This information is provided to help clarify and define the terminology associated with RCD products and their applications. Some of these terms are listed in the component datasheets. Many terms go beyond the specification of inductors. These terms describe issues associated with inductor design and performance, magnetic materials and theory and applications. A thorough understanding of these terms and definitions will aid in the selling, procurement and application of inductor products.



Alternating Current
A periodic current the average value of which over a period is zero. The equation for alternating current is the same as that for a periodic current except that I0 = O*

Ambient Temperature
The temperature of still air immediately surrounding a component or circuit. A typical method to measure ambient temperature is to record the temperature that is approximately 1/2 inch from the body of the component or circuit.

Ambient Temperature Sensor
Temperature sensor used to measure the temperature of the air that surrounds a component (the ambient temperature).

Ampere(s), the unit of electrical current. Current is defined as the amount of charge that flows past a give point, per unit of time. The symbol I is used for current in equations and A is the abbreviation for ampere.

Analog Temperature Sensor
Temperature sensor with a continuous analog voltage or current output that is related, usually linearly, to the measured temperature.

Armature Resistor
A resistor connected in series with the armature of a motor either to limit the inrush current or starting, the gradual short circuiting of which brings the motor to normal speed, or to regulate the speed by armature-voltage control.

Axial Inductor
An inductor constructed on a core with concentric leads on opposite ends of the core. Axial inductors are available for both power applications and RF applications, and are available in many core materials including the basic phenolic, ferrite and powdered iron types. Both rod and bobbin shapes are utilized. Axial inductors are very suitable for tape and reel packaging for auto placement. (see Inductor)


Bobbin Core
A core with the shape of a bobbin or spool which contains flanges. Bobbin cores are available with and without leads and in the axial and radial form. (see Axial Inductor and Radial Inductor)

Bobbin Wound Inductor
Bobbin wound inductors refers to a type or method of construction of winding inductors chokes and reactors. Toroidal coils are wound directly onto a toroidal core. The core may be coated or boxed to insulate it from the coil windings. In contrast, bobbin wound inductor coils are wound independently of the core. The coil must hold its shape or form until the coil is assembled onto the inductor core. One common method of doing this is to wind the coil onto a bobbin (also referred to as a spool), hence the term “bobbin wound winding inductor”.

Bracket Terminal Resistor
A resistor equipped with slotted metal and end j brackets that serve as a means of mounting and connecting to the resistor.

Burden Resistance
Resistance value connected across output terminals to produce required monitoring voltage for AC voltage measuring system.


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

A device, the primary purpose of which is to introduce capacitance into an electric circuit. Capacitors are usually classified, according to their dielectrics.

Ceramic Cores
Ceramic is one of the common materials used for inductor cores. Its main purpose is to provide a form for the coil. In some designs it also provides the structure to hold the terminals in place. Ceramic has a very low thermal coefficient of expansion. This allows for relatively high inductance stability over the operating temperature ranges. Ceramic has no magnetic properties. Thus, there is no increase in permeability due to the core material.

Ceramic core inductors are often referred to as “air core” inductors. Ceramic core inductors are most often used in high frequency applications where low inductance values, very low core losses and high Q values are required.

Circuit Topology
A generic term used to reference the different types of circuit configurations that may be implemented in a switch mode power supply.

The shortest distance through space between two live parts, between live parts and supports or other objects, or between any live part and grounded part.

A body so constructed from conducting material that it may be used as a carrier of electric current.

A body so constructed from conducting material that it may be used as a carrier of electric current.

Continuous Duty
A requirement of service that demands operation at a substantially constant load for an indefinitely long time.

Continuous Rating
Continuous Rating is the rating that defines the load which can be carried for an indefinitely long time.

Continuous-Duty Resistor
A resistor that is capable of carrying continuously the current for which it is designed without exceeding the specified temperature rise.

Convection is the motion resulting in a fluid owing to differences of density and the action of gravity.

