The engineer’s capacitor glossary: All terms and acronyms defined

The diagrams, equations and explanations of capacitor terms from A to Q (that’s quality factor, by the way).

Capacitors are a crucial component of any electrical circuit, and every engineer should know how they work and when to use them. But as with every electronic component, there’s a lot to understand beyond the schematic symbol.

The engineer’s complete guide to capacitors explains it all. Consider it your go-to reference on the many different types of capacitors (from the popular electrolytic capacitor to the specialized multilayer organic (MLO) capacitor) and how to select the right capacitor for all the most common circuit applications.

If you’re unfamiliar with any term used throughout this guide, you’ll find a clear explanation in this alphabetized glossary.

Aluminum electrolytic capacitors

Aluminum electrolytic capacitors are polarized. They have an anode electrode (+) made of pure aluminum foil, an electrolyte that acts as the cathode and a thin insulating layer of aluminum oxide that acts as the dielectric. Electrolytic capacitors have a higher charge capability (Q = CV) per unit volume than ceramic capacitors or film capacitors. Rather than using charge, the capacitance voltage product (CV) is used. Applications of aluminum electrolytic capacitors include audio, automotive, bypass, decoupling, filtering and motor starting capacitors.

Aluminum polymer capacitors

Aluminum polymer (electrolytic) capacitors are a polarized capacitor type that are based on an aluminum electrode with an aluminum oxide dielectric. These capacitors use a conductive polymer (solid) material instead of the traditional fluid electrolytes employed in aluminum electrolytic capacitors. This gives aluminum polymer capacitors a longer shelf life and a reduced ESR. Compared to aluminum electrolytic capacitors, aluminum polymer capacitors have an improved electrical performance, but with a greater sensitivity to operating environment. These capacitors are more expensive than aluminum electrolytic capacitors.

Aluminum polymer hybrid capacitors

Aluminum polymer hybrid capacitors use a combination of a liquid and conductive polymer to serve as the electrolyte and aluminum as the cathode. This approach is the best of both worlds: The polymer offers high conductivity and a correspondingly low ESR. The liquid portion of the electrolyte, meanwhile, can withstand high voltages and provide higher capacitance ratings due to its large effective surface area.

Buffer capacitor

A buffer capacitor is placed in parallel with electrical contacts to provide arc suppression.

Bypass capacitor

A bypass capacitor often provides a low-impedance path to ground. It can be used to keep noise out of a load. It can also be used to shunt a signal around a gain-setting resistor to modify a circuit’s voltage gain.

Capacitor networks/arrays

Capacitor networks, or arrays, contain two or more capacitors. The capacitors may be isolated from one another or have one terminal tied to a common bus. These devices can have either single or multiple capacitor values. Dielectric materials include ceramic, metallized polymer or metallized polypropylene. Capacitance values range from 10 pF to 80 µF with tolerances typically ranging from ±5% to ±20%. Maximum voltage ratings range from 6.3 V to 440 V. Capacitor arrays are available in SMT, through-hole and chassis-mount.

Coupling capacitor

A coupling capacitor is used to provide a low-impedance path to connect a signal from one point to another.

DC leakage current (DCL)

The DC leakage current specified at a given voltage and temperature.

Decoupling capacitor

Decoupling capacitors are usually connected between the DC power supply (e.g., VCC) and ground.

Dielectrics and more

Capacitors are usually classified primarily by their dielectric material, but there can be more. For example, ceramic capacitors use various ceramic materials as their dielectric. However, there are two major classifications: Class 1 and Class 2. Further, there are subgroups under Class 2: X5R, X6R and X7R. Aluminum electrolytic capacitors are formed using aluminum electrodes and an electrolyte solution.

One common distinction is between electrolytic and non-electrolytic capacitor types. Electrolytic capacitors use a dielectric material that is formed in-place electrochemically by oxidizing the surface of the electrode material, whereas non-electrolytic (often called “electrostatic” capacitors) use dielectric materials that are generally formed through various mechanical processes and are not a chemical derivative of the electrode material itself.

