In practice capacitors are often classified according to the material used as the dielectric with the dielectrics divided into two broad categories: bulk insulators and metal-oxide films (so-called electrolytic capacitors).
Capacitors have thin conducting plates (usually made of metal), separated by a layer of dielectric, then stacked or rolled to form a compact device.
Fixed value capacitorsEdit
Many types of capacitor are available commercially, with capacitances ranging from the picofarad range to more than a farad, and voltage ratings up to many kilovolts. In general, the higher the capacitance and voltage rating, the larger the physical size of the capacitor and the higher the cost. Tolerances in capacitance value for discrete capacitors are usually specified as a percentage of the nominal value. Tolerances ranging from 50% (electrolytic types) to less than 1% are commonly available. Another figure of merit for capacitors is stability with respect to time and temperature, sometimes called drift. Variable capacitors are generally less stable than fixed types.
Capacitors using bulk insulatorsEdit
The electrodes need round edges to avoid field emission. Air has low breakdown voltage, so any air inside a capacitor - especially at the edges - will reduce the voltage rating. Even closed air bubbles in the insulator or between the insulator and the electrode lead to gas discharge in High_Frequency applications.
- Air-gap: An air-gap capacitor has a low dielectric loss and offers good cooling. Large-valued tunable capacitors can be made this way. Good for resonating HF antennas.
- Ceramic: The main differences between ceramic dielectric types are the temperature coefficient of capacitance, and the dielectric loss. C0G and NP0 (negative-positive-zero, i.e. ±0) dielectrics have the lowest losses, and are used in filters, as timing elements, and for balancing crystal oscillators. Ceramic capacitors tend to have low inductance because of their small size. NP0 refers to the shape of the capacitor's temperature coefficient graph (how much the capacitance changes with temperature). NP0 means that the graph is flat and the device is not affected by temperature changes.
- C0G or NP0 — Typically 4.7 pF to 0.047 µF, 5%. High tolerance and temperature performance. Larger and more expensive.
- X7R — Typical 3300 pF to 0.33 µF, 10%. Good for non-critical coupling, timing applications. Subject to microphonics.
- Z5U or 2E6 — Typical 0.01 µF to 2.2 µF, 20%. Good for bypass, coupling applications. Low price and small size. Subject to microphonics.
- Ceramic chip: 1% accurate, values up to about 1 µF, typically made from Lead zirconate titanate (PZT) ferroelectric ceramic
- Glass — used to form extremely stable, reliable capacitors.
- Paper — common in antique radio equipment, paper dielectric and aluminum foil layers rolled into a cylinder and sealed with wax. Low values up to a few μF, working voltage up to several hundred volts, oil-impregnated bathtub types to 5,000 V used for motor starting and high-voltage power supplies.
- Polycarbonate good for filters, low tempco, good aging, expensive
- Polyester, Mylar®: (from about 1 nF to 1 μF) signal capacitors, integrators.
- Polystyrene: (usually in the picofarad range) stable signal capacitors.
- Polypropylene: low-loss, high voltage, resistant to breakdown, signal capacitors.
- PTFE or Teflon ™: higher performing and more expensive than other plastic dielectrics.
- Silvered mica: These are fast and stable for HF and low VHF RF circuits, but expensive.
- Printed circuit board: Finally, metal conductive areas in different layers of a multi-layer printed circuit board can act as a highly stable capacitor. It is common industry practice to fill unused areas of one PCB layer with the ground conductor and another layer with the power conductor, forming a large distributed capacitor between the layers, or to make power traces broader than signal traces.
An electrolytic capacitor is a type of capacitor with a larger capacitance per unit volume than other types, making them valuable in relatively high-current and low-frequency electrical circuits. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations, in rectifier output, and especially in the absence of rechargeable batteries that can provide similar low-frequency current capacity. They are also widely used as coupling capacitors in circuits where AC should be conducted but DC should not; the large value of the capacitance allows them to pass very low frequencies without carrying DC.
Aluminium electrolytic capacitors are constructed from two conducting aluminium foils, one of which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil act as cathode. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminium casing. The two most popular geometries are axial leads coming from the center of each circular face of the cylinder, or two leads or lugs on one of the circular faces. Both of these are shown in the picture.
