Glossary of Capacitor Terms

Illinois Capacitor provides this list of capacitor terminology to assist our customers, students and educators alike.  If you have a comment on a definition, or have a suggestion for additions, please let us know.

AC Ripple

The AC current flowing in the capacitor, so called because the associated AC voltage rides like ripple on water on the capacitor’s DC bias voltage. Ripple current causes heating in capacitors. The maximum permitted ripple current is how much can be permitted and still meet the capacitor’s load life specification.


The change in the characteristics of a capacitor over time, under specified conditions.
Aluminum Electrolytic Capacitor

Aluminum Electrolytic Capacitor

A capacitor made up of two aluminum electrodes seperated by paper saturated with an electrolyte. The dielectric is the oxide of the anode.

Ambient Temperature

The temperature of the environment, usually the still air surrounding the capacitor.


The positive electrode of an electrolytic capacitor.


A type of capacitor in which the two leads are connected to opposite ends of the main body of the component.  It is typically soldered in place horizontally to the circuit board or used for point-to-point wiring between terminals.


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

Breakdown (Puncture)

A disruptive momentary circuit path (discharge) through insulation layers of a capacitor. If plain dielectric film capacitors, this results in a failure. Metallized film types are self-healing and should repair the fault (in the capacitor is not being stressed by an over-voltage condition.

Breakdown Voltage

Working voltage that will cause the insulation between two conductors will break down. The voltage rating of the capacitor should be specified at least 50% ~ 100% greater than the circuit voltage.

Bypassing (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.


A measure of the energy storage capability of a capacitor at a given voltage usually expressed in farads, microfarads, nanofarads, or picofarads.

Capacitive Reactance (Xc)

The opposition to the flow of alternating or pulsating current by a capacitor measured in ohms. The imaginery component of the impedence of a capacitor.

Capacitive Reactance Formula


A passive circuit element capable of storing electrical energy and releasing it at a predetermined time and at a predetermined rate.

Capacitor in Parallel

When capacitors are connected in parallel, the total capacitance is the sum of the individual capacitors’ capacitances.  This also increases ripple-current tollerence. 

Capacitor in Series

Capacitors connected in series for a higher total voltage rating, although this configuration will have a lower total capacitance than any single one in the circuit.  This series circuit offers . The voltage drop across each capacitor is the sum of the total applied voltage.


The negative electrode of a capacitor.


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 coulomb is the International System of Units unit of electric charge. It is the charge transported by a constant current of one ampere in one second.


Situation where two circuits are connected together where only the AC voltage is passed.

Corona discharge

Corona is a small but locally intense electrical discharge that injects charge into the insulating film adjacent to edges of foil/metallization or a location where air is trapped between foil/metallization and the film. The discharge is caused by a voltage gradient large enough to ionize molecules in either the film or small air pockets. Each discharge does some small but cumulative damage to the film. This will result in a short circuit in a plain film/foil part. However, metallized film parts self-heal, but additional discharges will result in progressive capacitance loss. 


The amount of charge moving past a given reference point over time.


A decoupling capacitor is one that is used to decouple or separate one part of a circuit from another so that it is not affected by a different variable, such as voltage or current. (See Bypassing.)


An insulating or non-conductive medium between the plates of a capacitor.

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 electric field.

Dielectric Constant

The ratio of a capacitor's capacitance with a given dielectric to the same capacitor having a vacuum as a dielectric.

Dielectric Strength

The average RMS voltage gradient between two capacitor electrodes at the time of failure.

Dissipation Factor (D.F. or Tan delta)

A measure of the losses in a capacitor expressed as the ratio of the E.S.R. of a capacitor to its reactance at a specific frequency and temperature.

dv/dt Rating

The maximum voltage rise (or discharge) time a capacitor can withstand being damaged.


See supercapacitors

Electrolytic Capacitor

An  aluminum  electrolytic  capacitor  consists  of  a  wound  capacitor  element,  impregnated  with  liquid  electrolyte,  connected  to  terminals  and  sealed  in  a  can.    The element  is  comprised  of  an  anode  foil,  paper  separators  saturated  with  electrolyte  and  a  cathode foil.  The foils are high-purity aluminum and are etched with billions of microscopic tunnels to increase the surface area in contact with the electrolyte.


