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Application of capacitors

 

Applications of Capacitors
 
1.      Table of capacitor uses and applications
The most suitable way to summarize the various types of capacitor and the applications for which these electronic capacitors are suited is in a table.
 

Application
Suitable types
Reasons
Power supply smoothing
Aluminum electrolytic
High capacity, high ripple current
Audio frequency coupling
Aluminum electrolytic

Tantalum

Polyester / polycarbonate
High capacitance

High capacitance, small size

Cheap, but values not as high as electrolytic
RF coupling
Ceramic COG

Ceramic X7R

 

Polystyrene
Small, cheap, low loss

Small cheap, but higher loss than COG

Very low loss, but larger than ceramic
RF decoupling
Ceramic COG

 

Ceramic X7R
Small, low loss. Values limited to around 1000 pF

Small, low loss, higher values available than for COG types
Tuned circuits
Silver mica

Ceramic COG
Close tolerance, low loss

Close tolerance, low loss, although not as good as silver mica

This table gives the typical applications for which the various capacitor types may be used. However it is necessary to look at the exact requirements for any capacitor application in a circuit, and choose the capacitor type according to the needs and specifications available.
Summary
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.
 
2. Applications
The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge. That's why the electronic flash on a camera uses a capacitor -- the battery charges up the flash's capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly. This can make a large, charged capacitor extremely dangerous -- flash units and TVs have warnings about opening them up for this reason. They contain big capacitors that can, potentially, kill you with the charge they contain.
 
Capacitors are used in several different ways in electronic circuits:
  • Sometimes, capacitors are used to store charge for high-speed use. That's what a flash does. Big lasers use this technique as well to get very bright, instantaneous flashes.
  • Capacitors can also eliminate ripples. If a line carrying DC voltage has ripples or spikes in it, a big capacitor can even out the voltage by absorbing the peaks and filling in the valleys.
  • A capacitor can block DC voltage. If you hook a small capacitor to a battery, then no current will flow between the poles of the battery once the capacitor charges. However, any alternating current (AC) signal flows through a capacitor unimpeded. That's because the capacitor will charge and discharge as the alternating current fluctuates, making it appear that the alternating current is flowing.
2.      Hazards and safety
Capacitors may retain a charge long after power is removed from a circuit; this charge can cause dangerous or even potentially fatal shocks or damage connected equipment. For example, even a seemingly innocuous device such as a disposable camera flash unit powered by a 1.5 volt AA battery contains a capacitor which may be charged to over 300 volts. This is easily capable of delivering a shock. Service procedures for electronic devices usually include instructions to discharge large or high-voltage capacitors. Capacitors may also have built-in discharge resistors to dissipate stored energy to a safe level within a few seconds after power is removed. High-voltage capacitors are stored with the terminals shorted, as protection from potentially dangerous voltages due to dielectric absorption.
Some old, large oil-filled capacitors contain polychlorinated biphenyls (PCBs). It is known that waste PCBs can leak into groundwater under landfills. Capacitors containing PCB were labeled as containing "Askarel" and several other trade names. PCB-filled capacitors are found in very old (pre 1975) fluorescent lamp ballasts, and other applications.
High-voltage capacitors may catastrophically fail when subjected to voltages or currents beyond their rating, or as they reach their normal end of life. Dielectric or metal interconnection failures may create arcing that vaporizes dielectric fluid, resulting in case bulging, rupture, or even an explosion. Capacitors used in RF or sustained high-current applications can overheat, especially in the center of the capacitor rolls. Capacitors used within high-energy capacitor banks can violently explode when a short in one capacitor causes sudden dumping of energy stored in the rest of the bank into the failing unit. High voltage vacuum capacitors can generate soft X-rays even during normal operation. Proper containment, fusing, and preventive maintenance can help to minimize these hazards.
High-voltage capacitors can benefit from a pre-charge to limit in-rush currents at power-up of high voltage direct current (HVDC) circuits. This will extend the life of the component and may mitigate high-voltage hazards.
 
