CAPACITORS AND THEIR USES
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This article is not intended to be a definitive treatise on the subject, but is intended to give a general over-view, which I hope will be particularly interesting and helpful to the newcomer to Amateur Radio.
There are probably more different types of capacitors, or condensers as they used to be called, than any other type of electrical component, so I will limit the scope of this article to a brief description of the various types and their uses. The different types of capacitor have different circuit symbols, as shown below.
As with resistors, capacitors come in a large range of physical forms and may be either fixed or variable. Many are available with axial or radial leads, or are primarily intended for printed circuit mounting or are fitted with threaded bushes or solder tags. There are types with no leads at all that are either intended for surface mounting using metalled ends intended to be directly soldered onto copper tracks on a PCB, or in the case of bare discs, with surface metallising intended to be soldered directly to other components or copper track. There are encapsulated variants for use in adverse environmental conditions, high power types, sub-miniature surface mounting types, low self-inductance types, high voltage types, the list goes on and on.
But first, what is a capacitor? For the purposes of this article, we can regard a capacitor, other than a varicap diode or varactor diode, as two conductive plates separated by an insulator, the dielectric, or a stack of such devices connected in parallel. The capacity value is inversely proportional to the distance between the plates and is proportional to the number of plate pairs, the area of the plates and the dielectric constant, or permittivity, of the insulator. A vacuum, or for all practical purposes, air, has a dielectric constant of 1, with all other insulators having values greater than unity.
Capacity values are now stated in Farads (F), using the normal decimal sub- multipliers of pico (p), nano (n), and micro (µ), although "jars" were originally used (1.0µ = 900jars). As the farad is such a large unit, it is not normally necessary to use multipliers. Thus, for example, 0.1 picafarad is normally written as 0p1, 1.0 picafarad as 1p0, 1.5 picafarads as 1p5, 1000 picafarads as 1000p or 1n, 0.1 microfarads as 100n, 1.0 microfarad as 1µ and 1000 microfarads as 1000µ. Alpha-numeric marking is used for larger value types , whereas standard colour coding or alpha-numeric markings are used to indicate component values on low value types. The latter method is self evident and in the former method, a series of coloured bands is used to indicate value and tolerance, with the first band located near one end of the component and often wider than the other bands.
The Standard EIA Colour Code Table per EIA-RS-279 is as follows:-
Colour 1st band 2nd band 3rd band 4th band Temperature
(Multiplier) (tolerance) Coefficient
Black 0 0 ×1 ±1%
Brown 1 1 ×10 ±2% 100 ppm Red 2 2 ×100 50 ppm
Orange 3 3 ×1000 15 ppm Yellow 4 4 ×100000 25 ppm
Green 5 5 ×1000000 ±0.5% Blue 6 6 ×10000000 ±0.25%
Violet 7 7 ×100000000 ±0.1% Grey 8 8 ×1000000000 ±0.05%
White 9 9 ×10000000000
Gold ×0.1 ±5% Silver ×0.01 ±10%
Although it is theoretically possible to manufacture capacitors having any nominal value of capacity, the smaller value types are normally produced with "standard" picafarad values. These values are normally in the E12, also known as the 10% series (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82) or the E24, also known as the 5% series (10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91) series. Larger value types are normally manufactured with "decade" microfarad values, e.g. 1.0, 10, 100, 1000. Electrolytics are often manufactured with intermediate microfarad values, such as 2, 8, 16, 32, 50, 64, 100, 150, 200 and 500.
Temperature coefficients are stated in "parts per million per degree centigrade" and can be either positive or negative, although negative values are more common. For example, the capacity of a 100pf capacitor, with a stated temperature coefficient of N750 would reduce by 7.5pf if its temperature were to be raised by 100 degrees centigrade.
Before continuing, let us consider voltage, current and temperature ratings. Most people understand the voltage rating of a capacitor but many do not appreciate that capacitors also have series resistance, leakage resistance and maximum current ratings. These are not normally a problem when capacitors are used as coupling or de-coupling components in low power circuits. However, in applications such as reservoir and smoothing capacitors in power supplies, coupling and tuning capacitors in high power RF transmitters and high power pulse applications, such as are required for flash discharge tubes, the current rating and series resistance are very important, as currents of many amperes may be encountered.
