AC (Alternating Current)

Current which passes from the generator in one direction and then the other, alternately.

AC Ripple

The alternating current superimposed on the dc bias after rectification in a power supply.

Aerogel Capacitor

These capacitors use carbon aerogel to attain immense electrode surface area, can attain huge values, up to thousands of farads. EDLCs can be used as replacements for batteries in applications where a high discharge current is required, e.g. in electric vehicles. They can also be recharged hundreds of thousands of times, unlike conventional batteries which last for only a few hundred or thousand recharge cycles. However, 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.

Aluminum Electrolytic Capacitor

Aluminum Electrolytic Capacitors are compact but “lossy”. They 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. The electrolyte will tend to dry out in the absence of a sufficient rejuvenating voltage, and eventually the capacitor will fail. Bipolar electrolytics contain two capacitors connected in series opposition and are used for coupling AC signals. Bad frequency and temperature characteristics make them unsuited for high-frequency applications

Ambient Temperature

The temperature of the air or oil surrounding and in contact with the capacitor. To determine the maximum capacitor temperature, add this value to the internal working heat rise that the capacitor will develop.


The electrode through which a direct current enters the liquid, gas, or other discrete part of an electrical circuit; the positively charged pole of an electrochemical cell.


Intense luminous discharge formed by the passage of electric current across a space between electrodes.

Arcover Voltage

The minimum voltage required to create an arc between electrodes separated by an insulation under specified conditions. To increase the Arcover Voltage, clean the area between the terminals and improve the insulation medium between the terminals (by immersing the capacitor in oil).


Device for starting and regulating fluorescent and high intensity discharge lamps.


A situation where Direct Current (DC) is kept from a circuit element due to the high series impedance of a capacitor.

Breakdown (Puncture)

A disruptive discharge through insulation. If plain dielectric films are used, this is usually catastrophic. If metallized films are used, self-healing should maintain the capacitor’s integrity.

Breakdown Voltage

The voltage at which the insulation between two conductors will break down. This voltage should be at least 50% ~ 100% greater than the capacitor’s rated voltage.


Test designed to simulate conditions a product would encounter in actual operation. To shorten the duration of the test, it is conducted at maximum temperature and greater than rated voltage.

Bypass (De-Coupling)

The use of a capacitor to prevent the AC portion of a signal from a circuit element via a low impedance path in parallel with the circuit element.

Canadian Standards Association (CSA)

Group that generates product performance and/or safety standards for many Canadian industries.

Capacitance (Capacity)

That property of a system of conductors and dielectrics which permits the storage of electricity when potential difference exists between the conductors. Its value is expressed as the ratio of a quantity of electricity to a potential difference. A capacitance value is always positive.

Capacitive Reactance (Xc)

The opposition to the flow of alternating or pulsating current by a capacitor measured in ohms. The non-heating impedance component of the capacitor when AC flows: Xc = 1/(6.28fC)


An electrical/electronic part that stores electrical charges. In its simplest form it consists of two conducting surfaces separated by a dielectric.


The electrode through which an electric current leaves a liquid, gas, or other discrete part of an electric current; the negatively charged pole of an electrochemical cell.

Ceramic Capacitor

This capacitor is so named because it contains a ceramic dielectric. One type of ceramic capacitor uses a hollow ceramic cylinder as both the form on which to construct the capacitor and as the dielectric material. The plates consist of thin films of metal deposited on the ceramic cylinder. The other type of ceramic capacitor is manufactured in the shape of a disk. After leads are attached to each side of the capacitor, the capacitor is completely covered with an insulating moisture-proof coating. Ceramic capacitors usually range in value from 1 picofarad to 0.01 microfarad and may be used with voltages as high as 30,000 volts.


The amount of electricity present upon the capacitor’s plates. Also, the act of forcing of electrons onto the capacitor’s plates. See Coulomb.


The reciprocal of resistance. It is the ratio of current passing through a material to the potential difference at its ends.


Reciprocal of volume resistivity. Conductance of a unit cube of any material.


An electrical path which offers comparatively little resistance. A wire or combination of wires not insulated from one another, suitable for carrying a single electric current.


