Intro to Omnipolar Hall-Effect Switches

What is an omnipolar switch?

An omnipolar switch is like a unipolar switch except that it responds to either magnet pole. As either pole of a magnet nears the switch, it will activate.


Omnipolar Hall-Effect Switch Activation Patterns

MAGNET POLE FACING omNIPOLAR HALL-EFFECT SWITCH

Here are different views of omnipolar switch operation.  These are actual simulation results for a magnet facing the switch.  The cylinder magnet is magnetized along its axis.

  • 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 red dots on the magnet and switch show the reference points for positioning.  

Omnipolar switches respond to either magnet pole. 

 

MAGNET PARALLEL TO omNIPOLAR HALL-EFFECT SWITCH

Here are different views of omnipolar switch operation.  These are actual simulation results for a magnet  parallel to the switch.  The cylinder magnet is magnetized along its axis.

  • 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

The red dots on the magnet and switch show the reference points for positioning.  

Omnipolar switches respond to either magnet pole.


How does an omnipolar switch respond to a magnetic field?

You need to keep track of three things when working with a omnipolar Hall-effect switch.  You must read the data sheet for a particular switch to learn the details.

  • Direction of Magnetic Field:  the omnipolar switch only responds to the field in a particular direction.
  • Strength of Magnetic Field:  the omnipolar switch turns on and off at particular field strength levels.  It is not sensitive to polarity.
  • Operating Tolerances of Switch:  the strength levels differ for each switch due to natural manufacturing variation as well as environmental factors.
 

SWITCH REQUIREMENTS FOR MAGNETIC FIELD DIRECTION

In most omnipolar switches, the field needs to point perpendicularly through the face of the device.  

The magnet poles can be reversed for an omnipolar switch.

 

SWITCH REQUIREMENTS FOR MAGNETIC FIELD STRENGTH

Different models of omnipolar switches respond to different field strengths.  Some are more sensitive than others.    The images to the left show the effect of using the same magnet with different models of switches. 

More sensitive switches have larger activation regions.  Less sensitive switches have smaller activation regions.  Your particular application will determine what model of switch would be most appropriate.

 

THE EFFECT OF MAGNET AND SENSOR TOLERANCES

It is very important to keep track of magnet and sensor tolerances.  

The left image shows the response to "typical" magnet and sensor parameters.  In practice, this is probably close to what would be observed when constructing prototypes.

The right image shows the possible range of operation across magnet and sensor tolerances.  This is a good estimate of the range of behavior in a production run of a magnet and sensor design.  If you were to randomly select a magnet and sensor combination, the particular activate and deactivate regions would be somewhere between the green and gray regions.  Most would probably be near the "typical" regions.  However, some statistical fraction might be close to either the gray or the green tolerance regions.

 

Effect of Using Different Magnets with the Same Switch

Different magnets produce different shapes and strengths of magnetic fields.  This changes the operating regions around a Hall-effect switch.

It is critical that magnets and sensors be designed together.   Each sensor application will require matching a magnet design with the particular switch being used.


Electrical Properties of Omnipolar Hall-Effect Switches

You should note that Hall-effect switches do not "switch" in the sense that a standard mechanical switch does.  A Hall-effect 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.

The electrical performance of omnipolar switches varies greatly.   Some switches change current levels from "low" to "high".  Others change their voltage output.  Others use PWM changes.  Some manufacturers produce "inverted" versions of devices that give the opposite electrical change.   To repeat, you must read the data sheet for a particular device to know what "on" and "off" means. 

What is the best choice of electrical behavior?  It depends on your application.  There are many factors to consider beyond the scope of this article.


You should remember these key points.

  • The operation of the switch depends on the magnet design you use.
    • Omnipolar switches respond to either magnet pole.
    • 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 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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