BY VAUGHN D. MARTIN and JOHN STABLER
magnets are very powerful
compared to their own weight.
Three substances comprise this magnet: neodymium, iron, and boron —
Nd2Fe14B. They are now so economical they have replaced the more
heat-resistant samarium-cobalt
magnets. Samarian-cobalt’s Curie
temperature is about 800°C
compared to 310°C for rare earth
magnets. Exceeding Curie temperatures in ferromagnetic materials
nullifies their magnetic ability.
The toy industry uses millions of
these in magnetic products such as
building sets and jewelry. Apple’s
popular iPod music player uses
them for the transducers in the
earphones. A downside to these
magnets is their mechanical
brittleness. The edges are highly
susceptible to chipping. Fortunately,
this does not affect their ability to
function properly, whatsoever.
Another downside is that these
magnets are powerful enough to
irreparably damage the contents of
a floppy disc and erase data on
magnetic stripes of credit cards.
Neodymium magnets can also
magnetize color CRT shadow masks,
and physically deform them so
severely that even going through a
degaussing repair process is useless.
■ FIGURE 3. The
magnet’s travel
path related to
field strength.
Artwork courtesy of Allegro
Microsystems.
Total Effective
Air Gap or
TEAG). You
can measure
the strength of
the magnetic
field with a
Gaussmeter or
a calibrated
linear Hall Effect sensor. If you plot
field strength (magnetic flux density),
you will discover it is a function of
distance along the intended line of
travel of the magnet (see Figure 3).
The site www.coolmagnetman.
com/ magmeter.htm shows how to
use another Allegro Hall Effect sensor
to make your own Gaussmeter. If this
intrigues you, reference Allegro’s web
page at www.allegromicro.com/
demo/ 3515-6.htm and you will find a
calibrated Hall Effect sensor that you
can buy online for about $25. This
is an accurate, easy-to-use tool for
measuring magnetic flux densities.
The manufacturer individually cali-
brates these devices over their linear
magnetic span and furnishes you with
a reference curve and specific
sensitivity and quiescent outputs at
4.5V, 5.0V, and 5.5V.
You specify sensitivity in mV/G
for a linear device, or operate and
release points in Gauss for a digital
device. This allows you to determine
the critical distances (TEAG) for a
particular magnet and type of
motion (see Figure 4). Field strength
plots are not linear, especially when
you place the magnet off-axis from
the Hall Effect sensor’s centerline
(see Figure 5). I used an acrylic
sheet 80/1,000’s of an inch thick to
Grading and Field
Strength
Neodymium magnet strengths
are graded over a range of N24 to the
strongest of N54. The N number
represents the magnetic energy
product, in MegaGauss-Oersteds
(MGOe) (1 MG·Oe = 7,957 T·kA/m =
7,957 kJ/m3). Magnets included in the
kit are mid-range in strength.
A magnet’s field strength is
greatest at the pole face, and naturally
decreases with increasing distance
from the magnet (also referred to as
■ FIGURE 4. The magnet’s critical
separation distance. Artwork courtesy
of Allegro Microsystems.
April 2007 49
