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2016 Extra Class study guide: E6D - toroidal and solenoidal inductors,
piezoelectric devices

Posted: 23 Jan 2016 11:13 AM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email


There are lots of changes to this section. The questions on CRTs, LCDs, and
CCDs were eliminated, some questions on crystals from section E6E were
moved into this section, and questions were added on variable inductors,
transformers, and some things to consider when designing circuits with
inductors.Dan

E6D Toroidal and solenoidal Inductors: permeability, core material,
selecting, winding; transformers; piezoelectric devices

Solenoidal and toroidal inductors are both used in amateur radio equipment.
A solenoidal inductor is a coil of wire wound around a cylindrical core,
while a toroidal inductor is a coil of wire wound around a circular or
toroidal core. Solenoidal inductors often have just an air core, while
toroidal inductors are wound around a ferrite or powdered-iron core. A
primary advantage of using a toroidal core instead of a solenoidal core in
an inductor is that toroidal cores confine most of the magnetic field
within the core material. (E6D10)

The usable frequency range of inductors that use toroidal cores, assuming a
correct selection of core material for the frequency being used, is from
less than 20 Hz to approximately 300 MHz. (E6D07) Ferrite beads are
commonly used as VHF and UHF parasitic suppressors at the input and output
terminals of transistorized HF amplifiers. (E6D09)

An important characteristic of a toroid core is its permeability.
Permeability is the core material property that determines the inductance
of a toroidal inductor. (E6D06)

One important reason for using powdered-iron toroids rather than ferrite
toroids in an inductor is that powdered-iron toroids generally maintain
their characteristics at higher currents. (E6D08) One reason for using
ferrite toroids rather than powdered-iron toroids in an inductor is that
ferrite toroids generally require fewer turns to produce a given inductance
value. (E6D05)

To calculate the inductance of a ferrite-core toroid, we need the
inductance index of the core material. The formula that we use to calculate
the inductance of a ferrite-core toroid inductor is:

L = AL×N2/1,000,000

where L = inductance in microhenries, AL = inductance index in µH per 1000
turns, and N = number of turns

We can solve for N to get the following formula:

N = 1000 x √(L/AL)

Using that equation, we see that 43 turns will be required to produce a
1-mH inductor using a ferrite toroidal core that has an inductance index (A
L) value of 523 millihenrys/1000 turns. (E6D11)

N = 1000 x √(1/523) = 1000 x .0437 = 43.7 turns

The formula for calculating the inductance of a powdered-iron core toroid
inductor is:

L = AL×N2/10,000

where L = inductance in microhenries, AL = inductance index in µH per 1000
turns, and N = number of turns

We can solve for N to get the following formula:

N = 100 x √(L/AL)

Using that equation, 35 turns 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. (E6D01)

N = 1000 x √(5/40) = 100 x .353 = 35.3 turns

When designing circuits with ferrite-core inductors, you have to be careful
not to saturate the core. The definition of saturation in a ferrite core
inductor is that the ability of the inductor’s core to store magnetic
energy has been exceeded. (E6D12)

One problem that may occur in a circuit with inductors is self-resonance.
The primary cause of inductor self-resonance is inter-turn capacitance.
(E6D13) At some frequency, also called the “self-resonant frequency,� this
capacitance forms a parallel resonant circuit with the inductor.

Variable inductors are made by inserting a slug into an air-core inductor.
By varying the position of the slug, you vary the inductance. Ferrite and
brass are materials commonly used as a slug core in a variable inductor.
(E6D04) Brass is the type of slug material decreases inductance when
inserted into a coil. (E6D14)

Transformers

A transformer consists of two inductors that are closely coupled.
Connecting an AC voltage across one of the inductors, called the primary
winding, causes a current to flow in the primary, which then generates a
magnetic field. As the lines of this field cross the turns of the secondary
winding, it induces a current to flow in the secondary, and the voltage
across the secondary is equal to the voltage across the primary winding
times the number of turns in the secondary winding divided by the number of
turns in the primary winding. The current in the primary winding of a
transformer is called the magnetizing current if no load is attached to the
secondary. (E6D15)

Transformers are often used to match the output impedance of one circuit to
the input impedance of another. In this application, its importan not to
saturate the core of the transformer. The core saturation of a conventional
impedance matching transformer should be avoided because harmonics and
distortion could result. (E6D17)

In some applications, a transformers secondary winding may be subjected to
voltage spikes. In these applications, the designer may connect a capacitor
to absorb the energy in that voltage spike to prevent damage to the
transformer. The common name for a capacitor connected across a transformer
secondary that is used to absorb transient voltage spikes is snubber
capacitor. (E6D16)

Piezoelectric devices

Piezoelectric crystals are used in several amateur radio applications. They
are called piezoelectric crystals because they rely on the piezoelectric
effect, which is the physical deformation of a crystal by the application
of a voltage. (E6D03) The equivalent circuit of a quartz crystal is a
motional capacitance, motional inductance, and loss resistance in series,
all in parallel with a shunt capacitor representing electrode and stray
capacitance. (E6D02)

The post 2016 Extra Class study guide: E6D toroidal and solenoidal
inductors, piezoelectric devices appeared first on KB6NUs Ham Radio Blog.