A luminous discharge due to ionization of the gas surrounding a conductor around which exists a voltage gradient exceeding a certain critical value. A type of discharge–sometimes visible–in the dielectric of an insulation system caused by an electric field and characterized by the rapid development of an ionized channel which does not completely bridge the electrode. May be continuous or intermittent. Not a materials property, but related to the system, including electrodes.

Corona Resistance

The time that insulation will withstand a specified level field-intensified ionization that does not result in the immediate complete breakdown of the insulation.

Crest Factor (Lamp Current Crest Factor)

Ratio of peak lamp current to RMS or average lamp operating current.


A coulomb is the unit of electric charge. It is named after Charles-Augustin de Coulomb.1 coulomb is the amount of electric charge transported by a current of 1 ampere in 1 second. It can also be defined in terms of capacitance and voltage, where one coulomb is defined as one farad of capacitance times one volt of electric potential difference.


The use of a capacitor to connect two circuits. In using the capacitor, only the AC voltage (component) is passed.


Electrical leakage on a solid dielectric surface

Critical Voltage (of gas)

The voltage at which a gas ionizes and corona occurs, preliminary to dielectric breakdown of the gas.


The insulating material between the plates of the capacitor. The material is chosen for its ability to permit electrostatic attraction and repulsion to take place across it. The material will have the property that energy required to establish an electric field is recoverable in whole or in part, as electric energy. In other words, a good dielectric material is a poor conductor of electricity while being an effective supporter of electrostatic fields.

Dielectric Absorption

That property of an imperfect dielectric whereby there is an accumulation of electric charges within the body of the material when it is placed in an elec
tric field.

Dielectric Constant

That property of a dielectric which determines the electrostatic energy stored per unit volume for unit potential gradient.

Dielectric Loss

The time rate at which electric energy is transformed into heat in a dielectric when it is subjected to a changing electric field.

Dielectric Loss Angle

The difference between ninety degress (90°) and the dielectric phase angle.

Dielectric Phase Angle

The angular difference in phase between the sinusoidal alternating potential difference applied to a dielectric and the component of the resulting alternating current having the same period as the potential difference.

Dielectric Power Factor

The cosine of the dielectric phase angle (or sine of the dielectric loss angle).

Dielectric Strength

The voltage which an insulating material can withstand before breakdown (puncture) occurs, usually expressed as a voltage gradient (such as volts per mil). The voltage figure used is the average RMS voltage gradient between two electrodes at the time of failure.

Dielectric Test

Tests which consist of the application of a voltage higher than the rated voltage for a specified time for the purpose of determining the adequacy against breakdown of insulating materials and spacings under normal conditions.

Discharge Lamp

A light producing device that depends on an electric arc, rather than a filament, to create illumination.

Displacement Current

A current which exists in addition to ordinary conduction current in ACcircuits. It is proportional to the rate of change of the electric field.

Disruptive Discharge

The sudden and large increase in current through an insulation medium due to the complete failure of the medium under the electrostatic stress.

Dissipation Factor

The tangent of the loss angle of the insulating material. A measure of the power factor (or losses) of a capacitor, given as D.F. = 6.28 fRC X 100%, where R is the equivalent series resistance (ESR) of the capacitor, f is the frequency (Hz.), and C is capacitance (Farads). Dissipation Factor varies with frequency and temperature.


Change in Voltage divided by Change in Time, usually expressed in Volts per uSec.


Electric Double Layer Capacitor is a next-generation energy storage device that will be used as an auxiliary power supply and the combined use with photovoltaics equipment and hybrid electric cars.also known as supercapacitors or ultracapacitors, have very high capacitance values but low voltage ratings. They 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.


A conductor, not necessarily metal, through which a current enters or leaves a capacitor.

Electromagnetic and Electronic Ballasts

An Electromagnetic Ballast (Magnetic Ballast) is one that uses a Core & Coil assembly to transform electrical current to start and operate fluorescent and high intensity discharge (HID) lamps. An Electronic (High Frequency) Ballast is one that, with the help of electronic components, transforms current at high frequency to operate discharge lamps. Electronic ballasts typically operate at frequencies at or above 20,000 Hz.