A current-conducting solution between the electrodes of a capacitor used to replenish the dielectric in an aluminum electrolytic capacitor.


An abbreviation for Electromagnetic Interference. The generation of unwanted RF noise, which may be generated by the operation of a switching power supply or other electrical or electronic equipment.  Also called RFI

Equivalent Series Resistance (E.S.R.)

The sum of all the internal resistances of a capacitor measured in ohms. Expressed mathematically as ESR = D.F.*Xc.


The basic unit of a measure of a capacitor. A capacitor charged to 1 volt with a charge of 1 coulomb would have a capacitance of 1 farad. 1 microF = .000001 Farads.

Film Capacitors

A capacitor made up of two metal plates seperated by a plastic(polymer) dielectric material.


The rate at which an alternating or pulsating wave repeats itself expressed in cycles per second (C.P.S.) or hertz (Hz).


The unit of measure of the number of cycles -per-second of an AC signal, as an indication of frequency.

Hold-Up Capacitor

A capacitor that is functioning to provide output voltage for a brief period after the removal of input voltage.

Impedance (Zc)

The total opposition offered to alternating or pulsating current measured in ohms. Impedence is the vector sum of the resistive and reactive components of a capacitor expressed mathematically as

Impedance Formula

Insulation resistance

The ratio of the DC voltage applied to the terminals of a capacitor and the resultant leakage current flowing through the dielectric and over its surface after the initial charging current has ceased expressed in megohms or as time constant megohm x microfarads.

Leakage Current

Measure of the stray direct current flowing through capacitor after DC voltage is impressed on it.

MTBF (Mean Time Between Failure)

MTBF (Mean time between failures) is the most commonly used reliability rating used today. Aluminum electrolytic capacitors do not failure in a manner where MTBF can be used accurately. Instead of using MTBF, Load Life is the usual reliability rating. 

Metallized Capacitor

A capacitor where a thin layer of metal is vacuum-deposited directly onto the dielectric.

Operating Temperature

The temperatures range, usually stated in Celcius, in which a capacitor can operate within rated specifications.


A voltage applied to a capacitor that is above its rated operating voltage. In a dielectric withstand test, capacitors are overvoltage-tested (Hi-Pot tested) at 1.5X or 2X its rated voltage.


Some capacitors, such as most aluminum electrolytics have polarity, limiting the direction in which current tends to flow. Other specialized electrolytics (such as motor start, film and ceramic capacitors) are non-polar.


A capacitor in which both leads are connected to one end of the main component. Radial-leaded capacitors are typically mounted vertically to a circuit board.

Ripple and Noise

The combined amplitude of the AC components on the DC output of a power supply usually expressed in millivolts peak-to-peak or RMS.

Ripple Current

The total amount of alternating and direct current that can be applied to a capacitor under specific conditions without causing a failure. (See AC Ripple)

Snubber Capacitor

Capacitors used independently or with other circuit elements to suppress voltage transients in electrical circuits.


Supercapacitors also called ultracapacitors or Electric double layer capacitors (EDLC) are capacitors made up of 2 metal plates with a carbon dielectric material and a conductive electrolyte.

Surge Voltage (SV)

The maximum DC voltage a capacitor can tolerate under any circumstances for a short period of time without suffering any damage.

Temperature Coefficient

The expected change in capacitance value with variations in temperature. 

Temperature Derating

When a capacitor is operated at a lower ambiant temperature than rated specifications, its load life can be extended.  The amount of improvement can be calculated.


The maximum deviation from a nominal capacitance value at specific conditions expressed as a percentage of nominal capacitance.


The force or electrical pressure which causes current to flow through a conductor.

Ultra Capacitor

See supercapacitors

Working Voltage (WVDC)

The maximum DC voltage applied to a capacitor for continuous operation at maximum rated temperature.

X Capacitor

A safety agency approved capacitor designed to be connected across the supply lines to suppress normal-mode elecrical interference.

New Y Capacitor

A safety agency approved capacitor connected from line to chassis ground for EMI suppression.