Electrolytic capacitors offer high capacitance per unit volume, are polarized, low cost, high-loss and exhibit poor parameter stability. In contrast, non-electrolytic device types tend to be bulky for their ratings, are non-polar, relatively expensive, low-loss and (with exceptions) exhibit fair to excellent parameter stability.

Dielectric Withstand Voltage (DWV)

This is the maximum voltage that can be applied to a capacitor without producing voltage breakdown in its dielectric.

Dissipation Factor (DF)

The dissipation factor is associated with capacitors. It is the reciprocal of Q (DF = 1/Q).

Electric Double Layer Capacitors (EDLC)

This is a more descriptive name for supercapacitors.

Electrolytic capacitors

See “Dielectrics and more”.

Electrostatic capacitors

See “Dielectrics and more”.

EMI/EMC capacitor

These are capacitors used to prevent electromagnetic interference (EMI) and to implement electromagnetic control (EMC). Feedthrough capacitors are used commonly for this purpose. Ceramic and film capacitors are the fundamental types used for Class X and Class Y capacitors. Ceramic capacitors provide higher capacitor values in a smaller volume.

Energy storage capacitor

Capacitors store potential energy in an electric field. This is true in low-energy signal processing applications as well as large-energy backup systems in power grids and in renewable energy systems.

Equivalent Series Inductance (ESL)

This represents any inductance associated with the capacitor lead connections and the dielectric. See image for “Equivalent Series Resistance (ESR)”.

Equivalent Series Resistance (ESR)

This represents the resistive losses in the capacitor lead connections and plates. It is also taken to be the minimum resistance when the series resonant circuit (ESR, Cnominal, and ESL) is at resonance. See also “Equivalent Series Inductance (ESL)”.

Capacitor model (equivalent circuit).

Capacitor model (equivalent circuit).

Feedback capacitors

Capacitors are used to form negative feedback in op amp integrators. Feedback capacitors are also incorporated to limit the frequency of an op amp amplifier to a value below that determined by its gain-bandwidth product. In both cases the capacitors should have low leakage current and have adequate precision.


Low-pass, high-pass, band-pass and band-reject filters can be either implemented using only passive or passive devices with active devices (such as op amps). These realizations usually involve relatively low energy levels. In power supply applications, large-valued filter capacitors are used to smooth out the pulsating DC produced by the rectifier stage. (See “Power conditioning”.) They are also found across the inputs and outputs of DC links.

Frequency compensation capacitors

Capacitors in conjunction with resistors are used to modify the phase shift and/or amplitude of a transfer function as a function of frequency to provide an adequate phase margin. The phase margin controls the stability of a system (for example, freedom from oscillation) and establishes its dynamic response (for example, overshoot, undershoot and settling time).

Gain-bandwidth product

The gain-bandwidth product is a constant (fT) when an amplifier system is rolling off with frequency at a slope of -20dB/decade. In an amplifier circuit the closed-loop corner frequency (fH) is fH = fT/Av where AV is the closed-loop voltage gain.

Layered polymer aluminum capacitors

These capacitors use conductive polymer as the electrolyte and have an aluminum cathode. Some polymer capacitors have ESR values as low as 3 mΩ.

Leakage resistance

This is a large resistance in parallel with the nominal capacitance. Ideally it is infinite. See the capacitor model in “Equivalent Series Resistance (ESR)”.


Many capacitors have strong wear mechanisms that limit their useful life. A lifetime specification provides an indication of a capacitor’s expected service life under specified operating conditions. Definitions of service life vary. One common definition is the length of service under specified conditions (which usually are near rated maximum values) within which 50% of fielded devices can be expected to fail. Some service life specifications are more stringent, while others may be more lenient.


Microphony is the tendency to respond to acoustical vibrations (20 Hz to 20 kHz) or to convert vibrations to acoustical noise. Ceramic capacitors exhibit this effect.