Electrolytic Capacitor

Electrolytic Capacitor is used where a large amount of capacitance is required. As the name implies, an electrolytic capacitor contains an electrolyte. This electrolyte can be in the form of a liquid (wet electrolytic capacitor). The wet electrolytic capacitor is no longer in popular use due to the care needed to prevent spilling of the electrolyte. A dry electrolytic capacitor consists essentially of two metal plates separated by the electrolyte. In most cases the capacitor is housed in a cylindrical aluminum container which acts as the negative terminal of the capacitor. The positive terminal (or terminals if the capacitor is of the multisection type) is a lug (or lugs) on the bottom end of the container. The capacitance value(s) and the voltage rating of the capacitor are generally printed on the side of the aluminum case.

EMI (Electromagnetic Interference)

Electrical interference (noise) generated by electrical and electronic devices.

ESR (Equivalent Series Resistance)

A resistive element of the capacitor model found in both the AC and DC domains. Contributing factors: electrodes, leads, dielectric. This value can change with frequency, time, etc. A measure of the total lossiness of a capacitor which includes the leads, electrodes, dielectric losses, leakage (IR) and most important, the end spray connecting the leads to the metallized film. The lower the ESR the higher the current carrying ability the capacitor will have.


A farad is defined as the amount of capacitance for which a potential difference of one volt results in a static charge of one coulomb. It has the base SI representation of s4 • A2 • m-2 • kg-1. Since an ampere is the rate of electrical flow (current) of one coulomb per second, an alternate definition is that a farad is the amount of capacitance that requires one second for a one ampere flow of charge to change the voltage by one volt.

Film Capacitor

Made from high quality polymer film (usually polycarbonate, polystyrene, polypropylene, polyester (Mylar), and for high quality capacitors polysulfone), and metal foil or a layer of metal deposited on surface. They have good quality and stability, and are suitable for timer circuits and for high frequencies.

Fixed Capacitor

Fixed Capacitor is constructed in such manner that it possesses a fixed value of capacitance which cannot be adjusted. A fixed capacitor is classified according to the type of material used as its dielectric, such as paper, oil, mica, or electrolyte.

Flame Resistance

Ability of the material to extinguish a flame once the source of heat is removed.


Measure of the material’s ability to support combustion.


A disruptive discharge around or over the surface of a solid or liquid insulator.


The number of complete cycles or vibrations per unit of time. Rate of alternation in an AC current. Expressed in cycles per second or hertz (Hz).


Parts which are connected so that when the installation is complete, it is substantially of the same potential as the earth.


An integral multiple of the fundamental frequency (60 Hz) that becomes a component of the current.

Harmonic Distortion

Distortion of an AC waveform caused by multiples of the fundamental frequency (harmonics). Odd triplet harmonics (thirds, ninths, etc.) may result in large currents on the neutral line in a four-wire Wye three-phase system.

Heat Sink

Any device that absorbs and draws heat off a hot object, radiating it into the surrounding atmosphere.


(Hz) A term replacing cycles-per-second as an indication of frequency.


Tending to absorb moisture.


An effect in which the magnitude of a resulting quantity is different during increases in the magnitude of the cause than during decreases due to internal friction in a substance and accompanied by the production of heat within the substance. Electric hysteresis occurs when a dielectric material is subjected to a varying electric field as in a capacitor in an alternating current circuit.


The total opposition that a circuit offers to the flow of alternating current or any other varying current at a particular frequency. It is a combination of resistance R and reactance X, measured in ohms.


To fill the voids and interstices of a material with a compound.


A surge of unidirectional polarity.


Material having a high resistance to the flow of electric curre
nt, which prevents leakage of current from a conductor.

Insulation Resistance

The ratio of the applied Voltage to the total current between two electrodes in contact with a specific insulator. A measure of the resistance to a DC current flow through the capacitor under steady state conditions. Values for film and ceramic capacitors are usually expressed in megohm-microfarads for a given design and dielectric. The actual resistance of the capacitor is obtained by dividing the megohm-microfarads by the capacitance.


A material of such low electrical conductivity that the flow of current through it can usually be neglected.

Internal Fuse

A device that disconnects capacitor operation from power distribution system in the event of excessive current.

