Hall effect

The Hall effect refers to the potential difference (Hall voltage) on opposite sides of a thin sheet of conducting or semiconducting material in the form of a 'Hall bar' or a van der Pauw element through which an electric current is flowing, created by a magnetic field applied perpendicular to the Hall element. The ratio of the voltage created to the amount of current is known as the Hall resistance, and is a characteristic of the material in the element. Dr. Edwin Hall discovered this effect in 1879.

Applications

Hall effect devices produce a very low signal level. To apply this requires 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. Reed switch electrical motors using the hall effect IC is another application.

Related Topics:
Vacuum tube - Amplifier - 20th century - Integrated circuit

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Advantages over other methods

Hall effect devices when appropriately packaged are immune to dust, dirt, mud, and water. These characteristics make Hall effect devices superior for position sensing compared to alternative means such as optical and electromechanical sensing.

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The magnetic field may be that provided as a consequence of electrons flowing through a conductor. It is thus possible to create a non-contacting current sensor in which the conducting cable with current to be measured is threaded through a hole in the sensing device. The device has three terminals. Across two of these is applied a sensor voltage and from the third is taken a voltage proportional to the current being sensed. This has several advantages; no resistance (a "shunt") need be inserted in the primary circuit and also, the voltage present on the line to be sensed is not transmitted to the sensor, a characteristic which enhances the safety of measuring equipment.

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The range of a given feed through 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, with a portion of the current carried by a large wire flowing through a smaller parallel wire with the small wire passing through the sensor.

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Split ring clamp-on sensor

A variation on the ring sensor uses a split sensor which is clamped onto the line, rather than threading the line through the sensor, enabling the device to be included in test equipment not permanently installed in the device being tested. This also simplifies the permanent addition of current sensing to existing circuits as they need not be dismantled to perform the installation.

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Analog 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.

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Power sensing

By sensing the current provided to a load and using the device applied voltage as a sensor voltage it is possible to determine the power flowing through 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 are capable of reading the true power produced or consumed by a device.

Related Topics:
Direct current - Alternating current

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Position and motion sensing

Applied to mechanical motion sensing and motion limit switches the Hall effect device can offer enhanced reliability in extreme environments. As there are no moving parts involved within the sensor or magnet, there is a far greater useful life expected than from electromechanical switches. Also, the sensor and magnet may be permanently and completely encapsulated in an appropriate material

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Automotive ignition and fuel injection

If the magnetic field is applied by a rotating magnet resembling a toothed gear then 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 "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 (under the control of a computer) determines the amount of fuel delivered.)

Related Topics:
Automotive - Distributor

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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.

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