Magnetics Business & Technology - November/December 2022 - 14

TECH TIPS
How to Pair a Magnetic Sensor with the Right Magnet for a Cost-Effective Solution
By Eduardo E. Montalvo-Ortiz, Field Applications Engineer, Coto Technology, Inc.
Due to their inherent " contactless-ness " , small size, low cost and
high reliability, solid-state magnetic sensors have become the
preferred solution for many products and applications over the
last few years. These same features also make them an attractive
alternative to other technologies such as electromechanical reed
switches. Unfortunately, many purchasers within the solid-state
magnetic sensor market use the cost of the sensor, itself, as the
predominant factor in the component selection.
Instead, it is important to keep in mind that in any magnetic sensing
solution, there are two equally important considerations: a
magnet to produce a magnetic field and a sensor that can best
detect it.
At first glance, it may seem easy to choose parts that will work well
together and meet cost requirements. However, that chosen combination
might not be the optimal one. By taking the time to fully
understand the environment in which the sensor and magnet will
need to operate and determine which specifications will make the
ideal combination, one can achieve additional benefits, such as a
reduction in cost and product size as well as addressing possible
supplier concerns.
For example, one common way to reduce magnet cost is to switch
to a sensor with a higher magnetic sensitivity (i.e., a sensor that
activates with a lower magnetic flux density). If the sensor is
" less demanding " in terms of the field it needs to work with, the
magnet can be swapped out for one that produces a lower field.
And, because magnet cost depends greatly upon its physical volume,
magnetic strength and the material composition, choosing a
smaller or weaker grade of magnet can lower cost, without sacrificing
reliable activation of the sensor.
Figure 1: Cylindrical magnet placed at a distance from a sensor. The
results shown in Table 1 are for the configuration. Red dotted lines
represent the magnetic field around the magnet. The axis of sensitivity
(where the sensor is most sensitive) is denoted in blue.
Table 1 shows the required distances at which two magnetic sensors
- one which operates at 30 Gauss and the other at 9 Gauss
- would be activated by different types of cylindrical magnets. Cases
1 and 2 both assume a magnet with a diameter of 8mm and a
thickness of 5mm, but Case 1 assumes a Neodymium Iron Boron
14 Magnetics Business & Technology * November/December 2022
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To illustrate this, consider an application where a TMR (Tunneling
Magnetoresistive) sensor, such as Coto Technology's RedRock
RR122 TMR sensor series, is paired with a cylindrical magnet in
the orientation shown in Figure 1. [For this example, it is assumed
that there are no other magnets or ferromagnetic components
nearby, other than the cylindrical magnet.]
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