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Discovered by Edwin Hall in 1879, Hall effect sensors (Hall voltage) are named because they refer to the potential difference on opposite sides of an electrical conductor. There is a current flowing through it created as a result of a magnetic field perpendicular to the current.
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Without getting too technical, the ratio of the voltage output to the power of the current and magnetic field, divided by element thickness gives you the Hall coefficient. Mostly this refers to the material the conductor is made of, as various materials/carriers affect output. You can't get any more basic than understanding the Hall effect is due to the nature of the current flow in a conductor. Suffice it to say there are many places and applications for Hall effect sensors.
Hall Effect Sensors & Automotive Ignitions
The most common place you will find Hall effect sensors are in distributors for ignition timing. They aren't limited to being just there however. You will find them in some crank and camshafts to monitor injection pulse timing and sense speed. Older vehicles used to have what is called breaker points. When the Hall effect sensor hit the market place, it replaced the breaker points.
Its function as a timing device is fairly complex and yet at the same time, when broken down into steps, brilliantly easy. A permanently mounted fixed magnet and a semiconductor Hall effect chip are mounted side by side, only separated by an air gap that forms the Hall effect sensor.
Next, a metal rotor with windows and tabs is mounted on a shaft and positioned so the windows and tabs pass through the air gap between the magnet and Hall chip. Two things happen here, the windows and tabs are shielded and yet also exposed to the magnetic field.
If you are trying to time the ignition, the metal rotor has a number of tabs and windows the same size for the number of engine cylinders. What this does is give you what is referred to as a uniform square wave output. This happens because the on/off action (the exposure of windows and tabs) is equally balanced. The engine's computer uses this signal to control ignition timing.
To simplify this process even further and prevent any malfunctions in the sensor, many Hall effect sensors have a built-in NPN transistor that has an open collector and a grounded emitter. This means rather than voltage being created at the sensor output wire, the transistor turns on, opening a circuit to ground through the output wire. As you can tell, it sounds complex, but it's really simple yet effective.
Wheel Rotation Sensing and Other Things
Knowing how fast the wheels of a vehicle are rotating is really useful when it comes to anti-lock brakes. These days though it's more than just ABS that gets the once over from Hall effect sensors. These sensors are being constantly developed to take over other vehicle functions. Most mechanics today either love to work on a car with these sensors, or wish the were working on one of the oldies, but goldies.
Technology stands still for no man, and in the case of Hall effect sensors, this is even more the case. Sensors like this can and will revoluntionize the market place in ways we hadn't even thought of. But it's not just cars and trucks using the Hall effect sensors, they're crucial for the industrial/off-highway market as well.
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