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Magnetics Glossary

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AC Coupling
Use of a special circuit to remove the static (dc) components from the input signal to the amplifier in an instrument, leaving only the components of the signal that vary with time.

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.

Analog-to-Digital Conversion (ADC)
The process of subdividing an analog signal into discrete time segments, comparing the signal over those time segments to discrete voltage levels, and reporting the results of the comparisons in the form of digital outputs of binary numbers having a resolution of n bits.

The relative decrease in amplitude of a given parameter. Attenuation measurements are common for voltage, current and power. It is usually expressed in units of decibels (dB). For a power ratio, one dB=10Log(Pl/P2). A dB is equal to 20Log(I1/I2) for current and 20Log(V1/V2) for voltage ratios.

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A range expressing the limiting frequencies for which a suitable fraction of the maximum output of an instrument is obtained, which for strain gage instrumentation is usually zero Hz (dc) to the frequency at which the output is attenuated to either -3dB (half power, reflecting 70.7% of the input signal) or -0.5dB (about 90% power, reflecting about 95% of the input signal).

Bobbin Core
A core with the shape of a bobbin or spool which contains flanges.

Boost Regulator(DC-DC)
A basic DC-DC switching converter topology that takes an unregulated input voltage and produces higher regulated output voltage. This higher output voltage is achieved by storing energy in an input inductor and then transferring the energy to the output by turning a shunt switch (transistor) on and off.

Bridge Voltage
Voltage impressed across the power corners of a Wheatstone bridge by the constant-voltage power supply.

Buck Regulator (DC-DC)
A basic DC-DC switching converter topology that takes an unregulated input voltage and produces a lower regulated output voltage. This output voltage is achieved by chopping the input voltage with a series connected switch (transistor) which applies pulses to an averaging inductor and capacitor.

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Close Magnetic Path
Magnetic core shapes designed to contain all of the magnetic flux generated from an excited winding(s). Inductors made with these core types are considered to be shielded inductors. Shielding, however, is a matter of degree. Common core shapes that are considered to have closed magnetic paths are toroids, E-cores, and most pot cores. Shielded bobbins also offer a high degree of shielding but most have an air gap to some degree. Common core shapes that are considered to have open magnetic flux paths are rod cores and unshielded bobbin cores.

Coefficient of Thermal Expansion
The ratio of (a) the change in length of a line segment in a body per unit of temperature change to (b) its length at a reference temperature.

Another name for inductors

Common-Mode Noise
Noise or electrical interference that is common to both electrical lines in relation to earth ground.

Common-Mode Voltage
A voltage (usually unwanted) appearing in common at both inputs of an instrument with respect to the output reference (usually ground).

Constant-Current Power Supply
Power source that, when attached to the power corners of a Wheatstone bridge , produces the same current for all values of bridge resistance.

Constant-Voltage Power Supply
Power source that, when attached to the power corners of a Wheatstone bridge , produces the same voltage for all values of bridge resistance.

Copper Loss
The power lost by current flowing through the winding. The power loss is equal to the square of the current multiplied by the resistance of the wire (1^2*R). This power loss is transferred into heat.

Core Losses
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 require careful core selection to yield the highest inductor performance by keeping the core losses to a minimum.

Core Saturation
See Saturation Current.

Curie Temperature
The temperature above which a ferrite core loses its magnetic properties. The core's permeability typically increases dramatically as the core temperature approaches the curie temperature which causes the inductance to increase. The permeability drops to near unity at the curie temperature which causes the inductance to drop dramatically. The curie point is the temperature at which the initial permeability has dropped to 10% of its original value at room temperature.

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DC-DC Converter
A circuit or device that converts a DC input voltage to a regulated output voltage. The output voltage may be lower, higher or the same as the input voltage. Switching regulator DC-DC circuits most often require an inductor or transformer to achieve the regulated output voltage. Switching regulator circuits can achieve a higher level of power efficiency when compared to non-switching techniques (Also see Boost Regulator and Buck Regulator).

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 and it is usually specified as a maximum rating.

Differential-Input Voltage
The maximum voltage that can be applied across the input terminals of a strain gage instrument without causing damage to the instrument.

Differential-Mode Noise
Also known as normal-mode noise, it is an electrical interference that is not common to both electrical lines but present between both electrical lines.

Distributed Capacitance
In the construction of an inductor, each turn of wire or conductor acts as a capacitor plate. The combined effects of each turn can be presented as a single capacitance known as the distributed capacitance. The 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 (Also see SRF)

Dynamic Loads
Loads that vary significantly with time as measurements are being made.

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Eddy Current Losses
Eddy current losses are present in both the magnetic core and the winding of an inductor. Eddy currents in the winding (or conductor) contribute to two main types of losses: losses due to proximity effects and skin effects. As for the core losses, an electric field around the flux lines in the magnetic field is generated by alternating magnetic flux. This will result in eddy currents if the magnetic core material has electrical conductivity. Losses result from this phenomenon since the eddy currents flow in a plane that is perpendicular to the magnetic flux.

