Capacitor types

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).

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 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.

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.

Capacitor	Polarized Capacitor	Variable Capacitor

Capacitor symbols

Types of dielectric

Structure of a surface mount (SMT) film capacitor.

• Air-gap: An air-gap capacitor has a low dielectric loss. Large-valued, tunable capacitors that can be used for resonating HF antennas can be made this way.
• 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, (PET film): (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.
• Electrolytic capacitors have a larger capacitance per unit volume than other types, making them valuable in relatively high-current and low-frequency electrical circuits, e.g. in power-supply filters or as coupling capacitors in audio amplifiers. High-capacity electrolytics, also known as supercapacitors or ultracapacitors, have applications similar to those of rechargable batteries, e.g. in electrically powered vehicles;
• Printed circuit board: 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.
• In integrated circuits, small capacitors can be formed through appropriate patterns of metallization on an isolating substrate.
• Vacuum: Expensive, housed in glass or ceramic body, typically rated for 5kV - 30kV. Typically used in high power RF transmitters because the dielectric has virtually no loss and is self-healing. May be fixed or adjustable.

Fixed capacitor comparisons

Capacitor type	Dielectric used	Advantages/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 impregnants. Absorbed moisture degrades perfomance 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 superceded by metalized film capacitors
PET film or Mylar 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. Mylar capacitors have almost completely replaced paper capacitors for most DC electronic applications. Operating voltages up to 1600VDC and operating temperatures up to 125ºC. Low moisture absorbtion.	Temperature stability is poorer than paper capacitors. Inappropriate for RF applications due to excessive dielectric losses.
Kapton Capacitor	Kapton polyimide film	Similar to PET film, but significantly higher operating temperature (up to 250ºC).	Higher cost than PET. Temperature stability is poorer than paper capacitors. Inappropriate for RF applications due to excessive dielectric losses.
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 +85ºC. 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 almost zero temperature co-efficient. Can use full operating voltage across entire temperature range (-55ºC to 125ºC)	Maximum operating temperature limited to about 125ºC.
Polypropelene Plastic Film Capacitors	Polypropylene	Has become the most popular capacitor dielectric. Extremely low dissipation factor, higher dielectric strength than polycarbonate and may polyester films, low moisture absorbtion, 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 low dielectric losses.	More susceptible to damage from transient overvoltages or voltage reversals than oil-impregnated Kraft paper for robust pulsed power 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	Polytetrafluoroethylene	Lowest loss solid dielectric. Operating temperatures up to 250ºC, 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 200ºC. 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	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 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 with respect to temperature, and capacitance changes significantly with applied voltage.
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 tantalum capacitors have much better characteristics than their wet counterparts.	Higher cost than aluminum electrolytic capacitors
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 higer 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 excesive 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-idealities of practical capacitors

Q factor, dissipation and tan-delta

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.

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 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.

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 0.1 µF rated for high frequencies and a large electrolytic rated for lower frequencies, and occasionally, an intermediate value capacitor.

Voltage

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.

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 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.

Dielectric absorption (soakage)

In the construction of long-time-constant integrators, it is important that the capacitor will not retain a residual charge when shorted. This phenomenon of unwanted charge storage is called dielectric absorption or soakage, and it effectively creates a memory effect in the capacitor. This is a non-linear phenomenon, and is also important when building very low distortion filters.

Non-linearity

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.

Leakage

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.)

Component values and identification

Standard values

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; picofarads were often called micro-microfarads (µµF) then.

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

The same series are used for resistors, where E24/E48/E96 series are additionally used for even lower-tolerance components.

Practical capacitor values (the E3 and E6 series)

Why do electronic capacitors come in rather strange values multiples as (22, 47, 68, etc.) and not in all values? There is nothing particularly special about these values. Industry cannot produce every conceivable value of capacitor and for larger values, (1000μF, say) an extra 47μF doesn't make much difference.

The choice of values produced is a compromise:
The E3 Series (10, 22, 47)
The E6 Series (10, 15, 22, 33, 47, 68)
Capacitors are made available in multiples of ten of the E6 series, or more commonly the E3 series. The E3 series is more common for capacitors due to the difficulty of producing accurate values of most types of capacitors (most capacitors are produced with relatively large tolerances).

Since 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 nominal value, they are normally available in E6 (or even just E3) series values only (e.g. 2200 µF, 3300 µF, 4700 µF) – the tolerance ranges overlap the intermediate values from the next higher series anyway.

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

Colour coding

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	—	±5%	F	500	—	—
Blue	6	—	—	—	—	−55 °C to +150°C	—
Violet	7	—	—	—	—	—	—
Grey	8	—	—	—	—	—	—
White	9	—	—	—	—	—	EIA
Gold	—	—	±0.5%*	—	1000	—	—
Silver	—	—	±10%	—	—	—	—

*Or ±0.5 pF, whichever is greater.

See also

• Capacitor
• Capacitance
• Capacitor plague (premature failure of some electrolytic capacitors)
• Supercapacitor
• Electronic Devices and Circuits
• Capacitor color code
• Inductor

External links

• Caltech: Practical capacitor properties
• FaradNet: The Capacitor Resource
• NessCap, maker of 5000 farad capacitors
• 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
• Electrolytic Capacitors

References

• Glenn Zorpette Super Charged: A Tiny South Korean Company is Out to Make Capacitors Powerful enough to Propel the Next Generation of Hybrid-Electric Cars, 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
• Basic Circuit Theory with Digital Computations, Lawrence P. Huelsman, Prentice-Hall, 1972
• Philosophical Transactions of the Royal Society LXXII, Appendix 8, 1782 (Volta coins the word condenser)
• A. K. Maini Electronic Projects for Beginners, "Pustak Mahal", 2nd Edition: March, 1998 (INDIA)
• Spark Museum (von Kleist and Musschenbroek)
• Biography of von Kleist