ELECTRONIC CAPACITORS

Electronic capacitors are one of the most widely used electronic components. These electronic capacitors only allow alternating or changing signals to pass through them, and as a result they find applications in many different areas of electronic circuit design. There are a wide variety of types of capacitor including electrolytic, ceramic, tantalum, plastic, sliver mica, and many more. Each capacitor type has its own advantages and disadvantages can be used in different applications.

The choice of the correct capacitor type can have a major impact on any circuit. The differences between the different types of capacitor can mean that the circuit may not work correctly if the correct type of capacitor is not used. Accordingly a summary of the different types of capacitor is given below, and further descriptions of a variety of capacitor types can be reached through the related articles menu on the left hand side of the page below the main menu.

In essence the construction of an electronic capacitor is very simple, although in practice a lot of research and development has been put into capacitor technology. The basic electronics components consist of two plates that are insulated from one another. In between them there is an insulating medium known as the dielectric. The value of the electronic capacitor is dependent upon the area of the plates, the distance between them and the dielectric constant of the material or dielectric between them. The greater the area of the plates, the closer they are together and the greater the value of the dielectric constant the greater the value of capacitance.

Today, electronic capacitors are able to provide relatively high levels of capacitance within components that occupy a small volume. This is achieved in a number of ways. One is to have several sets of plates, and another is to place the plates very close to one another, having a thin layer of dielectric placed between them. In addition to this special insulating dielectric materials have been developed to enable high levels of capacitance to be achieved.

The method of construction of these electronic components is also important. In some capacitors the plates may be flat, and normally these capacitors will have rectangular, or more exactly cuboid shapes. Some will be tubular and in these capacitors the plates will be wound round on each other. The reasons for these types of construction are normally dependent upon the way in which the capacitors must be manufactured. The final stage in the construction of an electronic capacitor is to place it in a protective casing. In some instances it may be dipped in an insulating coating, in others it may be contained within a metal can.

Some capacitors types are what are termed polar or polarised. When this is the case the electronic capacitor has a positive and a negative connection and it must be placed in circuit so that the voltage across it is in a particular sense. If the voltage is incorrectly placed across the component then it may be damaged. Fortunately many capacitors, and in particular low value ones are non-polar and can be placed in circuit either way round.

Although there is a large variety that are available the most commonly used are ceramic, plastic film types, electrolytic and tantalum. These names refer to the type of dielectric that is used within the capacitor.

Ceramic capacitor

Ceramic capacitors are normally used for radio frequency and some audio applications. Ceramic capacitors range in value from figures as low as a few picofarads to around 0.1 microfarads. In view of their wide range and suitability for RF applications they are used for coupling and decoupling applications in particular. Here these ceramic capacitors are by far the most commonly used type being cheap and reliable and their loss factor is particularly low although this is dependent on the exact dielectric in use. Their stability and tolerance is not nearly as good as silver mica types, but their cost is much less. In view of their constructional properties, these capacitors are widely used both in leaded and surface mount formats.

There are a number of dielectrics that can be used with ceramic capacitors. For low values a dielectric designated "C0G" is normally used. This has the lowest dielectric constant but gives the highest stability and lowest loss. Where higher values are required in a given size, a dielectric with a higher dielectric constant must be used. Types with designations X7R and for higher values, Z5U are used, however their stability and loss are not as good as the capacitors made with C0G dielectric.

Electrolytic capacitor

Electrolytic capacitors are the most popular type for values greater than about 1 microfarad. Electrolytic capacitors are constructed using a thin film of oxide on an aluminium foil. An electrolyte is used to make contact with the other plate. The two plates are wound around on one another and then placed into a can that is often aluminium.

Electrolytic capacitors are polarised, and care should be taken to ensure they are placed in circuit the correct way round. If they are connected incorrectly they can be damaged, and in some extreme instances they can explode.

Electrolytic capacitors have a wide tolerance. Typically the value of the component may be stated with a tolerance of -50% +100%. Despite this they are widely used in audio applications as coupling capacitors, and in smoothing applications for power supplies.

Electrolytic capacitors are available in both leaded and surface mount formats. The surface mount electrolytic capacitors are available in rectangular packages whereas the leaded versions are normally contained in a tubular aluminium can, each end being marked to show its polarity.

Tantalum capacitor

Ordinary aluminium electrolytic capacitors are rather large for many uses. In applications where size is of importance tantalum capacitors may be used. These are much smaller than the aluminium electrolytic capacitors and instead of using a film of oxide on aluminium they us a film of oxide on tantalum. Tantalum capacitors do not normally have high working voltages, 35V is normally the maximum, and some even have values of only a volt or so.

Like electrolytic capacitors, tantalum capacitors are also polarised and they are very intolerant of being reverse biased, often exploding when placed under stress. However their small size makes them very attractive for many applications. They are available in both leaded and surface mount formats.

