Intro to TMR Switches

Tunneling Magnetoresistance (TMR) Switches are polarity sensitive devices that respond to a magnetic field.  A correctly oriented magnet will turn them on and off.


Magnet Pole Facing Switch

This set of images shows different views of the activated and deactivated regions around a TMR switch.  The red arrow on the device shows the sensing direction of the device.

  • Switch On:  magnet inside of the green region
  • Switch Off: magnet outside of the gray region
  • Hysteresis: magnet outside of green and inside of gray

In the hysteresis region, the switch might be on or off.   

The blue dots on the magnet and switch show the reference points for positioning.  The red arrows show the direction of the field.  The arrows point from south to north.

If the magnet were switched so the south pole was facing down, the activate region would move to the other side of the device.  

 

Magnet Pole Facing Switch

This vector plot explains the above graphs.  The TMR switch will only respond to a field pointing in a single direction.  

When the magnet is located to the left of the device (in this vector graph), the field is pointing in the correct direction.  The switch will activate.

When the magnet is located to the right of the device (not shown), the field would be pointing in the wrong direction.  The switch will not activate.

This is why the activate pattern is asymmetric.

 

Magnet aligned PArallel with sensing axis

This set of images show different views of the activated and deactivated regions around a TMR switch.  Note that the magnet is pointing parallel to the device sensing direction.  (The next set of graphs show what happens if the magnet is reversed.)

  • Switch On:  magnet inside of the green region
  • Switch Off: magnet outside of the gray region
  • Hysteresis: magnet outside of green and inside of gray

In the hysteresis region, the switch might be on or off.   

The blue dots on the magnet and switch show the reference points for positioning.  The red arrows show the direction of the field.  The arrows point from south to north.  

 

MAGNET ALIGNED PARALLEL WITH SENSING AXIS

This vector plot explains the above graphs.  The TMR switch will only respond to a field pointing in a single direction.  

When the magnet is located to the left of the device (in this vector graph), the field is pointing in the correct direction.  The switch will activate.

When the magnet is to the right of the device, the field will again be pointing in the correct direction.  With either the N pole to the left or the S pole to the right, the switch activates.

This is why the activate pattern is symmetric.

 

Magnet Aligned Anti-Parallel along sensing direction

This set of images show different views of the activated and deactivated regions around a TMR switch.  Note that the magnet is pointing anti-parallel to the device sensing direction. 

  • Switch On:  magnet inside of the green region
  • Switch Off: magnet outside of the gray region
  • Hysteresis: magnet outside of green and inside of gray

In the hysteresis region, the switch might be on or off.   

The blue dots on the magnet and switch show the reference points for positioning.  The red arrows show the direction of the field.  The arrows point from south to north.  

 

MAGNET ALIGNED ANTI-PARALLEL ALONG SENSING DIRECTION

This vector plot explains the above graphs.  The TMR switch will only respond to a field pointing in a single direction.  

When the magnet is anti-parallel to the sensing direction and above the device, the field points in the correct direction for activation.

 

 

Magnet Perpendicular to Sensing Direction

This set of images show different views of the activated and deactivated regions around a TMR switch.  Note that the magnet is pointing perpendicular to the sensing direction.

  • Switch On:  magnet inside of the green region
  • Switch Off: magnet outside of the gray region
  • Hysteresis: magnet outside of green and inside of gray

In the hysteresis region, the switch might be on or off.   

The blue dots on the magnet and switch show the reference points for positioning.  The red arrows show the direction of the field.  The arrows point from south to north.  


Electrical Properties of TMR Switches

You should note that TMR switches do not "switch" in the sense that a standard mechanical switch does.  A TMR switch changes its electrical output in some manner when it switches "on" and "off".  

You must read the data sheet for a particular device to understand what "on" and "off" mean for that device.


Key Things You Need to Know

  • A TMR switch is not like a mechanical switch.  Most are powered devices.  Its "on" and "off" states can be voltage, current, or resistance states.
  • The operation of the switch depends on the magnet design you use.
    • TMR switches are polarity sensitive.  The field must point in a particular direction.
    • The operating patterns depend on magnet orientation.
    • The operating patterns depend on magnet strength and size.
    • The operating patterns depend on the switch response to the field.
  • Individual switches and magnets have production and operation tolerances.  You must account for these!
  • You MUST read the data sheet to learn the magnetic, electrical, and mechanical properties of the switch you are using.
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