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

A synthetic, thermal-setting resin having excellent adhesion to a wide variety of materials, good resistance to chemical attack and water penetration, outstanding electrical properties, low or moderate curing temperatures, low shrinkage on curing, and good optical properties that is widely used in a variety of formulations for strain gage backings, adhesives, and protective coatings as well as photoelastic adhesives and model materials.

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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 temperature profile. The general composition of ferrites is xxFe2O4 where xx represents one or several metals. The most popular metal combinations are manganese and zinc (MnZn) and nickel and zinc (NiZn). These metals can be easily magnetized.

A circuit or device whose purpose is to control electrical energy at a given frequency or over a range of frequencies. Groups of passive components are commonly used to construct many types of filters. These passive components include resistors, capacitors and inductors.

Frequency Response
A measure of the effectiveness of an instrument to transmit signals applied to it in terms of their frequency, typically as the fraction of the maximum power obtained in the output signal at a specific frequency.

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The impedance of an inductor is the total resistance to the flow of current, including the AC and DC component. The DC component of the impedance is simply the DC resistance of the winding. The AC component of the impedance includes the inductor reactance. The following formula calculates the inductive reactance of an ideal inductor (i.e. one with no losses) to a sinusoidal AC signal.
Z= 2*Pi*f*L - L is in henries and f is in hertz. The equation indicates that higher impedance levels are achieved by higher inductance values or at higher frequencies. Skin Effect and Core Losses also add to the impedance of an inductor which are non-ideal components of the total impedance (See also Skin Effect and Core Losses).

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, i.e. air gap (Also 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 of the coil and the shape of the coil. Inductors most often have their inductances expressed in microhenries (uH).

A passive component designed to resist changes in current. Inductors arc 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 in current is known as inductance. The voltage induced across an inductor by a change of current is defined as:
Thus, the induced voltage is proportional to the inductance value and the rate of current change. (Also see Inductance).

Input Line Filter
A power filter placed on the input to a circuit or assembly that attenuates noise introduced from the power bus. The filter is designed to reject noise within a frequency band. Typically these filters arc low-pass filters meaning they pass low frequency signals such as the DC power and attenuate higher frequency signal which consist of mainly noise. Band pass or low pass filters are commonly made up of inductor and capacitor combinations. (Also see Noise, Attenuation, EMI and Pi-Filter).

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Kool Mu ®
Kool Mu ® is a magnetic material that has an inherent distributed air gap. The distributed air gap allows the core to store higher levels of magnetic ~ 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.

Kool Mu ® material is an alloy that is made up of basically nickel and iron powder (approx. 50%of each) and is available in several permeabilities. It has a higher permeability than powdered iron and also lower core losses. Kool Mu ® is required to be pressed at a much higher pressure than powdered iron material. The manufacturing process includes an annealing step that relieves the pressure put onto the powdered metals which restores their desirable magnetic properties. Thus, the powdered particles require a high temperature insulation as compared to powdered iron. Kool Mu ® performs well in switching power applications. The relative cost is significantly higher than powdered iron.

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Laminated Cores
Cores constructed by stacking multiple laminations on top of each other. The laminations are offered in a variety of materials and thicknesses. Some laminations are made to have the grains oriented to minimize the core losses and give higher permeabilities. Each lamination has an insulated surface which is commonly an oxide finish. Laminated cores are used in some inductor designs but are more common in a wide variety of transformer applications.

Litz Wire
Wire consisting of a number of separately insulated strands that are woven or bunched together such that each strand tends to take all possible positions in the cross section of the wire as a whole. The current through each individual strand is divided equally since this wire design equalizes the flux linkages and reactance of the individual strands. In other words, a litz conductor has lower AC losses than comparable solid wire conductors which becomes important as the operating frequency increases (Also see Skin Effect).

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Magnetic Wire
Wire used to create a magnetic field such as those in magnetic components (inductors and transformers). Magnet wire is nearly 100% copper and must be made from virgin copper. It Is offered with a number of different organic polymer film coatings.

MPP Core
MPP is an acronym for molypermalloy powder. It 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.

The basic raw materials are nickel, iron and molybdenum. The ratios are: approximately 80% nickel, 2%-3% molybdenum, and the remaining is iron. The manufacturing process includes an annealing step as discussed in the Kool Mu ® definition. MPP stores higher amounts of energy and has a higher permeability than Kool Mu ®.

Cores are offered in 10 or more permeability selections. The core characteristics allow inductors to perform very well in switching power applications. Since higher energy can be stored by the core, more DC current can be passed through the inductor before the core saturates. The cost of MPP is significantly higher than Kool Mu ®, powdered irons and most ferrite cores with similar sizes (Mso see Saturation Current and Kool Mu ®).

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

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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:

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Power Loss = (DCR)(1^2)dc
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

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.

