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Subelement E6
CIRCUIT COMPONENTS
Section E6D
Toroidal and Solenoidal Inductors: permeability, core material, selecting, winding; transformers; Piezoelectric devices
How many turns will be required to produce a 5-microhenry inductor using a powdered-iron toroidal core that has an inductance index (A L) value of 40 microhenrys/100 turns?
  • 35 turns
  • 13 turns
  • 79 turns
  • 141 turns

The approximation formula for a powdered-iron toroid is: \[N = 100 \times \sqrt{\frac{L}{A_L}}\]

Where:
\(N\) is the number of turns
\(L\) is the inductance in microhenries
\(A_L\) is the inductance index
     (and must be in microhenries per 100 turns to
      be consistent with the units for \(L\))
.

So, \begin{align} N &= 100 \times \sqrt{\frac{L}{A_L}}\\ &= 100 \times \sqrt{\frac{5}{40}}\\ &= 100 \times \sqrt{0.125}\\ &= 35.4\\ &\approx 35\text{ turns} \end{align}

The multiplier for henries in the formula doesn't matter as long as both \(L\) and \(A_L\) use the same multiplier. For example, \(L\) can be henries if \(A_L\) is henries per 100 turns; \(L\) can be millihenries as long as \(A_L\) is millihenries per 100 turns.

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What is the equivalent circuit of a quartz crystal?
  • Motional capacitance, motional inductance, and loss resistance in series, all in parallel with a shunt capacitor representing electrode and stray capacitance
  • Motional capacitance, motional inductance, loss resistance, and a capacitor representing electrode and stray capacitance all in parallel
  • Motional capacitance, motional inductance, loss resistance, and a capacitor representing electrode and stray capacitance all in series
  • Motional inductance and loss resistance in series, paralleled with motional capacitance and a capacitor representing electrode and stray capacitance

Remember:

series RLC + shunt capacitance.

The only answer that includes the mention of a shunt capacitor is the correct choice.

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Which of the following is an aspect of the piezoelectric effect?
  • Mechanical deformation of material by the application of a voltage
  • Mechanical deformation of material by the application of a magnetic field
  • Generation of electrical energy in the presence of light
  • Increased conductivity in the presence of light

Ever used piezo ignition for your gas stove or oven? https://en.wikipedia.org/wiki/Piezo_ignition

This is using the same property: you press a button (usually hard!) to deform a crystal and as a result you get a nice little spark due to a (brief) high voltage.

So if you deform the crystal, you get a voltage. You can also apply a voltage to deform the crystal.

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Which materials are commonly used as a slug core in a variable inductor?
  • Polystyrene and polyethylene
  • Ferrite and brass
  • Teflon and Delrin
  • Cobalt and aluminum

As a variable inductor is turned, the slug moves into the air space inside the coil, changing its magnetic properties and thus the quantity of inductance. Inserting a ferrite slug increases the inductance. Conversely, a brass slug decreases the inductance as it goes in. Both can be useful in building a tuned circuit. Ferrite is found in all sorts of electromagnetic applications including inductors, transformers, chokes, solenoids, etc.

Cobalt and aluminum are metals but they don't have properties ideal for an inductor. Polystyrene, polyethylene, Teflon, and Delrin are all plastics and wouldn't do anything at all in an inductor.

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What is one reason for using ferrite cores rather than powdered-iron in an inductor?
  • Ferrite toroids generally have lower initial permeability
  • Ferrite toroids generally have better temperature stability
  • Ferrite toroids generally require fewer turns to produce a given inductance value
  • Ferrite toroids are easier to use with surface mount technology

Because ferrite toroids have a higher permeability than powdered-iron toroids, the inductance for a given number of turns is increased. Therefore, smaller inductors and inductors with fewer turns are possible using ferrite toroids.

Last edited by jsharris1229.

Memory tip:
F errite T oroids = F ewer T urns

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What core material property determines the inductance of a toroidal inductor?
  • Thermal impedance
  • Resistance
  • Reactivity
  • Permeability

Permeability is a measure of the response of a given material to a magnetic field. The measure is relative to the magnetic field strength observed with no core. A higher permeability will result in a higher inductance for a constant number of turns on the toroid. It is measured in henries per meter. Air has a permeability of 1. So, the permeability of the material used in the core of the toroid will have the biggest impact on its inductance.

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What is the usable frequency range of inductors that use toroidal cores, assuming a correct selection of core material for the frequency being used?
  • From a few kHz to no more than 30 MHz
  • From less than 20 Hz to approximately 300 MHz
  • From approximately 10 Hz to no more than 3000 kHz
  • From about 100 kHz to at least 1000 GHz

This is simply memorizing that toroidal cores support a frequency range up to 300 MHz.

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What is one reason for using powdered-iron cores rather than ferrite cores in an inductor?
  • Powdered-iron cores generally have greater initial permeability
  • Powdered-iron cores generally maintain their characteristics at higher currents
  • Powdered-iron cores generally require fewer turns to produce a given inductance
  • Powdered-iron cores use smaller diameter wire for the same inductance

Powdered-iron cores have better temperature stability but the permeability is lower. Ferrite cores have higher permeability but the temperature stability is not as good. If temperature stability is more important than size, then iron- core may be desireable.

