Types of capacitor

Practical 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).

Contents

Capacitor construction

Capacitors have thin conducting plates (usually made of metal), separated by a layer of dielectric, then stacked or rolled to form a compact device.

Many types of capacitors are available commercially, with capacitance ranging from the picofarad, microfarad range to more than a farad, and voltage ratings up to hundreds of 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.

The electrodes need round edges to avoid field electron emission. Air has a low breakdown voltage, so any air inside a capacitor - especially at plate edges - will reduce the voltage rating. Even closed air bubbles in the insulator or between the insulator and the electrode lead to gas discharge, particularly in AC or high frequency applications. Groups of identically constructed capacitor elements are often connected in series for operation at higher voltage. High voltage capacitors need large, smooth, and round terminals to prevent corona discharge.

Types of dielectric

Capacitor Polarized
Capacitor		 Variable
Capacitor
Capacitor symbols

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 impregnant. 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 Has mostly been replaced by plastic film capacitors.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). Polyimides have the highest dielectric strength of any known dielectric. 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 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 and high frequency high power applications such as induction heating (often combined with water-cooling) 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 metalization 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 an 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.
Niobium Oxide Capacitors Niobium oxide is used in capacitors where a layer of Nb2O5 is formed around NbO grains as the dielectric. . .
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.

Variable capacitors

Main article: Variable capacitor

Variable capacitors may have their capacitance intentionally and repeatedly changed over the life of the device. They include capacitors that use a mechanical construction to change the distance between the plates, or the amount of plate surface area which overlaps, and variable capacitance diodes that change their capacitance as a function of the applied reverse bias voltage.

Variable capacitance is also used in sensors for physical quantities, including microphones, pressure and hygro sensors.

Non-ideal properties of practical capacitors

Breakdown voltage

Main article: Breakdown voltage

The breakdown voltage of the dielectric limits the power density of capacitors. For a particular dielectric, the breakdown voltage is proportional to the thickness of the dielectric.

If a manufacturer makes a new capacitor with the same dielectric as some old capacitor, but with half the thickness of the dielectric, the new capacitor has half the breakdown voltage of the old capacitor.

Because the plates are closer together, the manufacturer can put twice the parallel-plate area inside the new capacitor and still fit it in the same volume (capacitor size) as the old capacitor. Since the capacitance of a parallel-plate capacitor is given by:

C \approx \frac{\varepsilon A}{d}

this new capacitor has 4 times the capacitance as the old capacitor.

Since the energy stored in a capacitor is given by:

E_\mathrm{stored} = \frac{1}{2} C V^2,

this new capacitor has the same maximum energy density as the old capacitor.

The energy density depends only on the dielectric. Making a few thick layers of dielectric (which can support a high voltage, but results in a low capacitance), or making many very thin layers of dielectric (which results in a low breakdown voltage, but a higher capacitance) has no effect on the energy density.

Q factor, dissipation and tan-delta

Capacitors have Q (quality) factor (and the inverse, dissipation factor, D or tan-delta) which relates capacitance at a certain frequency to the combined losses due to dielectric leakage and series internal resistance (also known as ESR) dissipation factor (dielectric loss). The lower the Q, the lossier the capacitor. Aluminum electrolytic types have typically low Q factors. High Q capacitors tend to exhibit low DC leakage currents. 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.

Equivalent series resistance (ESR)

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 (non-superconducting) physical devices are constructed of materials with nonzero electrical resistance, which means that all real-world components contain some resistance in addition to their other properties. A low ESR capacitor typically has an ESR of 0.01 Ω. Low values are preferred for high-current, pulse applications. Low ESR capacitors have the capability 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.

Equivalent series inductance (ESL)

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 (100 nF) capacitor rated for high frequencies and a large electrolytic rated for lower frequencies, and occasionally, an intermediate value capacitor.

Maximum voltage and current

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, peak current, fault current, and percent voltage reversal are further considerations. Typically the voltage is 66% of the rated voltage. A voltage higher than that, usually reduces the life expectancy depending on manufacturer. The time for a voltage to discharge is 6 time constants.

Temperature dependence

Another major non-ideality is temperature coefficient (change in capacitance with temperature) which is usually quoted in parts per million (ppm) per degree Celsius.

Aging

When refurbishing old (especially audio) equipment, it is a good idea to replace all of the electrolyte-based capacitors. After long storage, the electrolyte and dielectric layer within 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. Deteriorating power supply filter capacitors are a frequent cause of hum in aging audio equipment.

Non-polarised capacitors also suffer from aging, changing their values slightly over long periods of time.

In high voltage DC applications, accumulated capacitor stress due to in-rush currents at circuit power-up can be minimized with a pre-charge circuit.

Dielectric absorption (soakage)

Some types of dielectrics, when they have been holding a voltage for a long time, maintain a "memory" of that voltage: after they have been quickly fully discharged and left without an applied voltage, a voltage will gradually be established which is some fraction of the original voltage. For some dielectrics 10% or more of the original voltage may reappear. This phenomenon of unwanted charge storage is called dielectric absorption or soakage, and it effectively creates a hysteresis or memory effect in capacitors.

The percentage of the original voltage restored depends upon the dielectric and is a non-linear function of original voltage.[2]

In many applications of capacitors dielectric absorption is not a problem but in some applications, such as long-time-constant integrators, sample-and-hold circuits, switched-capacitor analog-to-digital converters, and very low-distortion filters, it is important that the capacitor does not recover a residual charge after full discharge, and capacitors with low absorption are specified[3]. For safety, high-voltage capacitors are often stored with their terminals short circuited.