Tantalum capacitors are more expensive than aluminum-based capacitors, and generally only usable at low voltage, but they have much higher capacitance per unit volume and thus are popular in miniature applications such as cellular telephones.
Electrolytic capacitors have a polarity, unlike most capacitors. This is due to the fact that the aluminum oxide layer is held in place by the electric field, and when reverse-biased, it dissolves into the electrolyte. This allows a short circuit between the electrolyte and the aluminum. The liquid heats up and the capacitor may explode. The aluminium oxide layer is the dielectric, and the thinness of this layer, along with its ability to withstand an electric field strength of the order of 109 volts per metre, is what produces the high capacitance. Modern capacitors have a safety valve on one circular face to vent the hot gas/liquid, but the rupture is still loud. The correct polarity is indicated on the packaging by a stripe with minus signs and possibly arrowheads, indicating the lead that should be more negative than the other.
This is the only reason for the polarity requirement. Electrolytics will behave like any other capacitor if reverse biased, up to the point that they are destroyed. Most survive with no DC bias or with only AC, and can even withstand a reverse bias for a period of time, but circuits should be designed so that there is not a constant reverse bias for any significant amount of time. A constant forward bias also increases the life of the capacitors.
Electrical behavior of electrolyticsEdit
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Electrolytic capacitance values are not as tightly-specified as with bulk dielectric capacitors. Especially with aluminum electrolytics, it is quite common to see an electrolytic capacitor specified as having a "guaranteed minimum value" and no upper bound on its value. For most purposes (such as power supply filtering and signal coupling), this type of specification is acceptable.
As with bulk dielectric capacitors, electrolytic capacitors come in several varieties:
- Aluminum electrolytic capacitor: compact but lossy, these are available in the range of <1 µF to 1,000,000 µF with working voltages up to several hundred volts dc. The dielectric is a thin layer of aluminum oxide. They contain corrosive liquid and can burst if the device is connected backwards. Over a long time the liquid can dry out, causing the capacitor to fail. Bipolar electrolytics contain two capacitors connected in series opposition and are used for coupling AC signals.
- Tantalum: compact, low-voltage devices up to about 100 µF, these have a lower energy density and are more accurate than aluminum electrolytics. Compared to aluminum electrolytics, tantalum capacitors have very stable capacitance and little DC leakage, and very low impedance at low frequencies. However, unlike aluminum electrolytics, they are intolerant of voltage spikes and are destroyed (often exploding violently) if connected backwards or exposed to spikes above their voltage rating. Tantalum capacitors are also polarized because of their dissimilar electrodes. The cathode electrode is formed of sintered tantalum grains, with the dielectric electrochemically formed as a thin layer of oxide. The thin layer of oxide and high surface of the porous sintered material gives this type a very high capacitance per unit volume. The anode electrode is formed of a chemically deposited semi-conductive layer of manganese dioxide, which is then connected to an external wire lead. A development of this type replaces the manganese dioxide with a conductive plastic polymer (polypyrrole) that reduces internal resistance and eliminates a self-ignition failure. One vendor's web site refers to the advantage of this new design as "suppression of combustion" .
- Supercapacitor or Ultracapacitor : extreme high capacitance values up to ten farads but low voltage. They are based on the huge surface area of pucks of activated charcoal immersed in electrolyte, with the voltage of each puck being kept below 1 volt. Current is carried through the non-metallic but conductive granular carbon. See also: electrical double layer capacitor. The aerogel capacitor, using carbon aerogel to attain immense electrode surface area, can attain huge values, up to thousands of farads; these may be used as a replacement of rechargeable batteries in some applications.
Standard values for electrolyticsEdit
In the early days of electronics, components were often made to fit a specific need, the values of early capacitors were of arbitrary (usually integer) base numbers. The more common values included 1.0, 1.5, 2.0, 3.0, 5.0, 6.0, and 8.0 as base numbers, but they were not necessarily limited to these values. Values were generally in microfarads (µF) and could be multiplied by any power of ten.
Most electrolytic capacitors have a tolerance range of ±20%, meaning that the manufacturer guarantees that the actual value of the capacitor lies within ±20% of its labeled value. The tolerance is not usually stated on the capacitor and one must refer to the manufacturers data sheet to determine this.
