SLYA090 September   2024 TMAG5233

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Reed Switches
    2. 1.2 Hall-Effect Sensors
    3. 1.3 Tunneling Magneto Resistance (TMR) Sensors
  5. 2Design Considerations
    1. 2.1 Technology Complexity and Cost
    2. 2.2 Axis of Sensitivity
      1. 2.2.1 Hall-Effect Switches
      2. 2.2.2 TMR Switches
      3. 2.2.3 Reed Switches
    3. 2.3 Mechanical Constraints
    4. 2.4 Power Consumption
  6. 3Summary
  7. 4References

1.1 Reed Switches

Of the three technologies previously mentioned (reed switches, Hall-effect sensors, and TMR sensors), only reed switches are a zero power design.

Reed switches are normally constructed with two or three metal contacts made out of a ferromagnetic material shown in Figure 1-1. A reed switch with two metal contacts has a single contact point that closes when a sufficiently strong magnetic field is present while a three contact reed switch can have a common pin that switches between two paths depending on the magnet presence. For both cases, these contacts can have a normally closed or open position that is determined by the reed switch manufacturer.

Reed switches are also hermetically sealed at vacuum or filled with inert gas to prevent the build-up of contaminants or surface oxidation that can otherwise increase contact resistance or possibly even completely block electrical contact.

Typical Reed Switch Figure 1-1 Typical Reed Switch

A common use of the reed switch is to embed the switch behind a surface which is magnetically permeable and to control the switch through the interaction with a nearby magnetic field. Instead of placing a mechanical striker plate on the moving target, all that is needed is a permanent magnet that can move into the vicinity of the reed switch.

The reeds themselves are made from a ferromagnetic material which can channel the magnetic field. When placed correctly, the magnetic field can channel between both reeds which causes a small torque to be applied on each. This forces the two reeds towards each other and closes the electrical circuit. Notice in Figure 1-2 the concentrated magnetic flux density that results between the tips of the two reeds when a magnet is present.

Reed Switch Simulated Response Figure 1-2 Reed Switch Simulated Response

The specific placement and orientation of the magnet required to close the reed switch can vary based on the construction of the device. The resulting field between the two reeds must be strong enough to cause them to flex towards each other. When the magnet is placed too far from the junction, there is a possibility that the field cannot channel adequately through both reeds. Creating a horizontal offset of the magnet position reduces the magnitude of the concentrated field between the reeds in Figure 1-3.

Reed Switch Offset Simulation Figure 1-3 Reed Switch Offset Simulation

A sensitivity diagram can be provided by the reed switch manufacturer which must be followed to properly place the magnet for any switching application.