The series resistance of a capacitor is that intrinsic resistance existing in the terminal wires, the internal connections to the plates and in the plates themselves. Any current flowing in this resistance will generate heat, which could result in excessive temperature rise. Leakage resistance is that intrinsic resistance existing in parallel with the capacitor plates. Any voltage across the capacitor will cause current to flow in this leakage resistance with consequent generation of heat. With modern capacitors, except perhaps electrolytic types, series and leakage resistances are not usually a problem.
The voltage specified on any given capacitor is the "working voltage" but a test, peak or surge voltage may also be specified. Working voltage is the maximum continuous voltage that the capacitor will withstand for extended periods of time, although it is good engineering practice to assume an actual working voltage of about 75% of the stated value. The test, peak or surge voltage is that non- repetitive voltage that the capacitor will withstand for extremely limited periods of time.
The ability of all capacitors to maintain their values, pass current or withstand voltage is adversely affected by excessively low or high temperatures. Hence capacitors have temperature, as well as voltage and current, ratings. Electrolytic capacitors are particularly sensitive to temperature.
Large electrolytic capacitors used in smoothing circuits may have to pass several tens of amps of ripple current at the same time withstanding an applied DC voltage of scores, or even hundreds, of volts. A transmitter delivering 1000W into a 50ohm feeder via a DC blocking capacitor will require that capacitor to pass a current of 4.47A.
The last "parasitic" element to be considered is self-inductance. This appears in series with the capacitive element of the component and becomes increasingly important as the operating frequency increases, becoming of paramount importance in UHF and microwave applications, where a capacitor can actually become a resonant circuit.
Let us now consider the
different types of capacitor that are available and their applications.
These types are categorised according to the dielectric used and most of these can be sub-divided into many different types.
Fixed vacuum capacitors are basically non-adjustable versions of the vacuum variable (see below). Versions are available with capacities of up to 1000pF at working voltages of up to 50kV. They are capable of carrying RF currents in excess of 30A. Needless to say, these capacitors are extremely expensive. The photos below are reproduced with the kind permission of Dave Knight G3YNH. Further information and a host of other images may be found on his web-site.
Fixed air dielectric capacitors are rare and are normally limited to very high power transmitter applications. They usually consist of flat metal plates separated by glass or ceramic insulators. Working voltages up to hundreds of kV can be obtained, with capacity values of up to several hundred pF. The series resistance is extremely low and the leakage resistance is extremely high. Consequently, the Q is also very high. This type of capacitor would be a purpose built item for a specific application.
Paper capacitors are now virtually obsolete but were very common prior to the advent of the more modern plastic film types in the early 1960s. They employed metal foil, to which the leads were connected, on either side of a thin paper strip. When a sufficient length of strip had been manufactured, depending on the required value of the finished capacitor, it was wound into a tube which was then inserted into an outer case, often of cardboard but sometimes consisting of a sealed metal tube. The entire assembly was then wax impregnated before the ends of the outer case were sealed.
Some more specialised types were housed in sealed metal cans, often fitted with terminals. These capacitors were sometimes manufactured as multiple units, where two or more individual capacitors were mounted in a single can.
They were used for general purpose coupling, de-coupling and smoothing purposes, with values ranging from about 1000pF to several microfarads. Working voltages between 100V and several kV were available. The value tolerance was usually ±20%. Some types suffered from an effect called "migratory placticisers", where the paper insulation becomes brittle, which causes it to crack and break down after prolonged use. It is never advisable to apply voltages greater than half the working voltage to this type of capacitor and, if used in a mains filter, the DC working voltage should be at least three times the applied RMS voltage, unless the component is specifically designed for AC applications.
This type of capacitor exhibits fairly low-Q and poor temperature stability. Even so, block paper capacitors housed in sealed metal cans and having values of 4µF or more, were often used instead of the much smaller but, at the time, much less reliable, electrolytic alternatives.
Motor start, motor run and power factor correction capacitors are usually metalised paper types, although special AC rated, reversible electrolytic types exist for motor applications. Metalised paper capacitors for these applications are usually housed in sealed metal cans. These capacitors are designed to carry fairly large AC currents and therefore have substantial terminals, rather than wire connections.