Ion, Ionization

An electrified portion of matter of sub-atomic, atomic, or molecular dimensions such as is formed when a molecule of gas loses an electron (when the gas is stressed electrically beyond the critical voltage) or when a neutral atom or group of atoms in a fluid loses or gains one or more electrons. Ionization is the dissociation of an atom or molecule into positive or negative ions or electrons. Restrictively, the state of an insulator whereby it facilitates the passage of current due to the presence of charged particles usually induced artificially.

Joule (watt second)

Joule = 0.5 X Capacitance (Farads) X Voltage^2. The Joule is a measure of the amount of energy delivered by one WATT of power in one second or 1 million watts of power in one microsecond. The Joule rating of a surge protection device is the amount of energy that it can absorb before it becomes damaged.

Mica Capacitor

This is made of metal foil plates that are separated by sheets of mica (the dielectric). The whole assembly is encased in molded plastic. Since the capacitor parts are molded into a plastic case, corrosion and damage to the plates and dielectric are prevented. Also the molded plastic case makes the capacitor mechanically stronger. Various types of terminals are used on mica capacitors to connect them into circuits. These terminals are also molded into the plastic case.

Moisture Resistance

The ability of a material to resist absorbing moisture from the air or when immersed in water

Oil Capacitors (Self Healing)

These are often used in high-power electronic equipment. An oil-filled capacitor is nothing more than a paper capacitor that is immersed in oil. Since oil impregnated paper has a high dielectric constant, it can be used in the production of capacitors having a high capacitance value. Many capacitors will use oil with another dielectric material to prevent arcing between the plates. If arcing should occur between the plates of an oil-filled capacitor, the oil will tend to reseal the hole caused by the arcing. These are refered to as a self healing capacitor.


A voltage above the normal operating voltage of a device or circuit. In a dielectric withstand test, capacitors are overvoltage-tested (Hi-potted) at 1.5X or 2X its rated voltage to assure quality and workmanship.

Parallel Lamp Operation

Refers to ballasts that employ multiple-output current paths from a single ballast to allow lamps to operate independently of one another, allowing other lamps operated by ballast to remain lit should companion lamp(s) fail.

Partial Discharge

A partial discharge is an electric discharge that only partially bridges the insulation betwen conductors when the voltage stress exceeds a critical value. Partial discharges may, or may not, occur adjacent to a conductor. Partial discharge is often referred to as corona but the term corona is preferably reserved for localized discharges in cases around a conductor, bare or insulated, remote from any other solid insulation.

PCB (Polychlorinated Biphenyls)

Chemical pollutant formerly used in oil-filled capacitors which have been outlawed since the 1970’s and are no longer used in the capacitor and transformer industries.


Preferred term for dielectric constant.


An electrical condition determining the direction in which current tends to flow. The quality of having two opposite charges.

Polycarbonate Resins

Polymers derived from the direct reaction between aromatic and aliphatic dihydroxy compounds with phosgene or by the ester exchange reaction with appropriate phosgene derived precursors.


A resin formed by the reaction between a dibasic acid and a dihydroxy alcohol.


A thermoplastic material composed of polymers of ethylene.


A compound formed by polymerization which results in the chemical union of monomers or the continued reaction between lower molecular weight polymers.


To unite chemically two or more monomers or polymers of the same kind to form a molecule with higher molecular weight.


A plastic made by the polymerization of high-purity propylene gas in the presence of an organometallic catalyst at relative low pressures and temperatures.


A thermoplastic produced by the polymerization of styrene (vinyl benzene).

Potential Voltage

The work per unit charge required to bring any charge to the point at which the potential exists.


Similar to encapsulating, except that steps are taken to insure complete penetration of all voids in the object before the resin polymerizes.


Material used to completely surround and cover the capacitor winding. Potting compounds protect components, dampen sound, and dissipate heat.


The time rate at which work is done; equal to W/t where W is amount of work done in time t. Power will be obtained in watts if W is expressed in joules and t in seconds.

Power Factor

Measurement of the relationship between the AC source voltage and current. Formula: Power Factor equals Watts input divided by the product of Line Volts multiplied by Line Amps.


Force measured per unit area. Absolute pressure is measured with respect to zero pressure. Gauge pressure is measured with respect to atmospheric pressure.

