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Posted on 17 September 2020

Hall Effect Sensors

 

Hall effect

If we place a current carrying conductor or semiconductor in a perpendicular magnetic field B (fig. 1) then an electric field arises perpendicular to the I-B surface. This effect is known as the Hall-effect. This effect was discovered in 1879 by the American physicist Edwin Herbert Hall.

Hall effect

Figure 1. Hall effect

Consider a current composed of holes (I ) flowing in the positive x-direction. Due to the magnetic field the holes are forced towards the lower surface (2). Side 2 of the conductor is positive relative to surface 1, in other words an electric field E exists in the positive y-direction, and therefore perpendicular to the x-z surface (I-B surface).

A Hall voltage is generated, , between the electrodes 1 and 2.

In equilibrium the electric field will exercise a force (with opposite polarity) equal to the force of the magnetic field on the flowing charge carriers:

with q = charge and v = drift velocity. From this it follows:

If there are n holes per m3 then the charge density is:

If this charge is displaced with a drift velocity v (m/s) then the current density is:

The current density may also be written as: , so that:

With the Hall constant of the material, , this becomes:

Hall constant of different materials

Table 1. Hall constants

In contrast to metals some semiconductors (e.g. indium based) have an important RH value (see table 1).

From the possible applications follow.

Applications

  1. With a constant current a magnetic field can be measured:
  2. With constant magnetic field, e.g. from a permanent magnet, it is possible to measure currents:
  3. If I  is made proportional to the first input signal (V1) and B  is proportional to a second input signal (V2), then and we have a Hall-effect multiplier. In this way we can measure power:

Technical implementation

A thin layer of a few μm of semiconductor material placed on a ceramic substrate is sufficient to make a sensor for detecting magnetic fields.

The semiconductor can be InSb (= indiumantimonide). A constant current is caused to flow through the semiconductor. If a perpendicular magnetic field is present at the sensor, then there is an output voltage (VH).

In practical sensors this voltage is 0.2 to 1V/T , hereby: 1 T ( Tesla ) = 1Wb/m2.

A differential amplifier is usually integrated into the sensor to produce a useful output voltage.

 

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- who has written 7 posts on PowerGuru - Power Electronics Information Portal.

Professor Dr. Jean Pollefliet is the author of several best-selling textbooks in Flanders and the Netherlands

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