Intro to GMR Switches
What is a GMR Switch?
A Giant Magnetoresistive (GMR) switch turns on when a magnet is brought near. It is not polarity sensitive. When a magnet in the correct orientation approaches, the GMR switch will activate.
MAgnet Pole facing GMR switch
This set of images shows different views of the activated and deactivated regions around a GMR switch. The GMR switch is not polarity sensitive so either magnet pole will produce the same result. The GMR switch is direction sensitive in the plane of the switch face.
- 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.
These images show a switch with a sensitivity direction along the length of the sensor. Some GMR switches have a sensitivity direction across the length of the sensor. Read the data sheet of a particular device to learn how it works.
MAgnet PArallel to GMR SWitch face and aligned with sensitivity axis
These images show the activation regions when the magnet is parallel to the face of the switch and aligned with the sensitivity 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.
Note carefully that the magnet axis is parallel to the GMR sensing axis in these images.
MAGNET PARALLEL TO GMR SWITCH FACE AND ALIGNED across the SENSITIVITY AXIS
These images show the activation regions when the magnet is parallel to the face of the switch and aligned across the sensitivity axis.
When the magnet is inside the green region, the switch on. When the magnet is outside the gray region, the switch is off. The region between the green and gray is the hysteresis region where the switch might be on or off. The red dots on the magnet and switch show the reference points for positioning.
Note carefully that the magnet axis is perpendicular to the GMR sensing axis in these images.
How does a GMR Switch Respond to a Magnetic Field?
You need to keep track of two things when working with a GMR switch. You must read the data sheet for a particular switch to learn the details.
- Response to Magnetic Field Direction: the GMR switch only measures the field magnitude in a particular direction.
- Response to Magnetic Field Strength: the GMR switch turns on and off at particular field strength levels. (Note that production variation in switches means that there is a tolerance associated with these strength levels.)
SWITCH REQUIREMENTS FOR MAGNETIC FIELD DIRECTION
In most GMR switches, the field needs to point along a particular sensitivity axis. In many GMR switches, this direction is along the length of the sensor parallel to the face.
GMR switches are not polarity sensitive. Using either the north or south pole of the magnet will produce the same results.
You must read the data sheet to learn what the sensing axis is for a particular model of device.
SWITCH REQUIREMENTS FOR MAGNETIC FIELD STRENGTH
Different models of GMR 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.
Electrical Properties of GMR Switches
You should note that GMR switches do not "switch" in the sense that a standard mechanical switch does. A GMR 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 GMR switches varies greatly. Some switches change current levels from "low" to "high". Others change their voltage output. 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.
- A GMR switch is not like a mechanical switch. Most are powered devices. Its "on" and "off" states can be voltage, current, resistance, or PWM states.
- The operation of the switch depends on the magnet design you use.
- GMR switches respond to either magnet pole. They are not polarity sensitive.
- 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.