Plastic Film Dielectric
Plastic film capacitors employ a metal deposition, to which the leads are connected, on either side of a thin plastic film. When a sufficient length of strip has been manufactured, depending on the required value of the finished capacitor, it is wound into a tube, and either encapsulated in a plastic compound or covered in more film before sealing the ends. This type of capacitor has almost entirely replaced the paper dielectric type in coupling, de-coupling, timing and similar low to medium frequency applications. Specialised types are available for motor run and start, power factor correction, mains filtering, sample and hold, high stability RF tuning, high current pulse and many other applications. The plastic film can be mylar, polystyrene, polyester, polycarbonate or polypropylene, depending on application. PTFE is sometimes used as a dielectric but metal cannot easily be deposited on this material and its use is limited to very specialised applications. Values range from a few pF to several µF and working voltages range from around 50V to a few kV, depending on type and application. Value tolerances are normally better than ±10% for the higher values and better than ±2% for the smaller values. This type of capacitor exhibits medium to high-Q and has very good temperature stability, particularly in the lower values.
Ceramic capacitors are available in three main types, namely tubular, disc and surface mounting. They two former types employ metallising, to which the leads are attached, on either side of the ceramic and both are available in values ranging from less than 1pF to 1µF, although higher values are sometimes encountered. The surface mounting versions are similar to the disc type except that the leads are replaced by metalled end caps. Normally, the temperature stability and Q decreases as the value increases. The higher values are normally used as de-coupling capacitors but the lower values can be used as the capacitive element in LC tuned circuits. Components with values less than 100pF are available with closely controlled temperature coefficients of between P100 and N750, including NP0 and are often used to compensate for temperature drift in other components. Value tolerances are normally better than ±20% for the higher values and better than ±2% for the smaller values. The higher value types have large, ill-defined temperature coefficients. Working voltages depend on type and value and are typically 63V for the higher values and up to 250V for the smaller values. Special types are available, such as discs for use in the high voltage circuits of video monitors and TV receivers. These types are available with working voltages up to 5kV.
Special types of ceramic capacitors have been designed for use in high power RF transmitters. These can have working voltages up to more than 30kV and can carry more than 50 amperes of RF without damage. These types exhibit high temperature coefficients and are therefore not suitable for use in stable frequency determining applications.
Bare disc capacitors without leads or encapsulation are also available. In this type, the plates comprise metallising on either side of the disc and they are intended to be soldered directly to PCB tracks or other components. The main use of this type is in UHF and microwave applications, where low series inductance is very important. Values range from a few pF to about 1000pF, with value tolerances better than ±10% and working voltages of around 100V.
Another variety of ceramic capacitor is the monolithic or "Monobloc" type. These are multi-plate ceramic components where the plates and connection wires, dielectric and overall casing are combined into a single, homogeneous block, hence the name monolithic, or "single stone". Similar comments apply to these types as apply to ordinary ceramic discs, except high voltage and high current types are not available. This type of capacitor is also available in surface mounting versions.
The feed-through capacitor is a special type of ceramic capacitor, whereby a lead may be passed through a screened plate or chassis. They consist of a tubular ceramic capacitor, where one lead is connected to the metallising inside the tube and is passed through the tube to provide a connection at each end. The other plate is formed by metallising on the outside of the tube, which is either soldered to a connection ring or to a threaded bush. Those with the connecting ring are soldered directly to the chassis, whereas those with a threaded bush are fixed with a nut. The purpose of this type of capacitor is to provide a convenient method of passing a lead through a metal plate and efficiently de-coupling it to earth at the same time. They are often used as the input and output connections to sealed, metal cased, filter units. The range of values available is usually limited to 100pF, 500pF and 1000pF, although other values are occasionally encountered. The working voltage is usually 100V but high voltage versions for mains filtering are also manufactured.
Some versions are available which incorporate ferrite beads. These are usually manufactured as pi-section filters but other configurations are sometimes found. All types are available as multiple units.
Although fixed value glass dielectric capacitors are no longer manufactured, it is worth noting that in 1745, Pieter van Musschenbroek (1692-1761), invented the Leyden Jar. This device consisted of a glass jar with metal foil on the inner and outer surfaces which were not connected to each other. These two foils formed the two plates of a rudimentary capacitor. Originally, capacity was measured in the number of 'jars' of a given size. A typical Leyden jar had a capacity of between 50pF and 1nF. Over 150 years later, it was decided that a "standard" Leyden Jar would have a capacity of 1111.1111pF or 1µF = 900jars. These would appear to be very strange values to choose but there is a good basic theoretical reason for them. Anyone wishing to dig deeper could consult Volume 1 of the Admiralty Handbook of Wireless Telegraphy, 1938, or research the subject on the Internet. A typical Leyden Jar and a bank of such devices are shown below. The latter was used by Marconi in 1900 during his experiments with wireless (radio) transmitters.