Material placed within and around a coil to support the coil and to play a role in the magnetic flux properties of the inductor.

Core Geometry
Style or types of core structure. Core Geometries have been developed for use in different circuit applications.

Core Loss
Core losses are caused by an alternating magnetic field in the core material. The losses are a function of the operating frequency and the total magnetic flux swing. The total core losses are made up of three main components: Hysteresis, eddy current and residual losses. These losses vary considerably from one magnetic material to another. Applications such as higher power and higher frequency switching regulators and RF designs require careful core selection to yield the highest inductor performance by keeping the core losses to a minimum.

Creepage Distance
The shortest distance between conductors of opposite polarity or between a live part and ground as measured over the surface of the supporting material.

Current-Limiting Resistor
A resistor inserted into an electric circuit to limit the flow of current to some predetermined value.
Note: A current-limiting resistor, usually in series with a fuse or circuit breaker, may be employed to limit the flow of circuit or system energy at the time of a fault or short-circuit.

Current Output
Output is a current monitored by an ammeter.

Current Rating
The maximum current the inductor can handle at the rated temperature range. Expressed in milliamps (mA) or amps (A) maximum.

Current-Limiting Resistor
A resistor inserted into an electric circuit to limit the flow of current to some predetermined value.


DC Current
The unidirectional flow or movement of electric charge carriers. The intensity of the current can vary with time, but the general direction of movement stays the same at all times.

DCR (DC Resistance)
The resistance of the inductor winding measured with no alternating current. The DCR is most often minimized in the design of an inductor. The unit of measure is ohms (Ω), and it is usually specified as a maximum rating. This is an undesirable characteristic, which is a by-product of the wire or conductive material used. The lower the DCR, the more current an inductor will handle.

Dielectric Strength
The dielectric strength of an insulating material is the maximum potential gradient that the material can withstand without rupture. It is usually specified in volts per unit thickness.

Dielectric Test
A test which consists 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.

Direct Current
A unidirectional current in which the changes in value are either zero or so small that they may be neglected. A given current would be considered a direct current in some applications, but would not necessarily be so considered in other applications.

Distributed Capacitance
In the construction of an inductor, each turn of wire or conductor acts as a capacitor plate. The combined effect of each turn can be represented as a single capacitance known as the distributed capacitance. This capacitance is in parallel with the inductor. This parallel combination will resonate at some frequency which is called the self-resonant frequency (SRF). Lower distributed capacitances for a given inductance value will result in a higher SRF value for the inductor and vice versa. (see SRF)


Electronic Industries Alliance

Electromotive Force
The electromotive force is the agency causing the flow of current in a circuit. It is the electrical pressure (or drop) measured in volts.

EMI is an acronym for Electromagnetic Interference. It is unwanted electrical energy in any form. EMI is often used interchangeably with “Noise”.

EMI Frequency Range
Frequency range of interference the choke is to filter.

Epoxy Coated Inductor
Inductors that have been coated with epoxy as opposed to having a molded case, shrink wrapped tubing or left with an open construction body. Epoxy coated inductors typically have smooth edges and surfaces. The epoxy coat acts as an insulation. Both radial and axial styles can be found with epoxy coated surfaces.


The unit of capacitance of an electric condenser in which a charge of one coulomb produces a difference of potential of one volt between the poles of the capacitor.

Ferrite Core
Ferrite is a magnetic material which consists of a mixed oxide of iron and other elements that are made to have a crystalline molecular structure. The crystalline structure is created by firing the ferrite material at a very high temperature for a specified amount of time and profile. The general composition of ferrites is xxFe2O4 where xx represents one of several metals. The most popular metal combinations are manganese and zinc (MnZn) and nickel and zinc (NiZn). These metals can be easily magnetized.

Field Discharge Switch
A switch usually of the knife blade type having auxiliary contacts for connecting the field of a generator or motor across a resistor (field discharge) at the instant preceding the opening of the switch.