Some dielectrics have very low dielectric absorption, e.g., polystyrene, polypropylene, NPO ceramic, and Teflon. Others, in particular those used in electrolytic and supercapacitors, tend to have high absorption.

Voltage non-linearities

Capacitors may also change capacitance with applied voltage. This effect is more prevalent in high k ceramic and some high voltage capacitors. This is a small source of non-linearity in low-distortion filters and other analog applications.

Leakage

The resistance between the terminals of a capacitor is never truly infinite, leading to some level of d.c. 'leakage'; this ultimately limits how long capacitors can store charge. Before modern low-leakage dielectrics were developed this was a major source of problems in some applications (long time-constant timers, sample-and-holds, etc.).

Component values and identification

Standard values

Before 1960 electronic components values were not standardised. The more common, but not the only, values for capacitors were 1.0, 1.5, 2.0, 3.0, 5.0, 6.0, and 8.0 as base numbers multiplied by some negative or positive power of ten. Values in the nanofarad range and above were stated in microfarads (often incorrectly abbreviated as mF or mfd); lower values were stated in micro-microfarads (µµF, now called picofarads, pF).

In the late 1960s a standardized set of geometrically increasing preferred values was introduced. According to the number of values per decade, these were called the E3, E6, E12, etc. series

Series Values
E3 1.0 2.2 4.7
E6 1.0 1.5 2.2 3.3 4.7 6.8
E12 1.0 1.2 1.5 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2

In many applications capacitors need not be specified to tight tolerance (they often need only to exceed a certain value); this is particularly true for electrolytic capacitors, which are often used for filtering and bypassing. Consequently capacitors, particularly electrolytics, often have a tolerance range of ±20% and need to be available only within E6 (or E3) series values.

Other types of capacitors, e.g. ceramic, can be manufactured to tighter tolerances and are available in E12 or closer-spaced values (e.g. 47 pF, 56 pF, 68 pF).

Since the establishment of the SI in 1960, the range of prefixes used to specify capacitor values has expanded to include everything from pico- to kilo-, which is the range of commercially available capacitors. In some regions, however, certain prefixes can be less common than others; notably, in North America, use of millifarad and nanofarad is uncommon.

Capacitor markings

Capacitors, like most other electronic components, have markings in their bodies to indicate their electrical characteristics, in particular capacitance, tolerance, working voltage and polarity (if relevant). For most types of capacitor, numerical markings are used, whereas some capacitors, especially older types, use colour coding.

Numerical markings

On capacitors that are large enough (e.g. electrolytic capacitors) the capacity and working voltage are printed on the body without encoding. Sometimes the markings also include the maximum working temperature, manufacturer's name and other information.

Smaller capacitors use a shorthand notation, to display all the relevant information in the limited space. The most commonly used format is: XYZ J/K/M VOLTS V, where XYZ represents the capacitance (calculated as XY × 10Z pF), the letters J, K or M indicate the tolerance (±5%, ±10% and ±20% respectively) and VOLTS V represents the working voltage.

Polarised capacitors, for which one electrode must always be positive relative to the other, have clear polarity markings, usually a stripe or a "-" sign on the side of the negative electrode. Also, the negative lead is usually shorter.

Examples:

An electrolytic capacitor might be marked with the following information: 47µF 160V 105°C

A capacitor with the following text on its body: 105K 330V has a capacitance of 10 × 105 pF = 1 µF (±10%) with a working voltage of 330 V.

A capacitor with the following text: 473M 100V has a capacitance of 47 × 103 pF = 47 nF (±20%) with a working voltage of 100 V.

Colour coding

Main article: Electronic color code

Capacitors may be marked with 3 or more coloured bands or dots. 3-colour coding encodes most significant digit, second most significant digit, and multiplier. Additional bands have meanings which may vary from one type to another. Low-tolerance capacitors may begin with the first 3 (rather than 2) digits of the value. It is usually, but not always, possible to work out what scheme is used by the particular colours used. Cylindrical capacitors marked with bands may look like resistors.

Colour Significant digits Multiplier Capacitance tolerance Characteristic DC working voltage Operating temperature EIA/vibration
  Black 0 1 ±20% — — −55 °C to +70 °C 10 to 55 Hz
Brown 1 10 ±1% B 100 — —
Red 2 100 ±2% C — −55 °C to +85 °C —
Orange 3 1,000 — D 300 — —
Yellow 4 10,000 — E — −55 °C to +125 °C 10 to 2000 Hz
Green 5 — ±0.5% F 500 — —
Blue 6 — — — — −55 °C to +150 °C —
Violet 7 — — — — — —
Grey 8 — — — — — —
White 9 — — — — — EIA
Gold — — ±5%* — 1000 — —
Silver — — ±10% — — — —

*Or ±0.5 pF, whichever is greater.

See also

Notes

  1. ^ The abbreviation "MF" was used to indicate microfarads at the time; "MMF" was common for micro-microfarad = 10-12 F or picofarads.
  2. ^ "Modeling Dielectric Absorption in Capacitors" by Ken Kundert http://www.designers-guide.org/Modeling/da.pdf
  3. ^ "Understand Capacitor Soakage to Optimize Analog Systems" by Bob Pease 1982 http://www.national.com/rap/Application/0,1570,28,00.html

References

External links