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What you need to know about capacitors
Capacitors -- aka condenser -- of various types are the other main components to be found in vintage radio sets. In essence a capacitor consists of two metal plates in close proximity but separated by an insulator (usually called the dielectric) which might be air, paper; mica or some other material. If a battery is connected across the plates of a capacitor the EMF of the former tries its best to move electrons around but is frustrated by the insulation between the plates. After a small initial movement lasting only a fraction of a second the electrons build up on the plates; at one time this was thought to emulate the action of water vapor condensing on a cool surface, hence the name capacitor. According to the capacity of the capacitor, after a certain time it will accept no more electrons and is said to be fully charged. At this stage what is called a back EMF has been built up in the capacitor which is equal and opposite to the voltage of the battery. If the latter is now disconnected the electrons remain in situ in the capacitor; and if its internal insulation is perfect this will continue indefinitely. This may be demonstrated by a practical experiment in which a fair-sized capacitor is charged up from a battery or other DC source giving, say, 100 V. When this is removed from the capacitor the EMF within it may be discharged with quite a startling effect by shorting its terminal with a screwdriver blade, when a visible and audible spark will be produced.
This is all very well, but what real use is a capacitor in a radio set? In fact, capacitors have many different jobs to do, as we shall see in a moment, but first let’s examine how real-life capacitors are made and how their capacity is rated.
To do this we have to look briefly at another electrical unit called the coulomb. This is the amount of electricity that flows in a circuit when a current of 1 A passes for one second. The unit of capacity is defined as the size required to hold one coulomb when charged from a 1 V source. This unit was named the farad in honor of the great British scientist Michael Faraday, but whether anyone has ever made a capacitor of one farad capacity is open to doubt, since it would have to be of enormous physical size. In practice, therefore, the unit is taken as the microfarad or one-millionth of a farad, for which the Greek alphabet once again provides a handy abbreviation, u in this application meaning one millionth (we shall meet p again in various other roles and it is important to be able to identify in which sense it is being used). Farad is usually reduced to fd (or F), so the common form of microfarad is μ (Greek mu, μ). Even this is too large for many jobs in radio and another unit, a million-millionth of a farad, is used. In the early days of radio this was often written as mmfd, but around 1940 a new term started to come into use, the picofarad or pfd. This is synonymous with the mmfd. Some ten years later another new unit appeared called the nanofarads (nf) which is one ten-thousandth of a microfarad.
In a domestic radio receiver you may expect to find capacitors from about 5 pfd up to as much as 100 ufd. In most cases you will find that the capacity or value of a capacitor in such sets will either be printed directly onto it or will be found on a label glued around it. Remember that the annotation will differ according to the age of the set. For instance, a capacitor made around 1930 may bear the value 0.0002 mfd. In the later 1930s it might be labeled 200 mmfd and in the 1940s 200 pfd. Again, a capacitor of 0.001 mfd in 1930 might be 1000mmfd in the later 1930s, 1000pfd in the 1940s and 1 nfd in the 1950s. The Philips company was particularly addicted to using multiples of picofarads, even for large value capacitors such as 0.1 mfd. The typical Philips capacitor of the later 1930s, 1940s and early 1950s was a pitch covered tube with the value expressed in thousands of picofarads, but omitting any reference to the latter, so 0.1uF would have been expressed simply as look.
How are capacitors made? For many years the most common materials used were silver foil for the plates and paper impregnated with paraffin wax for the dielectric, the latter being sandwiched between two layers of the former. To save space the sandwich would be rolled into a tight cylindrical or curved- end rectangular shape and the whole then sealed in a molded plastic or metal case. The quality of the paper used had a direct bearing on the longevity of such capacitors; whilst cheap types might have lasted only a few years before starting to leak, that is having their insulation break down, it is quite common to find high quality paper capacitors still working well after 70 years’ use. The range of values covered by paper capacitors was from about 0.001 ufd up to about 16 ufd the larger values usually being of the ‘Mansbridge’ type depicted at the bottom right of the illustration above.
A better material for insulation is mica, which has a dielectric strength, or resistance to break down due to high voltage, of up to eight times that of waxed paper. It is also up to three times as efficient as a dielectric which means that a con denser using it needs less plate area and can thus be made much smaller physically. Mica capacitors were made from very low values of perhaps 5 pfd up to around 0.01uF. It was chosen for values up to about 1000pfd mainly because of the size advantage and the fact that the accuracy of the values (the tolerance) was good. Higher values of mica capacitors were used more for their excellent insulative properties.