Fixed Resistor
A resistor designed to introduce only one set amount of resistance into an electrical circuit.

Frequency Range
Range of frequencies over which the inductor is expected to operate.


The unit of inductance of a closed circuit in which an electromotive force of one volt is produced when the electric current traversing the circuit varies uniformly at the rate of one ampere per second.

Maximum isolation voltage from winding to winding or winding to core/ground.

HiPot Duration
Length of time in seconds to test device at the HiPot voltage.

Hot Spot
The point or location of maximum temperature on the external surface of a resistor.


The apparent resistance of an AC circuit, being the combination of both the resistance and reactance. It is equal to the ratio of the value of the EMF between the terminals to the current, there being no source of power in the portion under consideration. The unit of impedance is the ohm and is represented by Z.

Incremental Current
The DC bias current flowing through the inductor which causes an inductance drop of 5% from the initial zero DC bias inductance value. This current level indicates where the inductance can be expected to drop significantly if the DC bias current is increased further. This applies mostly to ferrite cores in lieu of powdered iron. Powdered iron cores exhibit “soft” saturation characteristics. This means their inductance drop from higher DC levels is much more gradual than ferrite cores. The rate at which the inductance will drop is also a function of the core shape. (see Saturation Current)

The property of a circuit element which tends to oppose any change in the current flowing through it. The inductance for a given inductor is influenced by the core material, core shape and size, the turns count and the shape of the coil. Inductors most often have their inductances expressed in microhenries (µH). The following table can be used to convert units of inductance to microhenries. Thus, 47 mH would be equal to 47,000 µH.

1 henry (H) = 106 µH
1 millihenry (mH) = 103 µH
1 microhenry (µH) = 1 µH
1 nanohenry (nH) = 10-3 µH

A passive component designed to resist changes in current. Inductors are often referred to as “AC Resistors”. The ability to resist changes in current and the ability to store energy in its magnetic field, account for the bulk of the useful properties of inductors. Current passing through an inductor will produce a magnetic field. A changing magnetic field induces a voltage which opposes the field-producing current. This property of impeding changes of current is known as inductance. The voltage induced across an inductor by a change of current is defined as:

V=L di/dt

Thus, the induced voltage is proportional to the inductance value and the rate of current change. (see Inductance)

Input Voltage(s)
The voltage applied to the primary winding.

Intermittent Duty
A requirement of service that demands operation for alternate intervals of (1) load and no-load; or (2) load and rest; or (3) load, no-load and rest; such alternate intervals being definitely specified.

Intermittent-Duty Resistor
A resistor capable of carrying for a short period of time the high overload current for which it is designed without exceeding the specified temperature rise.


MIL Resistor
A resistor built in accordance with Joint Army-Navy specifications.

Machine-Duty Resistor
A resistor for use in the armature or rotor circuit of a motor in which the armature current is almost constant.

Maximum Power Dissipation
An inductor’s ability to handle the heat generated by operating at maximum current at an ambient temperature, expressed in Watts (W) or milliwatts (mW). This is a function of the body area of the inductor and core material used.

Maximum Primary Current
Maximum current flowing through the conductor to be monitored.

A unit of resistance equal to one million ohms.

Metal Electro Leadless Face

Molded Inductor
An inductor whose case has been formed via a molding process. Common molding processes include injection and transfer molding. Molded inductors typically have well defined body dimensions which consist of smooth surfaces and sharper corners as compared to other case types such as epoxy coated and shrink wrap coatings. (see Inductor)

The method used to mount the component to the circuit board.

Multi-Section Resistor
A resistor having two or more electrically independent sections.


Unwanted electrical energy in a circuit that is unrelated to the desired signal. Sources of noise are mos often generated by some type of switching circuit. Common sources include switching voltage regulators and clocked signals such as digital circuits.

No Load Inductance
Inductance value under no load conditions expressed in micro henries.