Radio Frequency Interference (RFI)

Form of electromagnetic interference that can be radiated through the air.

Relative Humidity

Ratio of the quantity of water vapor present in the air to the quantity which would saturate it at any given temperature.


Property of a conductor that determines the current produced by a given difference of potential. The ohm is the practical unit of resistance.


The ability of a material to resist passage of electrical current either through its bulk or on a surface. The unit of volume resistivity is the ohm-cm, of surface resistivity, the ohm.


The amount-of radiation that will produce one electrostatic unit of ions per cubic centimeter volume.

Schering Bridge

An alternating current form of Wheatstone bridge, used for comparing capacitances or for measuring the phase angle of a capacitor by comparison with a standard capacitor.


A material whose resistivity is between that of insulators and conductors. The resistivity is often changed by light, heat, an electric field, or magnetic field. Current flow is often achieved by transfer of positive holes as well as by movements of electrons. Examples include germanium, lead sulfide, lead telluride, selenium, silicon, and silicon carbide. Used in diodes, photocells, thermistors, transistors, etc.

Series Lamp Operation

Refers to ballasts that employ a single current path passing through all lamps operated by the ballast If one lamp should fail, companion lamps operated by the same ballast will also extinguish.


Polymeric materials in which the recurring chemical group contains silicon and oxygen atoms as links in the main chain.

Sine Wave

A mathematical function used to represent voltage and current.


A disruptive discharge between electrodes of a measuring gap, such as a sphere gap or oil testi
ng gap.

Specific Gravity

The density (mass per unit volume) of any material divided by that of water at a standard temperature.


Another word for SuperCapacitors or Ultracapacitors


Another word for Ultracapacitors. Made from carbon aerogel, carbon nanotubes, or highly porous electrode materials. Extremely high capacity. Can be used in some applications instead of rechargeable batteries.

Surface Leakage

The passing of current over the boundary surfaces of an insulator as distinguished from passage through its volume.


A transient variation in the current and/or potential at a point in the circuit.

Tantalum Capacitor

Compact, low-voltage devices up to several hundred µF, these have a lower energy density and are more accurate than aluminum electrolytics. These capacitors are comprised of a permeable tantalum cneter section surrounded by tantalum pentoxide. A tantalum wire is inserted into the center section and then extends axially from teh component. There are many advantages of using tantalum capacitors over other types: They have higher volumetric efficiency (CV/cc); They have superior frequency characteristics; They are highly reliable and do not degrade over time. Tantalum capacitors do not lose capacitance like electrolytic capacitors. Unlimted shelf life.

Temperature Coefficient

The change in capacitance with temperature expressed linearly as parts per million per degree centigrade (PPM/°C), or as a percent change over a specified temperature range. Most film capacitors are not linear and TC is expressed in percent.

Thermal Conductivity

Ability of a material to conduct heat.

Thermal Expansion (Coefficient of Transients)

The fractional change in length (sometimes volume) of a material for a unit change in temperature.


High voltage surges through an electrical system caused by lightning strikes to nearby transformers, overhead lines, or the ground. May also be caused by switching of motors and compressors, as well as by short circuits or utility system switching.


A measure of the resistance of a fluid to flow (usually through a specific orifice).


Unit of Electromotive Force. It is the difference of potential required to make a current of one ampere flow through a resistance of one ohm.


Electromotive Force, potential, potential difference, or voltage drop, to designate electric pressure that exists between two points and is capable of producing a flow of current when a closed circuit is connected between the two points.

Voltage Sag

Drop in voltage levels of electrical distribution system which interferes with the operation of electrical and electronic equipment. Commonly called brownout. Results when demand for electricity exceeds capacity of the distribution system.

Ultra Broadband Capacitor (UBC)

A component that provides ultra-low insertion loss, flat frequency response and an excellent match over multiple octaves of frequency spectrum in a one piece configuration. They look like a tiny little box.


Also known as Supercapacitor, or Electrochemical Capacitor, or Electric Double Layer Capacitor (EDLC). Made from carbon aerogel, carbon nanotubes, or highly porous electrode materials. Extremely high capacity. Can be used in some applications instead of rechargeable batteries.