A 10 µF nominal capacitor, for example, may in reality be as low as 8 µF, whereas a capacitor marked 8 µF may be as high as 9.6 µF. This causes an overlap in actual capacitance value of 1.6 µF. For most practical applications, having a ±20% (or even higher) tolerance does not adversely affect the functionality of the device. The chosen nominal values help the manufacturer by reducing the number of different capacitors he has to produce to cover a desired range and allows lower cost manufacture.
In around 1970, a standardized set of capacitor base numbers (1.0, 2.2, 3.3, 4.7) was introduced having increasing distance between them to avoid unnecessary overlapping of tolerance-considered values. The value of any modern electrolytic capacitor may be derived from multiplying one of these base numbers by a power of ten. Using this method, values ranging from 0.1 to 4700 are common in most applications. Values are generally in microfarads (µF).
The left numbers are the nominal (marked) values. The center numbers are the differences based on ±20% tolerance. The numbers on the right illustrate the range of actual values by applying the tolerance.
- Capacitor plague (premature failure of some electrolytic capacitors)
Electric double-layer capacitors (EDLCs)Edit
These devices, often called supercapacitors or ultracapacitors for short, are capacitors that use a molecule-thin layer of electrolyte, rather than a manufactured sheet of material, as the dielectric. As the energy stored is inversely proportional to the thickness of the dielectric, these capacitors have an extremely high energy density. The electrodes are made of activated carbon, which has a high surface area per unit volume, further increasing the capacitor's energy density. Individual EDLCs have capacitances of hundreds or even thousands of farads. For example, the one manufacturing company offers units up to 5000 farads (5 kF) at 2.7 V, useful for electric vehicles and solar energy applications.
The electrode for EDLCS could also be made by transition metal oxides, eg. RuO2, IrO2, NiO, etc. Electrodes made by metal oxides store the charges by two mechanism: double layer effect, the same with active carbon, and pseudocapacitance, which can store more energy than double layer effects.
EDLCs can be used as replacements for batteries in applications where a high discharge current is required. They can also be recharged hundreds of thousands of times, unlike conventional batteries which last for only a few hundred or thousand recharge cycles. But capacitor voltage drops faster than battery voltage during discharge so a DC-to-DC converter may be used to maintain voltage and to make more of the energy stored in the capacitor usable. Shanghai is testing trolleybuses employing supercapacitors.
There are two distinct types of variable capacitors, whose capacitance may be intentionally and repeatedly changed over the life of the device:
- Those that use a mechanical construction to change the distance between the plates, or the amount of plate surface area which overlaps. These devices are called tuning capacitors or simply "variable capacitors", and are used in telecommunication equipment for tuning and frequency control. Small variable capacitors which are mounted directly to PCBs (for instance, to precisely set a resonant frequency at the factory and then never be adjusted again) are called trimmer capacitors.
- Those that use the fact that the thickness of the depletion layer of a diode varies with the DC voltage across the diode. These diodes are called variable capacitance diodes, varactors or varicaps. Any diode exhibits this effect, but devices specifically sold as varactors have a large junction area and a doping profile specifically designed to maximize capacitance.
Variable capacitance is sometimes used to convert physical phenomena into electrical signals.
- In a capacitor microphone (commonly known as a condenser microphone), the diaphragm acts as one plate of a capacitor, and vibrations produce changes in the distance between the diaphragm and a fixed plate, changing the voltage maintained across the capacitor plates.
- In process industry instruments, some types of pressure transmitter use a capacitor element to measure pressure and convert to an electrical signal.
