Intro to Unipolar Hall-Effect Switches
What is a unipolar switch?
A unipolar switch is a device that responds to a magnetic field. When the correct pole of a magnet is brought near, it turns on. When the magnet is moved away, it turns off. The actual details are more complicated, but this is the basic idea.
Unipolar Hall-Effect Switch Operation Patterns
MAGNET POLE FACING UNIPOLAR HALL-EFFECT SWITCH
Here are different views of unipolar 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.
Unipolar switches respond to only one magnet pole, either the south pole or the north pole. You must read the data sheet for a particular switch to learn which.
MAGNET PARALLEL TO UNIPOLAR HALL-EFFECT SWITCH
The activation and deactivation regions around a unipolar Hall-effect switch when the magnet is parallel to the switch.
Here are different views of unipolar 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.
Unipolar switches respond to only one magnet pole, either the south pole or the north pole. You must read the data sheet for a particular switch to learn which.
How does a unipolar switch respond to a magnetic field?
You need to keep track of three things when working with a unipolar Hall-effect switch. You must read the data sheet for a particular switch to learn the details.
- Direction of Magnetic Field: the unipolar switch only responds to the field in a particular direction.
- Strength of Magnetic Field: the unipolar switch turns on and off at particular field strength levels.
- 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
A 2-D side view of the field passing through a switch. The switch requires the field to point in a particular direction. This can work with either a south pole near the face or a north pole near the back.
In most unipolar switches, the field needs to point perpendicularly through the face of the device. It is common for many types of switches to respond to fields pointing from the back of the device toward the front. This can work by either using a south pole in front of the switch or a north pole behind the switch. Note that some switches do NOT operate this way. You must read the data sheet to know for sure.
Switch REquirements for Magnetic Field Strength
Different models of switches have different levels of sensitivity to magnetic fields.
Different models of unipolar 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
The tolerances of the magnet and sensor can cause a big change in activation regions.
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 Unipolar 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 unipolar 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.
- Unipolar switches respond to a single 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.