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© 2016 Littelfuse, Inc. www.littelfuse.com
Application Note:
Ampere*turn versus mT and Gauss
Introduction
What is the relationship between AT (Ampere*turn) and Tesla
or Gauss? What magnet strength will activate a 15 AT reed
switch at 5 mm? There is not an answer that is both simple
and accurate. Because the reed switch is made of ferrous
material, it affects the magnetic field into which it is placed.
That is, the magnetic field is different with and without a reed
switch present. The relationship between magnet strength,
reed switch sensitivity, and activation distance is dependent
on both the size and shape of the magnet and the position
and shape of the reed switch, including its leads.
Observations
The magnetic coupling of the magnet to the reed switch is
very important. For example, the length of the magnet and
the length of the reed switch leads will affect the result. This
is illustrated in the chart on the next page. As reed switch
lead material is removed, the reduced magnetic coupling will
decrease the sensitivity and decrease the activation distance.
However, depending on the magnet length and orientation
to the reed switch, removing reed switch lead material may
also increase the magnetic coupling and activation distance.
A simple but not very useful answer to the Tesla or Gauss
versus AT question is the classic formula for a long solenoid
(copper coil): B = μNI/L where B is flux density in Tesla,
μ = 4π10-7, N = number of turns, I = current [A], and L
= coil length [m]. In Gauss and inch units, this is B = NI/
(2.02L) where B is in Gauss and L = coil length [inch]. This
is not very useful because it applies to a coil rather than a
magnet, and it applies to a long or infinite coil. It is a good
approximation only when the length to diameter ratio of the
coil is greater than five.
Similarly, measuring a reed switch of known AT in a
Helmholtz Coil will relate AT to mT, but again this uses a
coil rather than a magnet. Again, this has limited applicability
in relating AT to mT for a typical reed switch / magnet
application.
There are methods for answering the magnet strength
versus switch sensitivity question, but they are not
simple. One method is to use computer modeling, but this
has the disadvantages of requiring expensive software,
accurate models and a good understanding of the magnetic
components involved. Another method is to solve a specific
problem experimentally using a range of reed switch
sensitivities and magnet sizes. Like the first method, this has
disadvantages as well as advantages.
As a starting point, a rule-of-thumb can be used that is
simple but not very accurate. The rule is that about 0.1 mT
(1 Gauss) per reed switch Ampere*turn will activate the reed
switch when both magnet poles are near the two ends of
the switch. For example, about 2 mT (20 Gauss) is needed at
a point in space near both poles of a magnet to activate a 20
AT reed switch at that same point. Activating from one end
using only one magnet pole will require about twice as much
magnetic field compared to activating using both poles. This
rule is an order-of-magnitude approximation. A specific case
could vary from this by a factor of 2 or more. See Fig.1 for
a graph showing specific relationships for specific magnets
and switches.
One might wonder why reed switches are specified using
units of Ampere*turn instead of mT or Gauss. The reasons
are practicality and convention. The most practical way
to generate the magnetic field for testing reed switch
sensitivity is to use a test coil. The only way to specify reed
switch sensitivity without also having to specify the test coil
used is to generate a large uniform magnetic field using a
large solenoid, Helmholtz coil, or similar coil geometry. This
is possible to do, but the coil sizes and currents needed
become unwieldy, and the benefit is negligible because it
does not solve any of the above-mentioned issues relating
the sensitivity measurement to realworld problems.
Figure 1.
Magnetic Flux Density VS. Pull-in for full length Littelfuse MDCG-4 Reed
Switches and Three Littelfuse Magnets
Magnet parallel to and centered on switch. Measured at center of switch after removing it.
0
10
20
30
40
0 10 20 30 40
Typical Re ed Sw itch Pull-In [Am pere-Turn]
Magnetic Flux Density [Gauss]
(1 mT = 10 G)
H-31 Magnet
H-33 Magnet
H-32 Magnet
Littelfuse, Inc.
8755 West Higgins Road, Suite 500
Chicago, IL 60631 USA
Phone: (773) 628-1000
Littelfuse.com