Mica capacitors are available in two types, namely plate-and-mica and silver-mica. Plate-and-mica capacitors consist of metal plates or pieces of metal foil separated by mica sheets clamped together and usually encapsulated to prevent ingress of moisture. Leads are attached to alternate plates. This type is mainly limited to use in high power applications such as transmitters and radar pulse modulators. They are often custom made to a specific value and working voltage. This type of capacitor is capable of carrying considerable currents and withstanding very high voltages.
Silver-mica capacitors are manufactured by depositing a layer of silver on either side of thin mica sheets and attaching a lead to each area of silvering. Several of these sheets are then clamped together in a stack and enclosed in either a plastic case or by encapsulation. This type of capacitor is capable of very good temperature stability but cannot carry the currents or withstand the voltages that are possible with the plate-and-mica type. Silver mica capacitors are available in values between 5pF and 0.1µF, with value tolerances between 1% and 10% and working voltages up to 250V, although higher working voltages are sometimes encountered. The temperature coefficient is normally assumed to be NP0. Silver- mica capacitors are high-Q, high stability components and are used in critical tuning and timing applications. This type of capacitor is also available in surface mounting versions.
Electrolytic capacitors (normally referred to as "electrolytics") employ either aluminium or tantalum and can use either liquid or solid construction techniques. The chemistry of electrolytic capacitors is very complex and beyond the scope of this article. Liquid types use etched foil "plates", separated by a porous insulator impregnated with electrolyte, whereas the solid type employ a solid "slug" of sintered metallic material. Both types are usually housed in metal containers equipped with wire ends, solder tags or screw terminals but plastic encapsulated and surface mounting versions are also available for the smaller types. The smaller sizes can usually be mounted onto tag-panels, or onto PCBs, by their lead-out wires, but the larger sizes are relatively heavy and require mounting clips or other methods of mechanical mounting. Solid types are more electrically and environmentally robust than their liquid counterparts and have lower leakage currents but are more expensive and less able to handle high AC currents and surges. Solid aluminium or tantalum electrolytic capacitors should never be connected directly across a low source impedance supply, as failure caused by high surge current at switch-on can occur.
Electrolytics require a DC polarising voltage and will pass a small leakage current. "Reversible" types are available, which are actually two identical capacitors connected back-to-back and mounted in a single can. Obviously, this type uses constituent capacitors that are not destroyed by a reversed polarising voltage. Catastrophic failure resulting in a rupture of the container can be very dangerous, as the electrolyte is corrosive and will damage surrounding components. The electrolyte used in liquid tantalum devices is sulphuric acid. Many of the larger aluminium electrolytics are equipped with pressure relief devices which prevent actual explosions but do not prevent electrolyte leakage in cases of failure.
Available values range from about 0.47µF to 1F (yes, one farad!), with working voltages between about 5V and 500V, depending on type. The higher working voltages are available only in liquid aluminium types, whereas the lowest are available only in solid tantalum types. Liquid aluminium electrolytics are capable of carrying high AC currents, with the highest value types capable of carrying many amps. Tantalum types are less tolerant of high AC currents but are more reliable and exhibit low leakage currents.
The capacity value tolerance is sometimes as high as -0+100% and ±20% is normal. The temperature stability is seldom specified, as electrolytics are never used where value stability, or even absolute value, are critical. However operational temperature range is important and is usually specified.
The main advantage of electrolytics over other types is the very high values of "farads-per-cubic-centimetre" that can be achieved. Indeed, it is not practicable, or even possible, to achieve values of hundreds or thousands of microfarads by other means. A special case is a range of components intended to act as a source of temporary "keep-alive" voltages in digital memory and processor applications. These devices usually have a capacity of around 1F and exhibit relatively low leakage currents considering their capacity. They are not true capacitors, neither are they true batteries, but can be regarded as using technology somewhere between that of electrolytic capacitors and storage accumulators.