A development of the mica capacitor was the replacement of silver foil plates by the spraying of silver onto the dielectric. Silver-mica capacitors could be made to fine limits of capacity and tolerance and were much used in the tuning stages of receivers where accuracy is important.
It was mentioned above that you might expect to find capacitors of up to 100 uF in a radio receiver, but the upper limit for paper capacitors was given as about 16 uF. A different way of making high value capacitors was introduced in the late 1920s and soon became popular. The electrolytic con denser uses, in place of a conventional dielectric, a film of oxide produced by passing current through a chemical solution. In essence one plate is the aluminum container in which the electrolyte is held, and the other is a rod of the same metal suspended within it. The case forms a cathode and the rod an anode, and current is able to pass from one to the other when a voltage is applied. The cathode must be connected to negative and the anode to positive.
When the voltage is first applied a considerable current will flow from the cathode through the electrolyte to the anode, but in a short space of time an oxide film is built up on the surface of the anode which acts as an insulator or dielectric. Since this is merely one or two atoms thick, the capacity between the electrolyte itself and the anode is high. By increasing the effective area of the anode, by etching or roughening its surface, or by making it in the form of a spiral, very high capacities may be achieved in capacitors of small overall dimensions.
Early electrolytic capacitors used a liquid for the electrolyte and thus had to be mounted upright on the chassis, but these were soon superseded by the ‘dry’ type with unrestricted mounting positions. Gradual improvements in design brought down the overall size of electrolytics, assisting the production of compact mains operated receivers.
Electrolytic capacitors for domestic radio purposes range in capacity from about 1 uF to 150uF with a wide variety of operating voltages from about 10V to 500 V. Those used in conjunction with the high voltage supplies used in tubes (valves) receivers are often marked with two voltages of which one is termed ‘working’ and the other either ‘surge’ or ‘peak’. This takes account of the fact that when a mains powered receiver is first switched on and until all the tubes (valves) have ‘warmed up’ the high voltage may be considerably above its normal value. A typical rating might be ‘250 V working, 350V surge’. This is another point to which we shall return in due course.
Leakage current and re-forming
When any electrolytic capacitor is in use a small current continues to pass through it to maintain the oxide film. Whilst this is of no account in smoothing and decoupling applications, it precludes the use of electrolytics for inter- tubes (valves) coupling, when the steady current would cause harm.
If an electrolytic capacitor lies unused for a fairly long period the oxide film gradually disappears, and before the capacitor can be used again it must be ‘re-formed’, i.e. have the film restored. This is achieved by passing a voltage through it that is very considerably less than the normal working voltage, in conjunction with some kind of current-limiting device and a meter to indicate the amount of current flowing. As the film builds up the current flow falls; the voltage is increased until the current falls again, until the point is reached where just a very small polarizing current passes at the normal working voltage.
Time spent on re-forming electrolytics is well spent since it may save the trouble and expense of finding new replacements. Further information on electrolytic capacitors will be given later in the sections on receiver servicing.
Capacitors and AC voltages
When AC is applied to a capacitor the EMF travels first in one direction then the other, having the effect of repeatedly charging and discharging the plates. This means that current flows in and out of the capacitor, although it does not actually pass through the dielectric. If this sounds a little obscure, what it really amounts to is that whilst a capacitor in good condition presents a complete block to DC it will allow a limited amount of AC current to pass through it, just how much depending on its capacity and the frequency of the applied AC. The amount of opposition presented by a con denser to AC is stated in ohms, but because only voltage flows and not current no power is developed as in resistors working with DC, so the term resistance is replaced by reactance. A con denser cannot be labeled as having any particular reactance because, as stated, this depends on the frequency. Only occasionally will anyone engaged in repairing vintage radio sets need to know reactance figures, because these lie in the set designer’s province. There is a formula by which they may be calculated but it will be much easier to look up the sort of tables in reference books that show them related to capacity and frequency. All you need to remember for now is that reactance decreases as capacity or frequency is increased.