3.      Fixed capacitor comparisons

Capacitor type
Dielectric used
Features/applications
Disadvantages
Paper Capacitors
Paper or oil-impregnated paper
Impregnated paper was extensively used for older capacitors, using wax, oil, or epoxy as an impregnate. Oil-Kraft paper capacitors are still used in certain high voltage applications. Has mostly been replaced by plastic film capacitors.
Large size. Also, paper is highly hygroscopic, absorbing moisture from the atmosphere despite plastic enclosures and impregnates. Absorbed moisture degrades performance by increasing dielectric losses (power factor) and decreasing insulation resistance.
Metalized Paper Capacitors
Paper
Comparatively smaller in size than paper-foil capacitors
Suitable only for lower current applications. Has been largely superseded by metalized film capacitors
PET film Capacitor
Polyester film
Smaller in size when compared to paper or polypropylene capacitors of comparable specifications. May use plates of foil, metalized film, or a combination. PET film capacitors have almost completely replaced paper capacitors for most DC electronic applications. Operating voltages up to 60,000 V DC and operating temperatures up to 125 °C. Low moisture absorption.
Temperature stability is poorer than paper capacitors. Usable at low (AC power) frequencies, but inappropriate for RF applications due to excessive dielectric heating.
Kapton Capacitor
Kapton polyimide film
Similar to PET film, but significantly higher operating temperature (up to 250 °C).
Higher cost than PET. Temperature stability is poorer than paper capacitors. Usable at low (AC power) frequencies, but inappropriate for RF applications due to excessive dielectric heating.
Polystyrene Capacitor
Polystyrene
Excellent 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. Ideal for low power RF and precision analog applications
Maximum operating temperature is limited to about +85 °C. Comparatively bigger in size.
Polycarbonate Plastic Film Capacitor
Polycarbonate
Superior insulation resistance, dissipation factor, and dielectric absorption versus polystyrene capacitors. Moisture pick-up is less, with about ±80 ppm temperature coefficient. Can use full operating voltage across entire temperature range (−55 °C to 125 °C)
Maximum operating temperature limited to about 125 °C.
Polypropylene Plastic Film Capacitors
Polypropylene
Has become the most popular capacitor dielectric [citation needed]. Extremely low dissipation factor, higher dielectric strength than polycarbonate and polyester films, low moisture absorption, and high insulation resistance. May use plates of foil, metalized film, or a combination. Film is compatible with self-healing technology to improve reliability. Usable in high frequency applications due to very low dielectric losses. Larger value and higher voltage types from 1 to 100 μF at up to 440 V AC are used as run capacitors in some types of single phase electric motors.
More susceptible to damage from transient over-voltages or voltage reversals than oil-impregnated Kraft paper for pulsed power energy discharge applications.
Polysulphone Plastic Film Capacitors
Polysulfone
Similar to polycarbonate. Can withstand full voltage at comparatively higher temperatures. Moisture pick-up is typically 0.2%, limiting its stability.
Very limited availability and higher cost
PTFE Fluorocarbon (TEFLON) Film Capacitors
Polytetra- fluoroethylene
Lowest loss solid dielectric. Operating temperatures up to 250 °C, extremely high insulation resistance, and good stability. Used in stringent, mission-critical applications
Large size (due to low dielectric constant), and higher cost than other film capacitors.
Polyamide Plastic Film Capacitors
Polyamide
Operating temperatures of up to 200 °C. High insulation resistance, good stability and low dissipation factor.
Large size and high cost.
Metalized Plastic Film Capacitors
Polyester or Polycarbonate
Reliable and significantly smaller in size. Thin metallization can be used to advantage by making capacitors "self healing".
Thin plates limit maximum current carrying capability.
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. Very resistant to corona damage
Unless properly sealed, susceptible to moisture pick-up which will increase the power factor and decrease insulation resistance. Higher cost due to scarcity of high grade dielectric material and manually-intensive assembly.
Metalized Mica or Silver Mica Capacitors
Mica
Silver mica capacitors have the above mentioned advantages. In addition, they have much reduced moisture infiltration.
Higher cost
Glass Capacitors
Glass
Similar to Mica Capacitors. Stability and frequency characteristics are better than silver mica capacitors. Ultra-reliable, ultra-stable, and resistant to nuclear radiation.
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. Available in voltages up to 15,000 volts
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. Available in voltages up to 50,000 volts.
Not as stable as Class-I type with respect to temperature, and capacitance changes significantly with applied voltage.
Aluminum Electrolytic Capacitors
Aluminum oxide
Very large capacitance to volume ratio, inexpensive, polarized. Primary applications are as smoothing and reservoir capacitors in power supplies.
Dielectric leakage is high, large internal resistance and inductance limits high frequency performance, poor low temperature stability and loose tolerances. May vent or burst open when overloaded and/or overheated. Limited to about 500 volts.
Lithium Ion Capacitors
Lithium ion
The lithium ion capacitors have a higher power density as compared to batteries and LIC’s are safer in use than LIB’s in which thermal runaway reactions may occur. Compared to electric double layer capacitor (EDLC), the LIC has a higher output voltage. They both have similar power densities, but energy density of an LIC is much higher.
New technology.
Tantalum Electrolytic Capacitors
Tantalum oxide
Large capacitance to volume ratio, smaller size, good stability, wide operating temperature range, long reliable operating life. Extensively used in miniaturized equipment and computers. Available in both polarized and unpolarized varieties. Solid tantalum capacitors have much better characteristics than their wet counterparts.
Higher cost than aluminum electrolytic capacitors. Voltage limited to about 50 volts. Explodes quite violently when voltage rating, current rating, or slew rates are exceeded, or when a polarized version is subjected to reverse voltage.
Electrolytic double-layer capacitors (EDLC) Supercapacitors
Thin Electrolyte layer and Activated Carbon
Extremely large capacitance to volume ratio, small size, low ESR. Available in hundreds, or thousands, of farads. A relatively new capacitor technology. Often used to temporarily provide power to equipment during battery replacement. Can rapidly absorb and deliver larger currents than batteries during charging and discharging, making them valuable for hybrid vehicles. Polarized, low operating voltage (volts per capacitor cell). Groups of cells are stacked to provide higher overall operating voltage.
Relatively high cost.
Alternating current oil-filled Capacitors
Oil-impregnated paper
Usually PET or polypropylene film dielectric. 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..
Limited to low frequency applications due to high dielectric losses at higher frequencies.
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. Physically larger than polymer dielectric counterparts.
Energy Storage Capacitors
Kraft capacitor paper impregnated with electrical grade castor oil or similar high dielectric constant fluid, with extended foil plates
Designed specifically for intermittent duty, high current discharge applications. More tolerant of voltage reversal than many polymer dielectrics. Typical applications include pulsed power, electromagnetic forming, pulsed lasers, Marx generators, and pulsed welders.
Physically large and heavy. Significantly lower energy density than polymer dielectric systems. Not self-healing. Device may fail catastrophically due to high stored energy.
Vacuum Capacitors
Vacuum capacitors use highly evacuated glass or ceramic chamber with concentric cylindrical electrodes.
Extremely low loss. Used for high voltage high power RF applications, such as transmitters and induction heating where even a small amount of dielectric loss would cause excessive heating. Can be self-healing if arc-over current is limited.
Very high cost, fragile, physically large, and relatively low capacitance.

