Figure 15. A special SMD coil that minimizes
its ESR and current blocking tendencies so that it
can drive high frequencies — even video — without
signals while maintaining picture quality. Figure 15 is a
surface mount special geometry Steward coil used in a
USB application. With the exception of varistors covered
in Part 1, coils are the only passive component that can
exhibit pronounced non-linear characteristics. They are
passive because they cannot generate energy. Values
range from fractions of milli-Henrys to fractions of Henrys.
You can temporarily store energy in magnetic fields
and inductors. Dramatic examples are an inductor firing
a spark plug, and to a lesser extent, our LED example.
Inductors, like capacitors, have ESR. Figure 16 shows
its effect as seen on a scope as a pulse generator drives
Inductor Frequency Ranges Versus
Structure And Core Materials
As previously mentioned, coils may use a metallic
core center. As a rule of thumb, use iron cores for
frequencies below 100 kHz. Use ferrite cores for
frequencies in the 10 MHz range; above 100 MHz,
the core is usually air and the coil is structurally
self-supporting. Check out file:///F:/air%20coils/Air-
Core-Inductor-Calculator.phtml.htm for an electronic
monograph to calculate air core coil inductance.
At low frequencies, an inductor may have hundreds of
turns, but above 1 MHz they only have a few turns. Most
inductors have a low DC resistance since they are wound
from copper wire. You will often see them if you look
under the amateur radio section cores and coil winding
machines of varying degrees of sophistication on eBay
(see Figures 17 and 18). Figure 19 shows a cut-away
exploded view of a toroid or device that goes on a coil
winder that holds the wire.
A monograph is a special graph with three lines purposely
graduated that allows a straight line to intersect any two of the
lines of the known values’ intersects. The third line crossing
them is the value of the related unknown variable.
trace of an
driven by a
A manual coil
A slightly more
manual coil winding
also holds bobbins.
Figure 19. A close-up/cut-away view showing what
a winding on a bobbin (wound indicator) looks like.
August 2009 63