Hall probe

A Hall probe contains an indium-compound semiconductor crystal such as indium antimonide, mounted on an aluminum backing plate and encapsulated in the probe head. The plane of the crystal is perpendicular to the probe handle. Connecting leads from the crystal are brought down through the handle to the circuit box. When the Hall probe is held so that the magnetic field lines are passing at right angles through the sensor of the probe, the meter gives a reading of the value of magnetic flux density (B). A current is passed through the crystal, which, when placed in a magnetic field, has a "Hall effect" voltage developed across it. The Hall effect is seen when a conductor is passed through a uniform magnetic field. The natural electron drift of the charge carriers causes the magnetic field to apply a Lorentz force (the force exerted on a charged particle in an electromagnetic field) to these charge carriers, resulting in charge separation, with a buildup of either positive or negative charges on the bottom or on the top of the plate. The crystal measures 5 mm square. The probe handle, being made of a non-ferrous material, has no disturbing effect on the field. A Hall probe should be calibrated against a known value of magnetic field strength. For a solenoid the Hall probe is placed in the centre.

Working principle

In a Hall-effect sensor, a thin strip of metal has a current applied along it. In the presence of a magnetic field, the electrons in the metal strip are deflected toward one edge, producing a voltage gradient across the short side of the strip (perpendicular to the feed current). Hall-effect sensors have an advantage over inductive sensors in that, while inductive sensors respond to a changing magnetic field that induces current in a coil of wire and produces voltage at its output, Hall-effect sensors can detect static (non-changing) magnetic fields.

In its simplest form, the sensor operates as an analog transducer, directly returning a voltage. With a known magnetic field, its distance from the Hall plate can be determined. Using groups of sensors, the relative position of the magnet can be deduced.

When a beam of charged particles passes through a magnetic field, forces act on the particles, and the beam is deflected from a straight path. The flow of electrons through a conductor forms a beam of charged carriers. When a conductor is placed in a magnetic field perpendicular to the direction of the electrons, they are deflected from a straight path. As a consequence, one plane of the conductor becomes negatively charged, and the opposite side becomes positively charged. The voltage between these planes is called the Hall voltage.

When the force on the charged particles from the electric field balances the force produced by the magnetic field, the separation of charges stops. If the current is not changing, then the Hall voltage is a measure of the magnetic flux density. Basically, there are two kinds of Hall-effect sensors: linear, which means that the output of voltage linearly depends on magnetic flux density; and threshold, which means that there is a sharp decrease of output voltage at some magnetic flux density. This experiment was the one to demonstrate that there are only negative charges free to move in a conductor. Before this, it was believed that positive charges move in a current-carrying conductor. This experiment is known as the Hall experiment.

Materials

The key factor determining sensitivity of Hall-effect sensors is high electron mobility. As a result, the following materials are especially suitable for Hall-effect sensors:

gallium arsenide (GaAs),
indium arsenide (InAs),
indium phosphide (InP),
indium antimonide (InSb),
graphene.

Advantages
A Hall-effect sensor may operate as an electronic switch.

Such a switch costs less than a mechanical switch and is much more reliable.
It can be operated at higher frequencies than a mechanical switch.
It does not suffer from contact bounce because a solid-state switch with hysteresis is used rather than a mechanical contact.
It is not affected by environmental contaminants, since the sensor is in a sealed package. Therefore, it can be used under severe conditions.

In the case of linear sensor (for the magnetic-field-strength measurements), a Hall-effect sensor:

can measure a wide range of magnetic fields,
can measure both sign and amplitude,
can be flat.

Disadvantages

Hall-effect sensors provide much lower measuring accuracy than fluxgate magnetometers or magnetoresistance-based sensors. Moreover, Hall-effect sensors drift significantly, requiring compensation.