Electrolytics are used for low frequency applications such as smoothing, de-coupling, coupling in low impedance circuits and non-critical time constants and other timing applications. Tantalum electrolytics, particularly the solid variety, are usable up to about 1MHz whereas aluminium types should not be used above about 500kHz. Applications involving high values of AC current always employ aluminium electrolytics. These capacitors are sometimes manufactured as multiple units, where two or more individual capacitors are mounted in a single can, with their negative connections brought out to one common terminal. Very occasionally, the positive connections are brought out to the common terminal.
Modern electrolytics are
very reliable components if they are used within their ratings and their inherent
potential problems are considered at the component selection stage of a design
exercise. Older aluminium electrolytics, particularly high value
types, suffered problems if not used for very long periods of time (several
years). In such cases, application of the full working voltage, even
to a brand new and unused component, could result in very high leakage current,
with consequent rise in temperature, which could lead to catastrophic failure
and physical explosion. However, such components can often be restored
to normal operation by applying the full working voltage but limiting the leakage
current to a safe level by a series resistance and slowly "reforming" the capacitor
over a period of several hours. The same method should be applied
to electrolytics incorporated into old equipment that is being refurbished after
many years of storage. Modern construction techniques have largely
overcome this problem but it is advisable to reform very large value capacitors,
following extended storage.
Variable capacitors can be sub-divided into four main categories, namely air- dielectric, ceramic dielectric, glass dielectric and mixed dielectric, the latter being further sub-divided into mica and plastic film. All types are available as pre-set trimmers or as components adjusted by means of a shaft and are either equipped with threaded bushes to enable them to be panel mounted, or are fitted with lugs to facilitate chassis mounting, or are fitted with tags or pins to facilitate direct PCB mounting.
The capacity value is normally specified as the maximum capacity achievable with the fixed and variable plates fully meshed. The minimum value, with the plates fully un-meshed, is also sometimes specified, as this is often an important consideration. Capacity tolerances are never stated on the component and temperature tolerances are seldom stated but both may be specified in the manufacturer's literature.
The vacuum variable capacitor is basically a very large "Philips" trimmer (see below) enclosed in a glass envelope from which all the air has been exhausted. Although similar in operation to a "Philips" trimmer, the vacuum variable has a much greater capacity swing and is often used as an operator control in large transmitters. Versions are available with capacities of up to 1000pF at working voltages of up to 50kV. They are capable of carrying RF currents in excess of 30A. Needless to say, these capacitors are extremely expensive.
Air dielectric capacitors do not employ any insulating material, other than air, between the fixed and moving plates. The fixed plates are supported on insulating mounts, usually ceramic.
Air dielectric variable capacitors can be sub-miniature, low value, trimmers or large, high value, transmitting components with all shapes, sizes and capacities in between. Values can vary from a few pF to several thousand pF.
The temperature coefficient is almost never specified but is normally assumed to be NP0. Generally speaking, the larger the variable capacitor, the higher will be the minimum achievable capacity and vice-versa. Air dielectric variables are available with working voltages varying from about 100V up to 10kV or more.
The relationship between angular rotation of the shaft and capacity value depends on the physical shape of the plates. Various rotation-versus-capacity laws are available, depending upon application. Probably the most common use of air dielectric variable capacitors, as opposed to small trimmers, is as tuning capacitors in radio receivers. For ease of use, it is convenient for the tuning scale to be as linear as possible. In a simple tuned circuit, the resonant frequency is inversely proportional to the reciprocal of the square root of the tuning capacity. Most variables of this type are therefore manufactured with a rotation-versus-capacity law that results in a substantially linear rotation-versus-frequency characteristic. Linear and other law devices have been produced for specialist applications.
Air dielectric capacitors are basically low loss (high-Q) components with good temperature stability, although unsealed types suffer from contamination due to atmospheric moisture and dust. These types of variable capacitor are normally used as the variable element in LC tuned circuits, particularly when high stability is required, such as in analogue receiver local oscillators or in transmitter VFOs. They are available as single units or multi-section ganged units that can comprise four or more separate sections. It is usually important that the capacity values of each section of a ganged unit are substantially equal at any given point of angular rotation of the shaft. To achieve this, one of the moving plates in each section is split into segments, each of which can be slightly bent away from or towards the adjacent fixed plate, thus facilitating incremental adjustment. The moving plates are usually, but not necessarily, electrically connected to the frame of the component.