Silver Mica Capacitor

Silver mica capacitors are not as widely used these days as they used to be. However these electronic components can still be obtained and are used where stability of value is of the utmost importance and where low loss is required. In view of this one of their major uses is within the tuned elements of circuits like oscillators, or within filters.

Values are normally in the range between a few picofarads up to two or possibly three thousand picofarads.

For this type of capacitor the silver electrodes are plated directly on to the mica dielectric. Again several layers are used to achieve the required capacitance. Wires for the connections are added and then the whole assembly is encapsulated.

Polystyrene Film Capacitor

Polystyrene capacitors are a relatively cheap form of capacitor. They are tubular in shape resulting from the fact that the plate / dielectric sandwich is rolled together. This adds some inductance and means that they are only suitable for relatively low frequency circuits, typically up to a few hundred kHz. In view of their relatively good tolerance levels they can be used in filter circuits, etc where values are of importance. They are generally only available as leaded electronics components.

Polyester Film Capacitor

Polyester film capacitors are used where cost is a consideration as they do not offer a high tolerance. Many polyester film capacitors have a tolerance of 5% or 10%, which is adequate for many applications. They are generally only available as leaded electronics components.

Metallised Polyester Film Capacitor

This type of capacitor is a essentially a form of polyester film capacitor where the polyester films themselves are metallised. The advantage of using this process is that because their electrodes are thin, the overall capacitor can be contained within a relatively small package. The metallised polyester film capacitors are generally only available as leaded electronics components.

Polycarbonate capacitor

Polycarbonate capacitors have earned a place as a reliable form of capacitor for use in a number of applications where performance is critical. The polycarbonate film is very stable and this enables high tolerance capacitors to be made which will hold their capacitance value over time. In addition they have a low dissipation factor, and they remain stable over a wide temperature range, many being specified from -55C to +125C.

In 2000 the Bauer Corporation announced they would be ceasing manufacture of the raw dielectric. As a result many of the manufacturers of polycarbonate ceased production. Fortunately there are a few smaller manufacturers of these capacitors, so they can still be obtained.

Polypropylene Capacitor

The polypropylene is sometimes used when a higher tolerance is necessary than polyester capacitors offer. As the name implies, this capacitor uses a polypropylene film for the dielectric. One of the advantages of the capacitor is that there is very little change of capacitance with time and voltage applied. They are also used for low frequencies, with 100 kHz or so being the upper limit. They are generally only available as leaded electronics components.

Summary of capacitor types

The table below gives and overview of the main characteristics of the various types of capacitor.

Capacitor types	Capacitance range	Accuracy	Temperature stability	Leakage	Comments & details
Electrolytic	0.1 µF - ~1 F	V poor	V poor	Poor	Polarised capacitor - widely used in power supplies for smoothing, and bypass where accuracy, etc is not required.
Ceramic	10 pF - 1 µF	Variable	Variable	Average	Exact performance of capacitor depends to a large extent on the ceramic used.
Tantalum	0.1 µF - 500 µF	Poor	Poor	Poor	Polarised capacitor - very high capacitance density.
Silver mica	1 pF - 3000 pF	Good	Good	Good	Rather expensive and large - not widely used these days except when small value accurate capacitors are needed.
Polyester (Mylar)	0.001 µF - 50 µF	Good	Poor	Good	Inexpensive, and popular for non-demanding applications.
Polystyrene	10 pF - 1 µF	V good	Good	V good	High quality, often used in filters and the like where accuracy is needed.
Polycarbonate	100 pF - 20 µF	V good	V good	Good	Used in many high tolerance and hash environmental conditions. Supply now restricted.
Polypropylene	100pF - 50 µF	V good	Good	V good	High performance and low dielectric absorption.
Teflon	100 pF - 1 µF	V good	V v good	V v good	High performance - lowest dielectric absorption.
Glass	10 pF - 1000 pF	Good	Good	V good	Excellent for very harsh environments while offering good stability. Very expensive.
Porcelain	100 pF - 0.1 µF	Good	Good	Good	Good long term stability
Vacuum and air	1 pF - 10 000 pF				Often used as variable capacitors in transmitters as a result of their very high voltage capability.

There is a huge number of different capacitor types and they are one of the most widely used electronic components. While different capacitors may have the same value, each different type of capacitor has its own properties and this will make this particular electronic capacitor suitable for a particular application. If the wrong type of capacitor is used, then it can make a circuit function incorrectly. As a result, choosing an electronic capacitor for a circuit means making more than the value calculations. Choosing the correct capacitor type is equally important.

Capacitor applications

The choice of capacitor for a particular application or use is of paramount importance. Even if the correct value is chosen for a particular capacitor application or capacitor use, the selection of the correct type is of equal importance.