Nominal Diameter
As applied to tubular resistors, this is the diameter of the ceramic tube expressed in inches and/or fractions thereof.

Nominal Length
As applied to tubular resistors, this is the length of the resistor base or core expressed in inches and/or fractions thereof.

Non-Inductive Resistor
A non-inductive power resistor is one in which the inductance and distributed capacitance are reduced to an absolute minimum.

Number of Primaries
Number of Primary Windings. Indicate if center tapped.

Number of Secondaries
The number of secondaries required.


The unit of measurement for resistance and impedance. Resistance is calculated by Ohm’s Law:

R = V/I
R = Resistance
V = Voltage
I = Current

An instrument for measuring electric resistance that is provided with a scale graduated in ohms.

Operating Frequency
Frequency at which a product will operate.

Operating Temperature Range
Range of ambient temperatures over which a component can be operated safely. The operating temperature is different from the storage temperature in that it accounts for the component’s self temperature rise caused by the winding loss from a given DC bias current. This power loss is referred to as the “copper” loss and is equal to:

Power Loss = I2 x DCR

This power loss results in an increase to the component temperature above the given ambient temperature. Thus, the maximum operating temperature will be less than the maximum storage temperature:

Maximum Operating Temperature = Storage Temperature – Self Temperature Rise

Output Current
Output Current expected to flow through secondary winding(s) under fully loaded conditions.

Output Voltage(s)
Output Voltage expected from secondary winding(s) under fully loaded conditions.


Percent Saturation
This is equal to 100% minus percent initial permeability. i.e. 20% saturation = 80% of initial permeability.

Periodic Duty
A type of intermittent duty in which the load conditions are regularly recurrent.

Periodic Rating
The rating which defines the load which can be carried for the alternate periods of load and rest specified in the rating, the apparatus starting cold and for the total time specified in the rating without causing any of the specified limitations to be exceeded.

Permeability (Core)
The permeability of a magnetic core is the characteristic that gives the core the ability to concentrate lines of magnetic flux. The core material, as well as the core geometry, affect the core’s “effective permeability”. For a given core shape, size and material, and a given winding, higher permeability magnetic materials result in higher inductance values as opposed to lower permeability materials.

Phenolic Core
Phenolic is a common material used for inductor cores. Many are made of polyester base that have high temperature characteristics. It is also common for phenolic cores to have high flammability ratings such as UL94V-0. Phenolic cores also provide high strength and are more economical than ceramic cores.

Phenolic has no magnetic properties. Thus there is no increase in permeability due to the core material.

Phenolic core inductors are often referred to as “air core” inductors and are most often used in high frequency applications where low inductance values, very low core losses and high Q values are required.

Powdered Iron Core
Powdered iron is a magnetic material that has an inherent distributed air gap. The distributed air gap allows the core to store higher levels of magnetic flux when compared to other magnetic materials such as ferrites. This characteristic allows a higher DC current level to flow through the inductor before the inductor saturates.

Powdered iron cores are made of nearly 100% iron. The iron particles are insulated from each other, mixed with a binder (such as phenolic or epoxy) and pressed into the final core shape. The cores are cured via a baking process. Other characteristics of powdered iron cores include: they are typically the lowest cost alternative and their permeabilities typically have a more stable temperature coefficient than ferrites. (see Saturation Current)

The time rate of transferring or transforming energy; the rate of doing work or expending energy.

Power Rating
Maximum power expected to process.

Power Resistor
A resistor capable of dissipating 5 watts or more.

Primary Inductance
Inductance in the primary winding.

Primary Winding
The winding connected to the source of energy.


RoHS (Restriction on Hazardous Substances)
European directive limiting or prohibiting the use of certain materials.