The butterfly capacitor is a form of rotary variable capacitor with two independent sets of fixed plates opposing each other, and butterfly-shaped moving plates between them. Rotating the shaft will vary the capacity between the moving plates and either set of fixed plates equally. Butterfly capacitors are used in symmetrical tuned circuits, such as those used in RF power amplifier stages using push-pull configuration or symmetrical antenna tuners where the moving plates need to be connected to RF (but not necessarily DC) ground. Since the RF current normally flows from one set of fixed plates to the other without going through wiper contacts, butterfly capacitors can handle large RF currents. In a butterfly capacitor, the moving plates can only cover a maximum angle of 90° since there must be a position without overlap of the fixed and moving plates corresponding to minimum capacity. Therefore a turn of only 90° covers the entire capacity range.
A variation of the butterfly capacitor is the split stator type. This type uses two separate sets of moving plates arranged axially behind one another but mounted on opposite sides of the shaft. These interleave with two sets of identical fixed plates. While split stator capacitors benefit from larger area plates than the butterfly type, as well as a rotation angle of up to 180°, the separation of rotor plates incurs some losses, since RF currents must pass through the rotor shaft instead of only flowing through the moving plates.
Yet another variation is the differential variable capacitor, which has two independent sets of fixed plates, where rotation of the shaft results in the capacity between one set of fixed plates and the moving plates increasing while that between the other set of fixed plates and the moving plates decreases. Therefore, the capacity between the two sets of fixed plates remains constant but the "electrical centre point" between the two sets of fixed plates varies with shaft rotation.
A derivative of the differential type is one where the temperature coefficient, rather than the capacity value, changes with shaft rotation. This is achieved by mounting both sets of fixed plates on a bi-metal strip. The relative position of the fixed and moving plates is therefore dependent on temperature. This type of component has a capacity value of about 50pF, but a temperature coefficient that is adjustable from positive to negative, through zero. This type of component is no longer commercially available but was marketed by Oxley under the trade name of "Tempatrimmer". An example of the use of this type of component can be found in the VFO circuit of the Yaesu FT221R VHF transceiver.
A type of air dielectric trimmer capacitor using concentric metal tubes was manufactured by Philips and became universally known as the "Philips beehive trimmer". The fixed plates consist of up to four concentric aluminium tubes, electrically connected together and concentrically mounted on a hollow ceramic tube. The moving plates consist of a similar arrangement of concentric aluminium tubes but having diameters such that they interleave with, but do not touch, the fixed tubes. The moving plates are mounted on a screwed rod projecting up the centre of the ceramic tube. Adjustment of capacity is achieved by varying the degree of interleaving between the fixed and variable plates. A cheaper commercial version was produced, in which the threaded rod was replaced by a course spiral rod, similar in form to old-fashioned barley sugar or the canopy supports on showmen's steam engines, mating with a spring arrangement on the moving plates. These cheap versions are not very stable and tend to be electrically noisy. Consequently, they are seldom used a frequencies above 50MHz. The high quality, and consequently more expensive, versions using a precision threaded rod and nut are much more stable and can be used at frequencies up to several hundred MHz. Philips beehive trimmers are available with maximum capacities from 2pF to 60pF and all types exhibit low minimum capacities, although their self-inductance is rather high.
Trimmer capacitors using a similar arrangement to the Philips variety, but much smaller and manufactured from gold plated beryllium-copper to very high physical tolerances, are produced for use up to and including microwave frequencies. These are very high-Q components and are consequently very expensive.
Ceramic dielectric capacitors are usually pre-set trimmers with maximum values of capacity between 1pF and 1000pF and working voltages up to about 250V. The fixed "plate" is normally a semi-circular metallised area on the underside of the ceramic base, located directly beneath a semi-circular area of metallising on the moving part of the component that forms the other "plate". Ceramic trimmers are medium-Q components and can exhibit reasonably good temperature stability. This type of trimmer is normally used in LC tuned circuits, where medium temperature stability is adequate, such as in receiver RF stages. The temperature stability is usually inversely related to the capacity value, i.e. the greater the capacity value, the poorer the stability. This type of trimmer is usually adjusted by means of a screw-driver slot in the moving plate.