 
4.      Tantalum capacitor advantages and disadvantages
Tantalum capacitors offer many advantages over other types of capacitor. This has meant that their use has risen considerably over the years, and now they are widely used in all forms of electronics equipment. The advantages of tantalum capacitors can be summarized:
•     Volumetric efficiency:   Tantalum capacitors offer a very high level of volumetric efficiency - much greater than many other types. In particular they are better than electrolytic capacitors which are their main rival.
•     Good frequency characteristics:   The frequency response of tantalum capacitors is superior to that of electrolytic capacitors. This means that they are more suitable for use in a number of applications where electrolytic could not be used.
•     High reliability:   Tantalum capacitors are more reliable than many other forms of capacitor. Provided they are operated within their ratings they are able to provide an almost unlimited life. Their use is not time limited as in the case of electrolytic capacitors.
•     Wide operating temperature range:   Tantalum capacitors are able to operate over a very wide temperature range. They are often specified for operating over the range -55C to +125C. This makes them an ideal choice for use in equipment for use in harsh environmental conditions.
•     Compatibility with modern production methods:   Modern production techniques often expose components to high temperatures during soldering as the whole assembly is heated by infra-red heat. Using conventional leaded components only the board surface was heated and the amount of heat conducted by the leads was usually insufficient to damage the components. Tantalum capacitors are able to withstand the temperatures of SMT production and are therefore ideal for use in many new electronics designs.
Tantalum capacitors have a number of disadvantages, and these need to be considered when using them in new designs.
•     Low ripple current ratings:   It is hardly surprising in view of their size, that tantalum capacitors do not have a high ripple current rating. They should not normally be used in areas that require any levels of current to be passed.
•     Not tolerant to reverse or excess voltage:   Tantalum capacitors do not like reverse or excess voltage. Even spikes can destroy them. If they are exposed to excess or reverse voltages then they can explode.
•     More expensive than other types:   Tantalum capacitors are more expensive than many other forms of capacitor. As a result their cost should be considered during the design phase as the other benefits may outweigh any increased costs.

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