Radial Inductor
An inductor constructed on a core with leads exiting from the same side of the inductor body as to be mounted in the same plane. Radial inductors most often refer to two leaded devices but technically include devices with more than two leads as well. Some common core shapes include rod cores, bobbins and toroids. (see Inductor)

Rated Current
The level of continuous DC current that can be passed through the inductor. This DC current level is based on a maximum temperature rise of the inductor at the maximum rated ambient temperature. The rated current is related to the inductor’s ability to minimize the power losses in the winding by having a low DC resistance. It is also related to the inductor’s ability to dissipate this power lost in the windings. Thus, the rated current can be increased by reducing the DC resistance or increasing the inductor size.

For low frequency current waveforms, the RMS current can be substituted for the DC rated current. The rated current is not related to the magnetic properties of the inductor. (see Incremental Current and Saturation Current)

A designated limit of operating characteristics of a machine, apparatus or device, based on definite conditions.

Note 1: Such operating characteristics as load, voltage, frequency, etc., may be given in the rating.

Note 2: The rating of control apparatus in general is expressed in volts, amperes, horsepower or kilowatts as may be appropriate, except that resistors are rated in ohms, amperes and class of service.

The imaginary part of the impedance. (see Impedance)

A device used for introducing reactance into a circuit for purposes such as motor starting, paralleling transformers and control of current.

A device which converts alternating current to unidirectional current by virtue of a characteristic permitting appreciable flow of current in only one direction.

Required Finished Dimensions
The dimensions of the space allotted for the finished product.

The (scalar) property of an electric circuit or of any body which may be used as part of an electric circuit which determines for a given current the rate at which electric energy is converted into heat or radiant energy and which has a value such that the product of the resistance and the square of the current gives the rate of conversion of energy. In the general case, resistance is a function of the current, but the term is most commonly used in connection with circuits where the resistance is independent of the current.

Resistance Method of Temperature Determination
This method consists in the determination of temperature by comparison of the resistance of the winding at the temperature to be determined with the resistance at a known temperature.

Resistance Tolerance
The resistance tolerance of a power resistor is the extent to which its resistance may be permitted to deviate above or below the specified resistance. Resistance tolerance is usually expressed in percent.

Resistance Conductor
A resistive conductor is a conductor used primarily because it possesses the property of high electric resistance.

The resistivity of a material is the resistance of a sample of the material having specified dimensions.

Resistive Core
The resistor core or base of a power resistor is the insulating support on which the resistive conductor is wound.

A device, the primary purpose of which is to introduce resistance into an electric circuit.


Maximum density of magnetic flux that can be present in a magnetic material.

Saturation Current
The DC bias current flowing through the inductor which causes the inductance to drop by a specified amount from the initial zero DC bias inductance value. Common specified inductance drop percentages include 10% and 20%. It is useful to use the 10% inductance drop value for ferrite cores and 20% for powdered iron cores in energy storage applications.

The cause of the inductance to drop due to the DC bias current is related to the magnetic properties of the core. The core, and some of the space around the core, can only store a given amount of magnetic flux density.

Beyond the maximum flux density point, the permeability of the core is reduced. Thus, the inductance is caused to drop. Core saturation does not apply to “air-core” inductors. (see Incremental Current and Permeability)

Secondary Current
Desired output AC current that represents max primary current.

Secondary Winding
The winding is the coil where energy is induced from the primary.

Secondary Voltage
Required output AC voltage to represent max primary current.

Self-Resonant-Frequency (SRF)
The frequency at which the inductor’s distributed capacitance resonates with the inductance. it is at this frequency that the inductance is equal to the capacitance and they cancel each other. The inductor will act purely resistive with a high impedance at the SRF point.

The distributed capacitance is caused by the turns of wire layered on top of each other and around the core. This capacitance is in parallel to the inductance. At frequencies above the SRF, the capacitive reactance of the parallel combination will become the dominant component.

Also, the Q of the inductor is equal to zero at the SRF point since the inductive reactance is zero. The SRF is specified in MHz and is listed as a minimum value on product data sheets.