In some instances one form of capacitor may work very well, but another capacitor type may cause the circuit to not work at all. It is therefore critical that the capacitor use or capacitor application is matched to the type or form of capacitor used.

Table of capacitor uses and applications

The most suitable way to summarise the various types of capacitor and the applications for which these electronic capacitors are suited is in a table.

Application	Suitable types with reasons details & comments
Power supply smoothing	Aluminium electrolytic   High capacity and high ripple current capability **
Audio frequency coupling	Aluminium Electrolytic:   High capacitance Tantalum:   High capacitance and small size Polyester / polycarbonate :   Cheap, but values not as high as those available with electrolytics
RF coupling	Ceramic COG:   Small, cheap and low loss Ceramic X7R:   Small and cheap but higher loss than COG, although high capacitance per volume Polystyrene:   Very low loss, but larger and more expensive than ceramic
RF decoupling	Ceramic COG:   Small, low loss, but values limited to around 1000 pF max. Ceramic X7R:   Small, low loss, higher values available than for COG types
Tuned circuits	Silver mica:   Close tolerance, low loss and stable, but high cost Ceramic COG:   Close tolerance, low loss, although not as good as silver mica

** Care must be taken to ensure that the ripple current rating of the capacitor meets the requirements of the capacitor application.

This table gives the typical capacitor applications or capacitor uses for areas where particular capacitors be used. However it is necessary to look at the exact requirements for any capacitor application in a circuit, and choose the capacitor according to the needs and specifications available.

Capacitors have many uses in electronic and electrical systems. They are so common that it is a rare electrical product that does not include at least one for some purpose.

Energy storage

A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery. Capacitors are commonly used in electronic devices to maintain power supply while batteries are being changed. (This prevents loss of information in volatile memory.)

Conventional capacitors provide less than 360 joules per kilogram of energy density, while capacitors using developing technologies could provide more than 2.52 kilojoules per kilogram. In car audio systems, large capacitors store energy for the amplifier to use on demand. Also for a flash tube a capacitor is used to hold the high voltage.

Energy storage is accomplished by devices or physical media that store some form of energy to perform some useful operation at a later time. A device that stores energy is sometimes called an accumulator.

All forms of energy are either potential energy (e.g. Chemical, gravitational, electrical energy, etc.) or kinetic energy (e.g. thermal energy). A wind-up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to operate a mobile phone, and a hydroelectric dam stores energy in a reservoir as gravitational potential energy. Ice storage tanks store ice (thermal energy) at night to meet peak demand for cooling. Fossil fuels such as coal and gasoline store ancient energy derived from sunlight by organisms that later died, became buried and over time were then converted into these fuels. Even food (which is made by the same process as fossil fuels) is a form of energy stored in chemical form.

Pulsed power and weapons

Groups of specially constructed, large, low inductance, high voltage capacitors (banks of capacitors) are used to supply huge pulses of current for various pulsed power applications. These applications include:

• Electromagnetic forming.
• Radar.
• Fusion research.
• Pulsed lasers (especially TEA lasers).
• Pulse forming networks.
• Marx generators.
• Particle accelerators.

Large reservoir capacitor banks are used as energy sources for the exploding-bridgewire detonators (EBW) or slapper detonators, also known as exploding foil initiators (EFIs), in nuclear weapons and other specialty weapons. Experimental research and work is taking place as a way of using banks of capacitors as power sources for electromagnetic armour and electromagnetic coil-guns and rail-guns.

Power conditioning

A 10,000 microfarad capacitor in a TRM-800 amplifier

Reservoir capacitors are used in power supplies where they smooth the output of a full or half wave rectifier. They can also be used in charge pump circuits as the energy storage element in the generation of higher voltages than the input voltage.
Capacitors are connected in parallel with the power circuits of most electronic devices and larger systems (such as factories) to shunt away and conceal current fluctuations from the primary power source to provide a "clean" power supply for signal or control circuits. Audio equipment, for example, uses several capacitors in this way, to shunt away power line hum before it gets into the signal circuitry. The capacitors act as a local reserve for the DC power source, and bypass AC currents from the power supply. This is used in car audio applications, when a stiffening capacitor compensates for the inductance and resistance of the leads to the lead-acid car battery.

Power factor correction

In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as three capacitors connected as a three phase load. Usually, the values of these capacitors are given not in farads but rather as a reactive power in volt-amperes reactive (VAr). The purpose is to counteract inductive loading from devices like electric motors and transmission lines to make the load appear to be mostly resistive. Individual motor or lamp loads may have capacitors for power factor correction, or larger sets of capacitors (usually with automatic switching devices) may be installed at a load center within a building or in a large utility substation.

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