is read off in microfarads. For example, a capacitor with the first two colours yellow and violet, with spot colour black, is 47µF. The third colour on the body of the capacitor gives the voltage rating, as follows: MULTIPLIER (µF) WHITE x.01 GREY x0.1 BLACK xl BROWN x10 RED x10D VOLTAGE RATING WHITE 3VW . / YELLOW 6.3VW BLACK 1DVW GREEN 16VW BLUE 20VW GREY 25VW PINK 35VW PO~J~~E ORANGE 40VW 2ND SIGNIFICANT FIGURE - \ BLACK 0 BROWN 1 RED 2 ORANGE 3 YELLOW 4 GREEN 5 BLUE 6 VIOLET 7 GREY 8 WHITE 9 CAPACITANCE IN uF \ POSITIV LEAD CAPACITANCE IN EIA CODE VOLTAGE RATING CAPACITANCE IN uF VOLTAGE RATING POSITIVE LEAD \. 33~ -INDICATES STRIPE 35V POLARITY POSITIVE LEAD Fig.3: tantalum capacitors are usually labelled with their value, voltage rating and a plus (+)sign to indicate polarity. However, some brands use the EIA code while others may be colour coded. white ........... 3VW yellow .......... 6.3VW black ........... l0VW green ........ ... 16VW blue .. ........... 20VW grey ............. 25VW pink ............ . 35VW orange ......... 40VW As can be seen, the colour code for tantalum capacitors bears little resemblance to that for plastic film capacitors, as listed last month. In fact , the only similarity is in the code for the first two significant figures for the capacitance value. It seems that colour coding of capacitors has now fallen into disuse, partly because the code is so confusing and partly because of the improved capability for printing values on such difficult objects as dipped tantalum capacitors. Super capacitors We mentioned these capacitors earlier. These are also known as "double layer" capacitors. They are packaged to look like electrolytic capacitors but they do not use the same dielectric principle as electrolytics and they are not used in FM tuners, TVs and VCR.s for storing station settings and a host of other electronic equipment where data needs to be stored in spite of the removal of mains power. The "double layer" capacitor is the first really new capacitor to be produced in the last 25 years or so although the principle has been known for over 100 years. The main constituents are activated carbon and a sulphuric acid solution as the electrolyte. The interface between the activated carbon and the sulphuric acid forms the electric " double layer" . A basic double-layer capacitor consists of two half-cells, each consisting of an activated carbon electrode saturated with sulphuric acid and separated by an ion permeable membrane. The two half-cells make up a non-polarised capacitor cell which has a low voltage rating and so between 8 and 15 of these cells are connected in series to give a practical capacitor with a voltage rating of 5V or 10V. Really, a double-layer capacitor is more like a battery than a capacitor and that is reflected in typical applications such as low power battery backup for microprocessors. They are not a substitute for conventional capacitors because they cannot handle substantial ripple current. Nor can they deliver substantial DC current because their internal resistance is high. As a substitute for batteries in low backup power circuits though, they are ideal. Barrier layer capacitors Tantalum capacitors are available in values up to about 100~F and with voltage ratings up to 50V. In most cases, you can substitute low-leakage electros for tantalums. polarised. They can be connected into circuit either way around. Made by companies such as NEC, they can have a capacitance up to 1 Farad and usually are available in only one voltage rating: 5V. At present, their main use is as a replacement for lithium and mercury cells in microprocessor controlled appliances. Thus they are Having described double layer capacitors we should mention " barrier layer" and "boundary layer" capacitors because they may be thought to be similar. They are not. In fact , barrier layer and boundary layer capacitors are a special type of ceramic capacitor, based on barium titanate. They have relatively high capacitance values for a ceramic capacitor, up to 0.47,-iF, but at low voltage ratings, typically 12V, 16V and 25V. These days they are tending to be displaced by the even smaller " multilayer " ceramics. ~ MAY1989 9