Fixed capacitor comparisonsEdit
|Capacitor type||Dielectric used||Advantages/applications||Disadvantages|
|Paper Capacitors||Paper or oil-impregnated paper||Extensively used for high voltage and high discharge current applications.||Large size. Paper being highly hygroscopic, it absorbs moisture from atmosphere despite plastic enclosures and impregnants thus increasing its power factor and decreasing its insulation resistance.|
|Metalized Paper Capacitors||Paper||Comparatively smaller in size and higher voltage rating||Suitable only for low current applications.|
|Polyester (MYLAR) Capacitor||Polyester film||Smaller in size as compared to paper capacitors of comparable specifications. These have almost completely replaced paper capacitors for most DC electronic applications with operating voltages up to 1600VDC and operating temperatures up to 125ºC. Moisture pick-up is lesser.||Temperature stability is poorer as compared to that of paper capacitors.|
|Polystyrene Capacitor||Polystyrene||Best general purpose plastic film capacitor. Excellent stability, low moisture pick-up and a slightly negative temperature coefficient that can be used to match the positive temperature co-efficient of other components.||Maximum operating temperature is only about +85ºC. Comparatively bigger in size.|
|Polycarbonate Plastic Film Capacitor||Polycarbonate||The insulation resistance, the dissipation factor and dielectric absorption are superior to polystyrene capacitors. Moisture pick-up is less and these have almost zero temperature co-efficient.|
|Polypropelene Plastic Film Capacitors||Polypropylene||Extremely low dissipation factor, very high dielectric strength, low moisture pick-up and high insulation resistance.||<center>-|
|Polysulphone Plastic Film Capacitors||Polysulfone||These can withstand voltage at comparatively higher temperatures. Moisture pick-up is typically 0.2% and it limits its stability.||<center>-|
|PTFE Fluorocarbon (TEFLON) Plastic Film Capacitors||Polytetrafluoroethylene||Operating temperatures of up to 170ºC, extremely high insulation resistance and good stability.||Large size and high cost.|
|Polyamide Plastic Film Capacitors||Polyamide||Operating temperatures of up to 200ºC. High insulation resistance, good stability and a low dissipation factor.||Large size and high cost.|
|Metalized Plastic Film Capacitors||Polyester or Polycarbonate||Reliable and significantly smaller in size.||Limited current carrying capability like the metalized paper capacitors.|
|Stacked Plate Mica Capacitors||Mica||Advantages of mica capacitors arise from the fact that the dielectric material (mica) is inert. It does not change physically or chemically with age and it has good temperature stability.||Unless properly sealed in a case, mica capacitors are susceptible to moisture pick-up which will increase the power factor and the insulation resistance.|
|Metalized Mica or Silver Mica Capacitors||Mica||Have the above mentioned advantages. In addition, they have much reduced moisture infiltration.||<center>-|
|Glass Capacitors||Glass||Similar to Mica Capacitors. Stability and frequency characteristics are better than mica capacitors.||High cost.|
|Class-I Temperature Compensating Type Ceramic Capacitors||Mixture of complex Titanate compounds||Low cost and small size, excellent high frequency characteristics and good reliability. Predictable linear capacitance change with operating temperature.||Capacitance changes with change in applied voltage, with frequency and with aging effects.|
|Class-II High dielectric strength Type Ceramic Capacitors||Barium titanate based dielectrics||Smaller than Class-I type due to higher dielectric strength of ceramics used.||Not as stable as Class-I type.|
|Aluminium Electrolytic Capacitors||Aluminium oxide||Very large capacitance to volume ratio, inexpensive.||Dielectric leakage is high, large internal inductances limit the high frequency performance, poor low temperature stability and loose tolerances. These have been found to burst open when overloaded.|
|Tantalum Electrolytic Capacitors||Tantalum oxides||Large capacitance to volume ratio, smaller size, good stability, wide operating temperature range, long reliable operating life. Extensively used in miniaturised equipment and computers. Available in both polarised and unpolarised varieties. Solid tantalums have much better characteristics than their wet counterparts.||<center>-|
|Alternating current oil-filled Capacitors||Oil-impregnated paper||Primarily designed to provide very large capacitance for industrial AC applications to withstand large currents and high peak voltages at power line frequencies. The applications include AC motor starting and running, phase splitting, power factor correction, voltage regulation, control equipment, etc..||<center>-|
|Direct current oil-filled capacitors||Paper or Paper-polyester film combination||Primarily designed for DC applications such as Filtering, Bypassing, Coupling, Arc suppression, voltage doubling, etc..||Operating voltage rating must be derated as per the curve supplied by the manufacturer if the DC contains ripple.|
|Energy Storage Capacitors||Kraft capacitor paper impregnated with electrical grade castor oil||Designed specifically for intermittent duty discharge applications. These are charged normally in milliseconds and made to discharge through a resistive or other critically damped load in a time period ranging from a few nanoseconds to a few hours. Typical applications include Biomedical equipment, high intensity flash tubes, masers, lasers and welders.||<center>-|
Non-idealities of practical capacitorsEdit
Q factor, dissipation and tan-deltaEdit
Capacitors have "Q" (quality) factor (and the inverse, dissipation factor or tan-delta) which relates capacitance at a certain frequency to the dielectric loss. The higher this figure, the more lossy the capacitor. Tan-delta is the tangent of the phase angle between voltage and current in the capacitor. This angle is sometimes called the loss angle. It is related to the power factor which is zero for an ideal capacitor.