Shielded Inductor
An inductor designed for its core to contain a majority of its magnetic field. Some inductor designs are self shielding. Examples of these are magnetic core shapes which include toroids, pot cores and E-cores. Magnetic core shapes such as slug cores and bobbins require the application of a magnetic sleeve or similar method to yield a shielded inductor.

It should be noted that magnetic shielding is a matter of degree. A certain percentage of the magnetic field will escape the core material. This is even applicable for toroidal cores as lower core permeabilities will have higher fringing fields than will high permeability toroidal cores.

Short-Time Rating
The rating that defines the load which can be carried for a short and definitely specified time, the machine, apparatus or device being at approximately room temperature at the time the load is applied.

A silicone coating meeting MIL-R-26 used on power type wirewound resistors.

Single-Wound Resistor
A resistor that has only one layer of resistance wire or ribbon wound around the insulating base or core.

Surface Mount
Components manufactured with solder feet that are soldered to pads on the circuit board providing the mounting and connection to the circuit. Electrical connection is made through soldering the component to these solder pads.

Still Air
Still air is considered air having no circulation except that created by the heat of the resistor which is being operated.


Tapped Resistor
A resistor with two or more steps.

Temperature Coefficient of Inductance
(Tc of L) is the value of inductance change as a function of temperature exposure, normally expressed in parts per million per degrees Celsius. This is a calculation comparing inductance at a reference temperature (25°C, room ambient) to the extremes and other temperatures within operating range. Can be called Percent Delta L or Temperature Stability; the lower the change the better for most applications.

Temperature Coefficient of Resistance
A measure of the increase or decrease in resistance of a resistive conductor due to change in temperature in parts per million (ppm). RT = Rr + [Rr(αT – αTr)] Where, RT = Resistance of conductor at temperature T Rr = Resistance of conductor at reference temperature Tr α = Temperature coefficient of resistance at reference temperature Tr

Temperature Rise
Temperature rise is the difference in temperature between the initial and final temperature of a resistor. Temperature rise is expressed in degrees C or F, usually referred to an ambient temperature. Temperature rise equals the hot spot temperature minus the ambient temperature.

Test Frequency
The industry/military standard for testing a range of inductances. It is not intended as the application frequency. Expressed in megahertz (MHz) or kilohertz (kHz).

Thermal Shock
Thermal shock consists of a sudden marked change in the temperature of the medium in which the device operates.

A device for converting heat energy into electrical energy consisting of a pair of dissimilar conductors so joined as to produce a thermo-electric effect. It is used with a millivoltmeter to measure temperature rise in apparatus.

Thru-Hole (or Through-Hole)
Component is manufactured with pins that mount the device through holes in the circuit board which are soldered.

Tolerance (%)
The tolerance is the allowable deviation from the nominal resistance value.

Turns Ratio (i.e. Ip:Is)
The ratio of the number of primary turns to the number of secondary turns expressed as Np:Ns.  Ratio of current monitored to current needed for sensing current.


Varying Duty
A requirement of service that demands operation at loads, and for intervals of time, both of which may be subject to wide variation.

Voltage (V or E)
The unit of measure is the volt. Is an expression of potential energy, representing he possibility or potential for energy release as the electrons move from one “level” to another, it is always referenced between two points.

Voltage Coefficient (VCR)
The unit of measure is in parts per million (ppm). Voltage coefficient defines the change in the value of the resistor that occurs as the voltage changes. The resistor is measured at two voltages and the deviation is then calculated. VCR is usually states as the change per volt (ex. 2ppm/v).

Voltage Range
Range of input voltage that is expected across primary winding.

Voltage Rating
Max output (voltage x current) without core saturation.


A unit of electric power. It is the power expended when one ampere of direct current flows through a resistor of one ohm.

Winding Pitch
The distance from any point on a turn of a resistive conductor to the corresponding point on an adjacent turn measured parallel to the long axis of the winding.