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Power Supplies A to Z : "I"

We've included this handy glossary to help with the terminology & abbreviations relating to power products. Pick a letter below to begin, or download the glossary to refer to it whenever you like.

A B C D E F G H I J K L M N O P Q R S T U V W X-Z #


Symbol for electric Current.


Acronym for Inter-Integrated Circuit. A simple, serial communications bus designed by Philips suitable for communications inside a system. Often used to turn on/off a power supply (or supplies) in a system. See Communications Port.

I²R Loss

See Copper Loss.


Abbreviation for Integrated Circuit.


Abbreviation for International Electrotechnical Commission.


Abbreviation of IEC System for Conformity testing and Certification of Electrical Equipment. See CB Scheme. (


Abbreviation for Institution of Electrical Engineers (


Abbreviation for Institute of Electrical and Electronic Engineers (


The standard defining the GPIB (parallel communications bus). See Communications Port.


Abbreviation for Institute of Engineering and Technology (


Abbreviation for Institution of Incorporated Engineers. Now part of the Institute of Engineering and Technology

Impedance (Z)

Total resistance to flow of an alternating current as a result of resistance and reactance.

Imperial Standard Wire Gauge

See Standard Wire Gauge.


See Insulated Metal Substrate.

Induced Current

Current that flows as a result of an Induced EMF(Electromotive Force).

Induced EMF

Voltage induced in a conductor in a varying magnetic field.

Inductance (L)

The inherent reactive property, measured in henrys, of an electric circuit or circuit element that opposes a change in current flow. Hence, inductance causes current changes to lag behind voltage changes. See also henry.

Inductive Reactance (XL)

Opposition to a changing current as a result of inductance:


A coil or component with the properties of inductance.

Schematic Symbol for an Inductor


A signal input to a power supply which will turn off the outputs (if the unit is fan cooled then this may also inhibit the fan). The signal has to be activated to make the outputs turn off, with nothing connected to the input, the power supply should operate. (Compare this with Enable).

Input Impedance

The impedance between the input terminals of a circuit or device, with the input disconnected.

Input Line Filter

A low-pass or band reject filter at the input of a power supply (PSU) that reduces line noise fed to the supply. This filter may be external to the power supply (PSU).

Input Pi Filter

See Pi Filter.

Input Surge

See Inrush Current.

Input Voltage Range

The range of input voltage values over which a power supply (PSU) or device will operate within its specified limits.

Inrush Current

The peak instantaneous input current drawn by a power supply (PSU) at turn on. In a switched mode power supply, there is no transformer reaction to limit the current that flows when the power supply is first switched on. An extremely large current can be drawn from the mains as the reservoir capacitors charge to the full voltage. To reduce this current to a sensible level, they often incorporate Inrush Current Limiting. This peak is usually restricted by inrush current limiting and only occurs for a short period of time(usually one cycle of ac) and occurs only if the power supply (PSU) is turned on at the peak of the ac cycle.

Inrush Current Limiting

The characteristic of a circuit that limits inrush current when a power supply (PSU) is turned on. Inrush Current Limiting is typically performed by either a thermistor (which when cold has a high resistance, when hot, a low resistance) connected in series with the ac supply, or by a relay which switches in a resistor until a defined period (a few milliseconds) has passed, or by a relay which bypasses a series resistor or NTC after the initial start-up period has passed. Inrush current limiting enables a power supply to meet IEC/EN61000-3-3 (flicker).

Instantaneous Value

The measured value of a signal at a given moment in time.

Institute of Electrical and Electronic Engineers (IEEE)

A US based professional organisation. The IEEE develops standards of definitions, test methods, symbols, units and safety in the field of electrical science and engineering.

Institute of Engineering and Technology

Institution formed in spring 2006 by the coming together of the IEE and the IIE.

Institution of Electrical Engineers (IEE)

A UK based professional organisation. The IEE develops standards of definitions, test methods, symbols, units and safety in the field of electrical science and engineering. Now part of the Institute of Engineering and Technology.

Institution of Incorporated Engineers (IIE)

Now part of the Institute of Engineering and Technology.

Insulated Metal Substrate

A Printed Circuit Board where the substrate is metal (usually aluminium but sometimes copper or other metal) covered with a thin layer of insulating material (epoxy based) and then a layer of copper (from which the required conducting tracks are created). IMS has excellent thermal conductance and is used where cooling is necessary (especially for Surface Mount Devices).


Non-conductive materials used to separate electrical circuits and to prevent electric shock.

Insulation Resistance

The resistance, usually measured in megaohms, of an insulating material.


Material that prevents the flow of electricity.

Integrated Circuit (IC)

Multiple components (both active and passive) manufactured on a single semiconductor substrate (usually silicon).

Integrated Magnetics

Multiple magnetic components assembled as a single assembly. Used on TDK-Lambda's EFE product range to increase efficiency and reduce the size of the power supply

Intermittent Duty

Operation for specified alternate intervals of load and no load. See also Continuous Duty.

Internal Impedance

The impedance exhibited by a power supply or electronic device.

Internal Resistance

See Internal Impedance

International Electrotechnical Commission (IEC)

Organisation that prepares and publishes international standards for electrical, electronic and related technologies. The standards cover safety, performance, EMC and environmental. The IEC does not perform any testing; this function is left to the national testing agencies. National standards are frequently based on IEC publications. See


1) A device that changes dc into ac (sometimes called dc-ac converters).

2) A circuit, circuit element or device that inverts the input signal.


A “United States-based trade association dedicated to furthering the competitive excellence and financial success of its members worldwide, who are participants in the electronic interconnect industry."Formerly known as 'Institute for Printed Circuits' and 'Institute for Interconnecting and Packaging Electronic Circuits'. See (

IR Drop

See Voltage Drop.

Iron Core

A general class of core that contains iron. Sometimes used to refer to cores made only from steel laminations. See also Core, Ferrite, Powdered Iron Core.


The electrical separation between two circuits, or circuit elements.

Isolation Transformer

A transformer (usually with a one-to-one turns ratio) used as a safety device to isolate a device under test from earth. Alternatively used to block the transmission of DC signals whilst allowing the ac to pass. See also Step-Down Transformer, Step-Up Transformer, Transformer.

Isolation Voltage

The maximum ac or dc specified voltage that may be continuously applied between isolated circuits.

A B C D E F G H I J K L M N O P Q R S T U V W X-Z #

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