Glossary of Capacitor Terms

Capacitor: A capacitor is a storage tank for electrical energy. In DC applications it blocks DC current and in AC applications it regulates AC current.

Capacitance: Capacitance is the property of a capacitor that defines its ability to store an electrical charge (or energy) when a given voltage is applied. The international unit of measurement for capacitance is Farad (or microfarads), which is named after the famous English inventor Michael Faraday (1791-1867).

Dielectric: Dielectric is the material used as the insulating medium between the plates of a capacitor.

Dissipation Factor (DF): This is the energy dissipated (loss) between charging and discharging a capacitor usually expressed in %. This energy is in the form of heat, therefore, the capacitor with the lowest DF will run much cooler in high current applications.

EIA – 456 – A: ANSI EIA standard “Metallized Film Dielectric Capacitors for AC Application.” This standard describes the requirements for metallized film dielectric capacitors, dry or non-PCB liquid filled and sealed in metal cases on non-metal cases made of self extinguishing material.

Equivalent Series Resistance (ESR): This is the internal resistance to current flow resulting from the interconnection of the terminals to the conductive material (electrodes) in the capacitor. This is expressed in ohms or milliohms. A low ESR is desirable because a high resistance generates heat and is directly related to DF.

Impedance: This is the resistance to the flow of AC current measured in ohms. With low impedance, less internal heat is generated in the capacitor.

Inductance: Inductance is an undesirable intrinsic component of a capacitor field that adversely affects the capacitors ability to handle high current. It is a circuit element that opposes a change in current flow that causes current changes to lag behind voltage changes measured in Henrys (Hn). Henry is defined as a unit of inductance in which an induced electromotive force of one volt is produced when the current is varied at the rate of 1 ampere per second.

Insulation Resistance (IR): 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.

Ripple Current: This applies to a DC application with a superimposed AC voltage. The ripple current rating is the maximum AC current the capacitor can handle.

Rated Temperature: The maximum operating temperature where the capacitor can operate without derating.

Temperature Coefficient (TC): 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.