A type of sensor that measures the strength of a magnetic field.

A steady current is passed through a thin plate. If a magnetic field passes perpendicular to the plate, then the distribution of the electrons passing through the plate will be skewed, so that there is a higher concentration on one side than the other. This will create a voltage difference that is roughly, but not quite, porportional to the strength of the field. A Schmitt trigger can be tacked on the end to provide a threshold, if you are interested in binary response.

Hall effect sensors can be used as encoders for the distributor in a car engine. A sensor is placed on one side of the path of a distributor's vane, and a magnet on the other. When the vane is not in place, a normal magnetic field passes through the sensor. When the vane passes between the two elements, the magnetic field is short circuited, and the sensor sends a signal.

While they don't have the same resolution, hall effect sensors have a practical advantage over optical encoders in that they can get greasy without inhibiting performance.

The Hall effect sensor is a magnetic field sensor that is capable of detecting both static and dynamic fields. Hall effect sensors are made from a semiconductor material with a high Hall constant (like Indium-antimony or indium-arsenic). There are several types of sensors available, some have a linear output according to field strength. Others may have a output that turns on when the field comes over a spesific value. Pure Hall effect elements without built in amplification also exist.

The Hall effect.

No magnetic field:

 electron     ___/___                 /
  flow       /      /             ---
      -->---/------/--->---      |\  | (no voltage)
           /______/              `-o-'
              /                   /

In a magnetic field(electron flow deflected):

                - _____________________
 electron     ___/___                 /
  flow       / ..   /             ---
      -->---/-'  `-/--->---      |  /| voltage
           /______/              `-o-'difference
              /                   /
            + `------------------/

The hall effect sensor is connected to a DC power supply. Without a magnetic flux present the electrons flow evenly and straight through the conductor. But when the sensor is subjected to a magnetic field, the field will attract or repel the electrons according to polarity(This is due to the electrons charge). This means that one side of the plate has a higher density of electrons, and this causes an elecrtrical potential difference between the sides of the plate. This potential are almost propotional to the strength of the field.

Common places to find these sensors is in disk drives and cooling fans where they are used to measure rotational speed. They are also used in DC amphere meters of the clamp type.

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