Another type of ceramic trimmer consists of a ceramic tube with either a tight fitting metal tube, or metallising, over the outer surface, which forms the fixed "plate". The moving "plate" consists of a closely fitting screw or piston inside the ceramic tube. Adjustment is achieved by altering the amount the screw or piston projects within the outer "plate". The metal parts of these components are usually silver plated. In high quality versions of this type, intended for use at UHF or microwave frequencies, the stability and Q can be moderately high. Cheaper versions are manufactured but these tend to be rather unstable and electrically noisy.
Sheets of mica can be placed between the fixed and moving plates of any air-spaced variable capacitor to increase both the maximum capacity and the breakdown voltage. These then become mixed dielectric variable capacitors, as the dielectric is a combination of air and mica. However, the use of mica as a dielectric material has now been largely superseded by plastic film.
However, mica may still be encountered in compression trimmers. Compression trimmers consist of a stack of thin metal plates separated my sheets of mica, in which alternate plates are electrically connected together. One set of plates is the "fixed plate" and the other set is the "moving plate", although neither set is truly "fixed" or truly "moving". Altering the force pressing the plate and mica sandwich together varies the capacity between the fixed and variable plates and this is usually achieved by means of a screw. These components are not particularly stable and are prone to problems with moisture or dirt ingress. They are medium-Q devices and are available with maximum capacities of up to 500pF, although the minimum capacity is high and the range of adjustment is low. This type of capacitor is seldom used above 50MHz.
Plastic Film Dielectric
Sheets of plastic film can be placed between the fixed and moving plates of any air-spaced variable capacitor to increase both the maximum capacity and the breakdown voltage. These then become mixed dielectric variable capacitors, as the dielectric is a combination of air and plastic film. Modern components of this type are almost exclusively trimmers and exhibit medium-Q and fairly good temperature stability and are available in a variety of shapes, sizes and maximum capacities for use in similar applications to their air-spaced counterparts.
Examples of this type of capacitor are to be found in practically every cheap portable transistor radio, where they are used, not as trimmers but as the main tuning capacitor.
The more mature readers of this article will be familiar with the "reaction condensers" used in TRF receivers of the 1920s and 1930s, which were almost invariably mixed dielectric types using air and Paxolin or celluloid as the dielectric. Paxolin is not a film, nor is it a plastic, but the principle is the same. This type of capacitor was a rather crude, low-Q, device but was perfectly adequate for purpose, bearing in mind the materials and construction practices available at the time.
Incidentally, modern versions are still manufactured, although the dielectric is now modern plastic film and variants are available with working voltages up to several kV.
Another type of trimmer consists of a glass tube with metallising, over the outer surface, which forms the fixed "plate". The moving "plate" consists of a closely fitting piston inside the glass tube. The metal parts of these components are usually gold or silver plated. Adjustment is achieved by altering the amount the piston projects within the outer "plate". These trimmers are high quality, expensive components, exhibiting high-Q and good stability, and are intended for use at UHF or microwave frequencies.
Voltage Controlled Capacitors (Varicaps and Varactors)
Voltage controlled capacitors,
or varicaps, are actually semi-conductor diodes, where the capacity is dependent
upon the width of the depletion layer, which is determined by the reverse bias
applied to the diode. Increasing the reverse bias causes the depletion
layer to increase in width, which decreases the effective capacity and vice
versa. These diodes are used in voltage controlled oscillators and
in electronically tuned radio receivers. It is necessary to ensure that
any RF signals that appear across the diode never exceed the reverse bias and
hence these diodes are only suitable for relatively low power applications,
although power varicaps, or varactors, are available for frequency multiplier
applications at VHF and UHF, where DC bias is not used. The operation
of varactors is beyond the scope of this article. Devices having
minimum capacities of around two picofarads, and maximum capacities of hundreds
of picofarads are available. Physically, these devices resemble ordinary
signal and power diodes and it is worth noting that for operation at up to a
few tens of MHz, ordinary power diodes can often be used as varicaps but bear
in mind that this is an unspecified mode of operation and hence repeatability
cannot be guaranteed.
This article is intended to give an overview of the subject and none of the items has been dealt with in depth. Further information is available from capacitor manufacturer's literature or from the Internet.
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