For certain purposes in radio receivers, particularly in connection with tuning (see the next section), it is necessary to have variable capacitors, the capacity of which may be altered at will between certain limits. The most popular type has remained essentially the same right through the tubes (valves) radio era up to the present day. It consists of two sets of plates, usually of aluminum, occasionally of brass, one of which is held firmly in position on a framework with the other mounted on a movable spindle fitted on the same framework. Individual plates in each set are spaced a short distance apart, making it possible for the set mounted on the spindle to slide within the fixed set. When the two sets are fully en meshed the capacity of the capacitor is at its highest, and when the two sets are fully apart it is at its lowest. Generally speaking variable capacitors used for tuning in stations have a maximum capacity of 500 pfd or 0.0005 uF (although some may be rather less) and a minimum capacity of a few pfd, for it can never fall quite to zero even with the capacitor wide open. In order to achieve accurate settings, in practice the variable tuning capacitors in receivers are usually provided with some sort of reduction gearing giving slow-motion drive. Only in the cheapest or most compact sets will you find the tuning knob attached directly to the spindle of the capacitor.
The type of variable capacitor just described is referred to as air-spaced or air-dielectric. Another type, called solid dielectric, used thin wafers of insulating material between the plates, permitting them to have much closer spacing and also a greater capacity for a given number of plates and overall size. This type was occasionally used for tuning in compact receivers but was more usually employed to control reaction, an effect we shall meet a little later. For this reason all solid dielectric variable types tend to be called ‘reaction capacitors’. They usually have a maximum capacity of about 300 pfd (0.0003uF)
For certain fine-adjustment purposes physically small variable capacitors of limited range called trimmers are employed. The usual method of construction is to have two sets of plates made of some springy metal, interleaved with wafers of mica, fitted onto a small ceramic base. Typically this would be about the size of a normal postage stamp, from which the once-common name post age-stamp trimmer was derived. The plates and the mica wafers all have a centre hole through which a small bolt may pass. This may screw into a threaded brass bush set into the ceramic base or it may be fixed into the latter, face outwards and have a nut fitted on the outer end. In either case adjustment consists of tightening the screw or nut to press the plates together to increase the capacity, or loosening them off to reduce it. It was common practice for the adjusting screw or nut to be sealed in position with quick-setting paint once the correct capacity had been obtained. This not only guarded against alteration in capacity due to vibration, etc., it also warned service engineers that someone had been twiddling the trimmer, perhaps without the necessary knowledge!
Yes, but what do capacitors do?
A good question, and the answer to which is, lots of things. First of all, let’s look again at what we have just discussed, the ability of a capacitor to block DC but to pass AC. In any and every receiver the need arises for just this property, as in coupling and decoupling. The most common form of coupling is when the output of one tubes (valves) in a receiver has to be passed on to another. For reasons which will be explained in another section this consists of a mixture of AC which needs to travel as unimpeded as possible and DC which must be blocked to prevent its damaging the next valve. Clearly, a capacitor is ideal for this work. We have already mentioned decoupling in Section 3 in connection with the use of resistors to drop voltages fed to tubes (valves) and the need to separate the sections of a receiver to prevent instability. The latter is caused if AC voltages in one part of the set enter another, so the places where they are likely to occur, such as the lower ends of voltage dropping resistors, are connected via capacitors to the metal chassis on which the set is built. These effectively short unwanted AC voltages out whilst not affecting the DC voltages. There are other uses of decoupling which will be discussed when we start to look a receiver design in general.
The ability of a capacitor to hold a charge is exploited in the power supply sections of mains operated receivers in which it is necessary to change the incoming AC from the supply line to DC suitable for the tubes (valves) by means of a rectifier (of which again more later). Large electrolytic capacitors are used to ‘smooth’ the DC output from the rectifier to make it as nearly as possible the same as current obtained from batteries.
A capacitor may be called upon to do two jobs at once. If we ‘look again at Figure 3 a rudimentary symbol for a capacitor, marked C and C’, will be seen at the bottom, connected across the points P and P’. Without this capacitor, each time the points open the self-inductance of the primary coil would cause it to build up an EMF great enough to cause heavy sparking at the points. This EMF would also be built up when the points close again, slowing down the build-up of current through the primary. Both these effects would reduce the amount of EMF induced into the secondary and its output would be lowered. With the capacitor across the points the EMF that would otherwise have caused the sparking is absorbed as a charge, which when complete is discharged back into the primary in the opposite direction. This in turn assists the induction EMF from primary to secondary and results not only in suppressing sparking at the points but also increase the voltage output from the secondary. Note that because the capacitor can charge only when the points open, the high voltage at the secondary is obtained when they ‘break’ and not when they ‘make’. Motorists with long experience of the traditional type of coil ignition will be aware, perhaps only too well, that if the capacitor fitted across the points in a distributor fails the engine just won’t run!
Having touched on the subject of capacitors being used in conjunction with the coils we come to the effects which are at the heart of radio transmission and reception, which are so import ant that they deserve a section to themselves.
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