A filter consisting of two capacitors connected in parallel with a series inductor. These filters are commonly found near DC-DC converters to filter ripple current and voltage.

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 (Also see Saturation Current.)

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The Q value of an inductor is a measure of the relative losses in an inductor. The Q is also known as the 'quality factor' and is technically defined as the ratio of inductive reactance to effective resistance and is represented by:


Since X and Re are functions of frequency, the test frequency must be given when specifying the Q. X typically increases with frequency at a faster rate than Re at lower frequencies, and vice versa at higher frequencies. This results in a bell shaped curve for Q vs frequency. Re is mainly comprised of the DC resistance of the wire, the core loss and the skin effect of the wire. Based on the above formula it can be shown that the Q is zero at the self resonant frequency since the inductance is zero at this point (Also see SRF.)

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RFI is an acronym for Radio-Frequency Interference. It is an older and more restrictive term that is used interchangeably with 'EMI' (Also see EMI.)

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 loss 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 (Also see Incremental Current and Saturation Current).

The imaginary part of the impedance (Also see Impedance).

Ripple Voltage
The periodic alternating voltage imposed on the voltage output of a switching voltage converter. The ripple voltage is normally specified as a peak-to-peak value.

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Saturation Current
The DC bias current flowing through tile 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 m 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 ~ 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 (Also see Incremental Current and Permeability).

SRF (Self Resonant Frequency)
The frequency at which tile 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 ~re 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, tile 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 (Also see Distributed Capacitance.)

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 B-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 to toroidal cores as lower core permeabilities will have higher fringing fields than will high permeability toroidal cores (Also see Closed Magnetic Path.)

SI Units
Units of measurement based on International System of Units, including seconds of time, meters of distance, and kilograms of mass. From these, newtons of force and joules of work and energy are derived. Temperature is measured on the Celsius scale.

Skin Effect
Skin effect is the tendency for alternating current to flow near the surface of the conductor in lieu of flowing in a manner as to utilize the entire cross-sectional area of tile conductor. This phenomenon causes the resistance of the conductor to increase. The magnetic field associated with the current in the conductor causes eddy currents near the center of the conductor which opposes the flow of the main current flow near the center of the conductor. The main current flow is forced further to the surface as the frequency of the alternating current increasing (Also see Litz Wire.)

Storage Temperature Range
Range of ambient temperatures over which a component can be stored safely (Also see Operating Temperature Range.)

Switching Frequency
The operating frequency of a switching regulator.

Switching Regulator
A circuit that is designed to regulate the output voltage, from a given input voltage, by using a closed control loop design. The most common switching regulator types involve a magnetic component, such as an inductor or transformer, that is used to store and transfer energy to the Output by having the current switched on and off (Also see Boost Regulator and Buck Regulator.)

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Tape Wound Cores
Cores made by rolling strips of alloy iron into a toroidal shape. The metal strips have a precisely controlled thickness which are coated with a very thin insulation material to prevent the metal in the layers to make contact with each other. The finished cores have an outside coating to protect the metal layers and they are offered in a variety of material mixes. Tape wound cores are capable of storing high amounts of energy and contain a high permeability. Their major disadvantage is that they are relatively expensive when compared to other core types (Also see Toroidal Inductor.)

Temperature Rise
The increase in surface temperature of a component in air due to the power dissipation in the component. The power dissipation for an inductor includes both copper and core losses.

Toroidal Inductor
An inductor constructed by placing a winding(s) on a core that has a donut shaped surface. Toroidal cores are available in many magnetic core materials within the four basic types: Ferrite, Powdered iron, Alloy and High Flux, and Tape Wound. Characteristics of toroidal inductors include: self shielding (closed magnetic path), efficient energy transfer, high coupling between windings and early saturation.

Any device or element for converting an input signal into an output signal of a different form, commonly a device for converting a physical variable (force, pressure, displacement, etc.) into electrical signals.

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Voltage Injection
Method of zero-balancing an instrument by injecting a voltage of opposite sign but like magnitude into the output from an unbalanced Wheatstone bridge .

Volt Microsecond Constant
The product of the voltage applied across the winding and the time for the magnetizing current to reach 1.5 times the linear extrapolation of the current waveform. This constant is a measure of the energy handling capability of a transformer or inductor. It is dependent upon the core area, core material (including the saturation flux density of the core), the number of turns of the winding and tile duty cycle of the applied pulse.

Volume Resistivity (Core)
The ability of a core to resist the flow of electrical current either through the bulk of the material or on its surface. The unit of the volume resistivity is Ohm-cm. Core volume resistivity becomes an issue in Inductor designs where the leads/termina1s come in contact with the core material. This type includes axial and radial inductors that have leads epoxied into the core. As for core materials, high permeability ferrites present the most concern as their volume resistivity is typically the lowest.

Under certain conditions, a low resistive path can be realized between two inductor terminals if they are in contact with a low resistivity core. The inductor, under these conditions, will lose its higher impedance characteristics.

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