The Hall effect was discovered in 1879 by Edwin Hall, a full 20 years before the discovery of the electron. At the time, no one really knew how to explain the phenomenon; it wasn’t until much later that the effect would find its way into commercial applications.
Edwin Hall, the discoverer of the Hall effect. Image used courtesy of John Hopkins University
Today the Hall effect is used in all sorts of electric systems, generally in the form of Hall-effect sensors. For the sake of this article, we’ll be looking at a specific type of Hall-effect sensor: the unipolar Hall-effect sensor.
The Hall Effect: A Brief Recap
Our discussion of the unipolar Hall-effect sensor first requires us to understand the Hall effect at its core. The Hall effect is the potential difference between two sides of a conducting material when exposed to a magnetic field.
Visual depiction of the Hall effect. Image used courtesy of Melexis
On a high level, it can be thought of like this: when electrons flow through a conductor, they travel in a mostly straight line. Now if you put that conductor in a magnetic field, the electrons are deflected from their straight line by the Lorentz Force. This unequal spatial distribution of electrons causes a potential difference to develop in the conductor.
This effect gets exploited in Hall-effect sensors to detect magnetic fields for a variety of reasons.
Unipolar Hall-Effect Sensors
A unipolar Hall-effect sensor uses the Hall effect to operate like a switch. The operating principle is as follows.
A magnet presenting a positive magnetic field (south pole) of sufficient strength (magnetic flux density) will cause the device to switch to its on-state. Once turned on, the unipolar IC will remain on until the magnetic field is removed and the IC reverts to its off state.
Unipolar switch regions of operation vs. magnetic flux density. Image used courtesy of Allegro Microsystems
The operation of these switches normally depends on the strength of the magnetic field, the direction of the field, and the device tolerances. In most unipolar switches, the field needs to point perpendicularly through the face of the package.
Magnetic field orientation is important to a unipolar switch’s operation. Image used courtesy of Steven R. Stuve
Applications of Unipolar Hall-Effect Sensors
This technology has found a home in many proximity-sensing devices.
One classic example application of this technology is a vehicle gear-shift lever. When the driver moves the lever, a magnet on the bottom of the lever is moved as well. As it changes location, different sensors are exposed to the magnetic field and their switch turns on. The others, outside of proximity, are turned off. In this way, a driver can control the mode of operation (i.e. drive, park, reverse, neutral) of the vehicle.
Unipolar Hall-effect sensors in a vehicle gear-shift lever. Image used courtesy of Allegro Microsystems
Other applications include detecting open vs. closed orientation of laptops or sliding smartphones.
Unipolar Hall-effect sensors in a laptop. Image used courtesy of Digi-Key
A New Dual Unipolar Switch
Today, companies are still working on improving and miniaturizing unipolar switches because of their wide range of applications.
Just this month, Diodes Incorporated released the AH3188, a new ultra-high sensitivity, low-power unipolar switch. This device includes dual unipolar outputs, meaning that the AH1388 can independently detect North and South poles.
Graphic of the AH1388. Image used courtesy of Diodes Incorporated
The device claims higher sensitivity to allow for smaller magnets, thus allowing for more compact, lower power, and flexible designs. The product is aimed at proximity-sensing designs and is currently in full production.
Use as a Small, Flexible Switch
Unipolar Hall-effect sensors are a useful technology that exploits the Hall effect to work as switches. Often found in proximity-sensing applications, these devices offer a small scale, flexible solution for designers.
With companies still developing and improving the technology, it seems likely that these devices will remain widely available.
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