Hall Effect Sensor

Introduction:

The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor, transverse to an electric current in the conductor and to an applied magnetic field
perpendicular to the current. It was discovered by Edwin Hall in 1879. For clarity, the original effect is sometimes called the ordinary Hall effect to distinguish it from other “Hall effects” which have different physical mechanisms. The Hall coefficient is defined as the ratio of the induced electric field to the product of the current density and the applied magnetic field. It is a characteristic of the material from which the conductor is made, since its value depends on the type, number, and properties of the charge carriers that constitute the current.

A Hall effect sensor is a device that is used to measure the magnitude of a magnetic field. Its
output voltage is directly proportional to the magnetic field strength through it. Hall effect sensors are used for proximity sensing, positioning, speed detection, and current sensing applications. A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications.

Working:

Hall Effect Sensors consist basically of a thin piece of rectangular p-type semiconductor material such as gallium arsenide (GaAs), indium antimonide (InSb) or indium arsenide (InAs) passing a continuous current through itself. When the device is placed within a magnetic
field, the magnetic flux lines exert a force on the semiconductor material which deflects the charge carriers, electrons and holes, to either side of the semiconductor slab. This movement of charge carriers is a result of the magnetic force they experience passing through the
semiconductor material.

Hall Effect Sensor

As these electrons and holes move side wards a potential difference isproduced between the two sides of the semiconductor material by the build-up of these charge carriers. Then the movement of electrons through the semiconductor material is affected by the presence of an
external magnetic field which is at right angles to it and this effect is greater in a flat rectangular shaped material. The effect of generating a measurable voltage by using a magnetic field is called the Hall Effect after Edwin Hall who discovered it back in the 1870’s with the basic physical principle underlying the Hall effect being Lorentz force. To generate a potential difference across the device the magnetic flux lines must be perpendicular, (90 degree) to the flow of currentand be of the correct polarity, generally a south pole.

The Hall effect provides information regarding the type of magnetic pole and magnitude of the magnetic field. For example, a south pole would cause the device to produce a voltage output while a north pole would have no effect. Generally, Hall Effect sensors and switches are designed to be in the “OFF”, (open).

Advantages of Hall Effect Sensor:

  • Hall Effect sensors are not affected by ambient conditions, such as dust, humidity, and vibrations and are due to are insensitive to some ambient conditions based on the principle that these sensors display a constant flow of an electrical current making their characteristics constant over time.
  • Hall Effect sensors do not have contact with neighboring mechanical parts, making these sensors strong and sensitive enough to detect movement. These sensors do not wear over time thus maintain quality and unlimited use.
  • Hall Effect sensors are built from semiconductor material that display low carrier density, hence conductivity is smaller and their voltage is larger.
  • Hall Effect sensors depends on carrier mobility, which eliminates any perturbations due to surface elements; thus, making these conductors reproducible and highly reliable.
  • Production of an output voltage signal independent of the rate of the detected field.

Disadvantages of Hall Effect Sensor:

  • The Hall Effect sensor is not capable of measuring a current flow at a distance greater than 10 cm; however, use of a magnet strong enough to generate a magnetic field wide enough may make this possible.
  • Hall Effect sensors work on the principle of a magnetic field, making it possible for external magnetic fields to interfere with this and bias the measurement of a current flow.
  • Temperature affects the electrical resistance of the element and the mobility of majority carriers and also the sensitivity of Hall Effect sensors.
  • Even with well-centered electrodes, the offset voltage still presents as an output voltage in the absence of a magnetic field.
  • An offset voltage occurs when there are physical inaccuracies and material nonuniformities. It can be as high as 100 mV for a 12V source. To solve this problem, an additional control electrode would need to be added and through this a necessary current can be injected to obtain a null output when no magnetic field is present.

Applications of Hall Effect Sensor:

Hall Effect sensors are considered as magnetic sensors with a wide range of applications. Some of the most popular and effective ways of utilizing Hall effect sensing devices are listed below for both analog and digital output sensors. Analog output sensor applications include:

  • Current sensing
  • Variable speed drives
  • Motor control protection/indicators
  • Power supply sensing
  • Motion sensing
  • Diaphragm pressure gage
  • Flow meters
  • Direct current electricity
  • Encoded switches
  • Rotary encoders

Digital output sensor applications include:

  • Wireless communication
  • Pressure sensors
  • Proximity sensors
  • Flow sensors
  • Valve position sensors
  • Lens position sensors
  • Shaft position sensors Summary