Motor starting capacitors

In the case of a single-phase source induction motor, a rotating magnetic field is not produced inherently. Consequently, the motor contains a start winding in addition to its main winding. An initial rotating magnetic field is developed using the start winding with a series-connected start capacitor. The current following through the start winding (with the capacitor) produces a 90-degree phase angle difference (ideally) compared to the current flowing through the main winding.

Due to this phase angle difference, a resultant rotating stator magnetic field is produced which will rotate the shaft in the desired direction. A centrifugal switch is attached in series with the start capacitor. When the motor reaches sufficient speed, the centrifugal switch opens to disconnect the capacitor and the start winding. The motor starting capacitor is usually a non-polarized electrolytic (see “Dielectrics and more”.) In single-phase motor applications, capacitors with values above 70 µF are starting capacitors.

Niobium oxide capacitors

Niobium oxide capacitors are a type of polarized (electrolytic) capacitor incorporating oxides of niobium as anode and dielectric materials alongside a manganese oxide cathode system. Developed in response to a tantalum shortage, their properties and behaviors are like conventional tantalum manganese dioxide (Ta-MnO2) capacitors, with a much narrower range of available capacitance and voltage values (limited to less than 10 V) and a greatly reduced likelihood of device failure resulting in ignition. Consequently, niobium oxide capacitors are regarded to be the safest capacitor technology.

Power factor correction capacitors

A typical AC power system can be modeled using a lumped resistor, a lumped inductor and a capacitor. These elements will be in parallel across the AC voltage source. Capacitive current is phase shifted 180o from inductive current. Consequently, capacitive current can be used to cancel inductive current. The goal is to make the equivalent AC power load as purely resistive as possible. Modern polypropylene film power capacitors are state of the art for power factor correction. High-temperature operation may require glass capacitors.

Power conditioning capacitors

Power conditioning capacitors are connected in parallel with the DC power supply. The power conditioning capacitors hold the DC power supply level during brief AC power line interruptions and insure the minimum instantaneous voltage is large enough to avoid voltage regulator dropout.

Pulsed power capacitors

Pulsed power capacitors are energy discharge capacitors designed to provide high peak discharge current, high energy density, low inductance and low equivalent series resistance.

Quality Factor (Q)

The definition of the quality factor (Q) of a capacitor is given below:

Definition of quality factor (Q).

Definition of quality factor (Q).

Relative permittivity

Relative permittivity measures a material’s capability to permit the establishment of an electric field, relative to that of a vacuum. It is also known as the material’s dielectric constant.

Resonant circuits capacitors

Resonant (tuned) circuits usually provide filtering and frequency selection in RF applications. These applications require capacitors that provide precision and stability. Class 1 (NPO/COG) ceramic capacitors and silver mica capacitors are often used.

Ripple current rating

The ripple current rating of a capacitor indicates the maximum AC current the capacitor should experience. Ripple current results in self-heating due to the capacitor’s ohmic and dielectric losses. The amount of current flow a given device can tolerate is finite and is influenced by environmental conditions.

Run capacitors

In single-phase motor applications, capacitors with values above 70 µF are starting capacitors. Run capacitors (typically 3 to 70 µF) are designed for continuous duty and are energized the entire time the motor is running.

Safety capacitors

Safety capacitors are placed across the AC power line to suppress electromagnetic interference (EMI) and high-frequency radio frequency interference (RFI). Should they fail, these capacitors are designed to fail in a safe mode, which means their failure will not lead to personal injury or equipment damage.

Safety rating

Safety capacitors are given an alphanumeric safety rating, such as X1, X2, Y1 and Y2, according to regulatory standards like IEC 60384-14. X certified devices are not expected to pose a shock hazard. X class capacitors connect line to line (for example, hot to neutral) and are designed to fail as short circuits, which causes the overcurrent protective device to open. Y class capacitors are certified for applications that may pose a shock hazard. They are connected from line to ground (for example, hot to ground or neutral to ground) and fail as open circuits.

