Everything about The Hall Effect totally explained
The
Hall effect refers to the
potential difference (
Hall voltage) on the opposite sides of an
electrical conductor through which an
electric current is flowing, created by a
magnetic field applied
perpendicular to the current.
Edwin Hall discovered this effect in
1879.
The ratio of the voltage created to the power of the product of the amount of current and the magnetic field (I*B), divided by the element thickness, is known as the
Hall coefficient. It is a characteristic of the material from which the conductor is made, as its value depends on the type, number and properties of the
charge carriers that constitute the current.
Theory
The Hall effect comes about due to the nature of the current flow in a conductor. Current consists of the movement of many small charge-carrying "particles" (typically, but not necessarily,
electrons). Moving charges experience a force, called the
Lorentz Force, when a magnetic field is present that isn't parallel to their motion. When such a magnetic field is absent, the charges follow an approximately straight, 'line of sight' path. However, when a perpendicular magnetic field is applied, their path is curved so that moving charges accumulate on one face of the material. This leaves equal and opposite charges exposed on the other face, where there's a scarcity of mobile charges. The result is an
asymmetric distribution of charge density across the hall element that's perpendicular to both the 'line of sight' path and the applied magnetic field. The separation of charge establishes an
electric field that opposes the migration of further charge, so a steady
electrical potential builds up for as long as the current is flowing.
For a simple metal where there's only one type of
charge carrier (electrons) the Hall voltage
VH is given by
»
Technological applications
So-called "
Hall effect sensors" are readily available from a number of different manufacturers, and may be used in various sensors such as fluid
flow sensors,
current sensors, and
pressure sensors. Other applications may be found in some electric
airsoft guns and on the triggers of electropnuematic
paintball guns.
Quantum Hall effect
For a two dimensional electron system which can be produced in a
MOSFET. In the presence of large
magnetic field strength and low
temperature, one can observe the quantum Hall effect, which is the
quantization of the Hall voltage.
Spin Hall effect
The Spin Hall effect consists in the spin accumulation on the lateral boundaries of a current-carrying sample. No magnetic field is needed. It was predicted by M.I. Dyakonov and V.I. Perel in 1971 and observed experimentally more than 30 years later, both in semiconductors and in metals, at cryogenic as well as at room temperatures.
Quantum Spin Hall effect
For HgTe two dimensional quantum wells with strong spin-orbit coupling, in zero magnetic field, at low temperature, the Quantum Spin Hall effect has been recently observed.
Hall effect in magnetic systems
In
ferromagnetic materials (and
paramagnetic materials in a
magnetic field), the Hall resistivity includes an additional contribution, known as the
Anomalous Hall Effect (or the
Extraordinary Hall effect), which depends directly on the
magnetization of the material, and is often much larger than the ordinary Hall effect. (Note that this effect is
not due to the contribution of the
magnetization to the total
magnetic field.) Although a well-recognized phenomenon, there's still debate about its origins in the various materials. The anomalous Hall effect can be either an
extrinsic (disorder-related) effect due to
spin-dependent
scattering of the
charge carriers, or an
intrinsic effect which can be described in terms of the
Berry phase effect in the crystal momentum space (
k-space).
Applications
Hall effect devices produce a very low signal level and thus require amplification. While suitable for laboratory instruments, the
vacuum tube amplifiers available in the first half of the
20th century were too expensive, power consuming, and unreliable for everyday applications. It was only with the development of the low cost
integrated circuit that the Hall effect sensor became suitable for mass application. Many devices now sold as "
Hall effect sensors" are in fact a device containing both the sensor described above and a high gain
integrated circuit (IC) amplifier in a single package. Recent advances have resulted in the addition of ADC (Analog to Digital) converters and
I²C (Inter-integrated circuit communication protocol) IC for direct connection to a microcontroller's I/O port being integrated into a single package, see
Advanced Hall Effect Current Transducer
.
Reed switch electrical motors using the hall effect IC is another application.
Hall probes are often used to measure magnetic fields, or inspect materials (such as tubing or pipelines) using the principles of
Magnetic flux leakage.
Advantages over other methods
Hall effect devices when appropriately packaged are immune to dust, dirt, mud, and water. These characteristics make Hall effect devices better for position sensing than alternative means such as optical and electromechanical sensing.
When electrons flow through a conductor, a magnetic field is produced. Thus, it's possible to create a non-contacting current sensor. The device has three terminals.