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What devices are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of a transistor HF amplifier?
  • Electrolytic capacitors
  • Butterworth filters
  • Ferrite beads
  • Steel-core toroids

The ferrite bead (small sphere of ferrite with a hole through it) is a very small core and acts as a filter to suppress higher frequency noise.

https://en.wikipedia.org/wiki/Ferrite_bead

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What is a primary advantage of using a toroidal core instead of a solenoidal core in an inductor?
  • Toroidal cores confine most of the magnetic field within the core material
  • Toroidal cores make it easier to couple the magnetic energy into other components
  • Toroidal cores exhibit greater hysteresis
  • Toroidal cores have lower Q characteristics

Because of the circular geometry of the toroidal core, it contains most of the magnetic field inside the core. This makes toroids well suited for use on circuit boards where magnetic field interference with other components is undesirable.

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How many turns will be required to produce a 1-mH inductor using a core that has an inductance index (A L) value of 523 millihenrys/1000 turns?
  • 2 turns
  • 4 turns
  • 43 turns
  • 229 turns

The equation for a ferrite toroid is:

\[N = 1000 \times \sqrt {\frac{L}{A_L}}\]

Where:

\(N\) is the number of turns;
\(L\) is the inductance in millihenries;
\(A_L\) is the inductive index (which must be given in millihenries per 1000 turns for this equation).

So,

\begin{align} N &= 1000 \times \sqrt {\frac{L}{A_L}}\\ &= 1000 \times \sqrt {\frac{1}{523}}\\ &= 43.7\\ &\approx 43\text{ turns} \end{align}

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What is the definition of saturation in a ferrite core inductor?
  • The inductor windings are over coupled
  • The inductor's voltage rating is exceeded causing a flashover
  • The ability of the inductor's core to store magnetic energy has been exceeded
  • Adjacent inductors become over-coupled

Ferrite greatly increases the inductance of a coil, allowing the use of smaller coils to do the same job compared to an air-core inductor. But ferrite can only store a limited amount of energy. If too much current is passed through the coil the ferrite will saturate and can't store any additional energy. The inductor then behaves as if the ferrite is not there, with a consequently much lower inductance value, until the energy is drained back out. This is usually bad although specialized applications take advantage of it.

Inductors store current, not voltage, so the answer about voltage is irrelevant. And the two answers about coupling are similar enough that they can't both be right.

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What is the primary cause of inductor self-resonance?
  • Inter-turn capacitance
  • The skin effect
  • Inductive kickback
  • Non-linear core hysteresis

Self-resonance happens when you have both inductance and capacitance in series.

Real world components have parasitics-- every component contains resistance, inductance, and capacitance in addition to the values it is designed to have.

In an inductor, adjacent wire acts similarly to the plates of a capacitor to create a small amount of parasitic capacitance between each turn. This combination of inductance and capacitance causes self-resonance.

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Which type of slug material decreases inductance when inserted into a coil?
  • Ceramic
  • Brass
  • Ferrite
  • Powdered-iron

Ferrite is found in many electromagnetic applications because it increases inductance, allowing smaller, lighter parts to do the same job. Powdered iron fits a similar role, also increasing inductance. Brass is pretty much only used in variable inductors because it reduces inductance compared to the air it displaces as you turn the slug. Normally you'd just make a smaller coil, but sometimes other constraints exist. Ceramic is inert and doesn't do anything.

Hint: Slug think bullet and brass is the common casing.

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What is current in the primary winding of a transformer called if no load is attached to the secondary?
  • Magnetizing current
  • Direct current
  • Excitation current
  • Stabilizing current

Even if there is no load, a small amount of current flows through a transformer when AC is applied. This current creates a magnetic field, and is called the magnetizing current.

If there is a load on the secondary, it acts to deplete the magnetic field and generally increases the amount of current flowing in the primary beyond the magnetizing current.

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What is the common name for a capacitor connected across a transformer secondary that is used to absorb transient voltage spikes?
  • Clipper capacitor
  • Trimmer capacitor
  • Feedback capacitor
  • Snubber capacitor

Any kind of circuit that absorbs transient spikes caused by switching action is called a "snubber".

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Why should core saturation of a conventional impedance matching transformer be avoided?
  • Harmonics and distortion could result
  • Magnetic flux would increase with frequency
  • RF susceptance would increase
  • Temporary changes of the core permeability could result

Inductors and transformers frequently have a core made of ferrite because it greatly increases the quantity of inductance compared to an air core. However, the ferrite can only hold a limited amount of energy. Once it is saturated it no longer boosts the inductance of the coil and thus the inductor behaves like a much smaller inductor. This results in a non-linear effect as the threshold is crossed, and any kind of non-linear behavior is sure to cause harmonics and distortion.

Flux, suseptance, and permeability are all words you might see in association with transformers and inductors, however none of them are intrinsically a problem so much as parameters you might measure.

-gxti

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