The number in a rating like X1 indicates a level of tolerance to surge voltages, as specified in the applicable regulatory standard. Devices may also carry multiple safety ratings, indicating their certification for use in different circumstances. For example, a capacitor with an X1Y2 safety rating may be used in applications requiring an X1 rating as well as those requiring a Y2 rating.


Self-healing is the ability of a metallized capacitor to clear a fault area where a momentary short occurs due to dielectric breakdown under an over-voltage condition.

Series Resonant Frequency (SRF)

The model for a capacitor includes its nominal capacitance, equivalent series inductance (ESL) and its equivalent series resistance (ESR). See the image in “Equivalent Series Resistance (ESR)”.

The series resonant frequency (SRF) occurs when the impedance is at its minimum ESR value and the capacitive reactance is canceled by the ESL. The series resonant frequency is defined as:

Silicon capacitors

Silicon (and thin-film) capacitors are fabricated using integrated circuit and solid-state device techniques. The extreme precision and quality control methods produce capacitors that are nearly ideal in terms of parameter stability. Silicon capacitors compete with ceramic capacitors but tend to be more expensive.

Snubber capacitors

A simple snubber circuit consists of a capacitor in series with a small-valued resistor. Some also incorporate a switching diode to minimize losses. The purpose of the snubber circuit is to slow the rate of rise in the voltage (dv/dt) across a solid-state switch. Snubbers are used to absorb energy to eliminate the voltage spikes and ringing caused by a switch opening under inductive loads. Polypropylene film capacitors are self-healing and often used in snubber circuits.


Supercapacitors (also called ultra capacitors or electric double layer capacitors) are specially designed capacitors that possess extremely large values of capacitance (such as 12,000 F). They can be recharged very quickly and are used primarily for energy storage.

Tantalum polymer capacitors

Tantalum polymer capacitors are dry tantalum polarized capacitors. A conductive polymer anode material is used instead of the manganese dioxide found in other dry tantalum devices. While more expensive, these outperform traditional tantalum electrolytic capacitors and have a more benign failure mode.

Temperature range

The operating temperature range is the range of temperatures for which a capacitor has been qualified for use. The storage temperature range is the range of temperatures for which a capacitor in a non-active state will not experience damage or irreversible parameter shifts. Low temperatures below the specified storage temperature range could result in mechanical damage and ultimate device failure.

Thin-film capacitors

Thin-film capacitors are two-pad SMT devices available in package sizes ranging from 0201 to 1210. The devices are composed of thin-film layers deposited on a substrate and separated by a dielectric. Available capacitance values range from 0.05 pF to 1500 pF. Voltage ratings run from 2.5 V to 100 V. Tolerances can be as low as ±0.01 pF for small-capacitance units and as large as ±20% for larger capacitance values.


A capacitor’s tolerance describes the limits of deviation from its nominal capacitance value under specified test conditions—particularly the AC test voltage and its frequency. In general, tolerance figures specify the steady-state deviation from the nominal value due to variability in manufacturing. The deviation from the nominal value is also affected by device operation over its specified operating temperature range. It should be noted that test conditions (temperature, frequency, amplitude and DC bias value of test voltage, among others) frequently have a strong influence on observed device parameters.

Voltage ratings

Capacitor voltage ratings indicate the maximum voltage that can be applied. The test conditions should be reviewed carefully. The context of the rating is significant; in some cases, it may indicate the maximum safe working voltage, while in others it may be like a semiconductor’s absolute maximum rating and an appropriate de-rating factor should be applied.

Volumetric efficiency

A measure of a capacitor’s capacitance relative to its physical size.

Wound polymer aluminum capacitors

These capacitors are also based on conductive polymers and aluminum, but they have a wound foil structure. Wound polymer capacitors cover a wider range of voltages and capacitance values than other types of polymer capacitors. Some have ESR values below 5 mΩ.


Working DC voltage, usually applied to polarized (electrolytic) capacitors.