A sensor voltage is applied across two terminals and the third provides a voltage proportional to the current being sensed. This has several advantages; no additional resistance (a
shunt, required for the most common current sensing method) need be inserted in the primary circuit. Also, the voltage present on the line to be sensed isn't transmitted to the sensor, which enhances the safety of measuring equipment.
Ferrite toroid Hall effect current transducer
Hall sensors can detect stray magnetic fields easily, including that of Earth, so they work well as electronic compasses: but this also means that such stray fields can hinder accurate measurements of small magnetic fields. To solve this problem, Hall sensors are often integrated with magnetic shielding of some kind. For example, a Hall sensor integrated into a ferrite ring (as shown) can reduce stray fields by a factor of 100 or better. This configuration also provides an improvement in signal-to-noise ratio and drift effects of over 20 times that of a 'bare' Hall device.
The range of a given feedthrough sensor may be extended upward and downward by appropriate wiring. To extend the range to lower currents, multiple turns of the current-carrying wire may be made through the opening. To extend the range to higher currents, a current divider may be used. The divider splits the current across two wires of differing widths and the thinner wire, carrying a smaller proportion of the total current, passes through the sensor.
The principle of increasing the number of 'turns' a conductor takes around the ferrite core is well understood, each turn having the effect of 'amplifying' the current under measurement. Often these additional turns are carried out by a staple on the PCB.
Split ring clamp-on sensor
A variation on the ring sensor uses a split sensor which is clamped onto the line enabling the device to be used in temporary test equipment. If used in a permanent installation, a split sensor allows the electrical current to be tested without dismantling the existing circuit.
Analogue multiplication
The output is proportional to both the applied magnetic field and the applied sensor voltage. If the magnetic field is applied by a solenoid, the sensor output is proportional to product of the current through the solenoid and the sensor voltage. As most applications requiring computation are now performed by small (even tiny)
digital computers, the remaining useful application is in power sensing, which combines current sensing with voltage sensing in a single Hall effect device.
Current sensing
By sensing the current provided to a load and using the device's applied voltage as a sensor voltage it's possible to determine the power dissipated by a device. This power is (for
direct current devices) the product of the current and the voltage. With appropriate refinement the devices may be applied to
alternating current applications where they're capable of reading the true power produced or consumed by a device.
Position and motion sensing
Hall effect devices used in motion sensing and motion limit switches can offer enhanced reliability in extreme environments. As there are no moving parts involved within the sensor or magnet, typical life expectancy is improved compared to traditional electromechanical switches. Additionally, the sensor and magnet may be encapsulated in an appropriate protective material.
Automotive ignition and fuel injection
If the magnetic field is provided by a rotating magnet resembling a toothed gear, an output pulse will be generated each time a tooth passes the sensor. This is used in modern
automotive primary
distributor ignition systems, replacing the earlier
contact breaker ('points', which were prone to wear and required periodic adjustment and replacement). Similar sensor signals are used to control multi-port sequential
fuel injection systems, where each cylinder's intake runner is fed fuel from an injector consisting of a spray valve regulated by a solenoid. The sequences are timed to match the intake valve openings and the duration of each sequence by the
Engine Control Unit (computer).
Wheel rotation sensing
The sensing of wheel rotation is especially useful in
anti-lock brake systems. The principles of such systems have been extended and refined to offer more than anti-skid functions, now providing extended vehicle
handling enhancements.
Electric motor control
Some types of
brushless DC electric motors use Hall effect sensors to detect the position of the rotor and feed that information to the motor controller.
Industrial applications
Applications for Hall Effect sensing have also expanded to the industrial/off-highway market, which now use Hall Effect Joysticks to control hydraulic valves, replacing the traditional mechanical levers. Such applications include; Mining Trucks, Backhoe Loaders, Cranes, Diggers, Scissor Lifts, etc. The leading manufacturer of Industrial Hall Effect Joysticks is
P-Q Controls, Inc.,which was one of the first companies to expand the use of Hall Effect sensing to such applications in the 1980's, and in fact holds exclusive patents for contactless sensing.
The Corbino effect
The
Corbino effect is a phenomenon similar to the Hall effect, but a disk-shaped metal sample is used in place of a rectangular one. A radial current through a circular disc subjected to a magnetic field perpendicular to the plane of the disk, produces a "circular" current through the disk.
Further Information
Get more info on 'Hall Effect'.
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