This is an effective resistance that is used to describe the resistive parts of the impedance of certain electronic components. The theoretical treatment of devices such as capacitors and inductors tends to assume they are ideal or "perfect" devices, contributing only capacitance or inductance to the circuit. However, all physical devices are constructed of materials with finite electrical resistance, which means that all real-world components contain some resistance in addition to their other properties. A typical ESR for a low esr capacitor is 0.01 Ω Low values are preferred for high-current, pulse applications. Since capacitors have such low ESRs, they have the capacity to deliver huge currents into short circuits, which can be dangerous.
For capacitors, ESR takes into account the internal lead and plate resistances and other factors. An easy way to deal with these inherent resistances in circuit analysis is to express each real capacitor as a combination of an ideal component and a small resistor in series, the resistor having a value equal to the resistance of the physical device.
ESL in signal capacitors is mainly caused by the leads used to connect the plates to the outside world and the series interconnects used to join sets of plates together internally. For any real-world capacitor, there is a frequency above DC at which it ceases to behave as a pure capacitance. This is called the (first) resonant frequency. This is critically important with decoupling high-speed logic circuits from the power supply. The decoupling capacitor supplies transient current to the chip. Without decouplers, the IC demands current faster than the connection to the power supply can supply it, as parts of the circuit rapidly switch on and off. Large capacitors tend to have much higher ESL than small ones. As a result, electronics will frequently use multiple bypass capacitors — a small 0.1 µF rated for high frequencies and a large electrolytic rated for lower frequencies, and occasionally, an intermediate value capacitor.
Important properties of capacitors are the maximum working voltage (potential, measured in volts) and the amount of energy lost in the dielectric. For high-power or high-speed capacitors, the maximum ripple current are further considerations.
Another major non-ideality is temperature coefficient (change in capacitance with temperature) which is usually quoted in parts per million (ppm) per degree Celsius.
When refurbishing old (especially audio) equipment, it is a good idea to replace all of the electrolyte-based caps. After long storage electrolytic capacitors may deteriorate; before powering up equipment with old electrolytics, it may be useful to apply low voltage to allow the capacitors to reform before applying full voltage. Non polarised capacitors also suffer from aging, changing their values slightly over long periods of time.
In the construction of long-time-constant integrators, it is important that the capacitor does not retain a residual charge when shorted. This phenomenon of unwanted charge storage is called dielectric absorption or soakage, and it creates a memory effect in the capacitor. This is a non-linear phenomenon, and is important when building very low distortion filters.
Capacitors may also change capacitance with applied voltage. This effect is more prevalent in high 'k' and some high voltage capacitors. This can be another small source of non-linearity when building low distortion filters.
Capacitors also have some level of parasitic resistance across the terminals which is called 'leakage'. This fundamentally limits how long capacitors can store charge. Historically, this was a major source of problems in some types of applications (long RC timers, sample-and-holds, etc.)
- Capacitor plague capacitor failures on computer motherboards
- Electronic Devices and Circuits
- Caltech: Practical capacitor properties
- FaradNet: The Capacitor Resource
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- General Atomics Electronic Systems, inc. High Voltage Pulsed Power Capacitors and Systems.
- Skeleton NanoLab, Research & Development of advanced capacitors
- Howstuffworks.com: How Capacitors Work
- CapSite 2006: Introduction To Capacitors
- "IEEE Spectrum", January, 2005 Vol 42, No. 1, North American Edition.
- "The ARRL Handbook for Radio Amateurs, 68th ed", The Amateur Radio Relay League, Newington CT USA, 1991
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- A. K. Maini "Electronic Projects for Beginners", "Pustak Mahal", 2nd Edition: March, 1998 (INDIA[])
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- Biography of von Kleist
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