Omnipolar and unipolar

Fig. 6: A1301/A1302 linear Hall effect sensor IC by Allegro

Fig. 3 shows the functional block diagram of an omnipolar (north- or south-pole) Hall effect switch. It is based on two Hall effect plates and a chopper-stabilised architecture. The single open-drain output switches on with either a north-pole or south-pole magnetic field of sufficient strength. When the magnetic flux density (B) is larger than operate point (Bop), the output switches on (output pin is pulled low). The output turns off when B becomes lower than the release point (Brp). It remains off when there is no magnetic field.

In case of a unipolar Hall effect switch (refer Fig. 4), the output switches on in the presence of a sufficiently strong south-pole magnetic field facing the marked side of the chip package. Similarly, the output switches off in the presence of a weaker south field and remains off with no field.

Unipolar types are optimised for applications of solidstate switches. Magnet proximity sensor for reed switch replacement in low-duty-cycle applications is a good example.

Transfer function

Fig. 7: Magnet-operated relay using MH183 unipolar Hall effect switch. The relay energises in the presence of a sufficiently strong south-pole magnetic field facing the marked side of the sensor MH183. Circuit is designed and tested by the author

The transfer function of a device describes its output in terms of its input. For analogue-output Hall effect sensors, the transfer function expresses the relationship between a magnetic field input (in gauss; unit of magnetic flux density equal to 1 maxwell per square centimetre) and a voltage output.

The principal input/output characteristics of the digital-output Hall effect sensor are operate point, release point and the differential. As the magnetic field is increased, there is no change in the sensor output until the operate point is reached. Once the operate point is reached, the sensor changes state to ‘on’. Further increase in the magnetic input beyond the operate point will have no effect.

If the magnetic field is decreased below the operate point, the output remains the same until the release point is reached. At this point, the sensor’s output returns to its original state (off).

Fig. 8: Open-loop Hall effect current sensor by Honeywell

The purpose of the differential between the operate and release points is to eliminate false triggering caused by minor variations in the input.

Practical applications
Hall effect sensors are widely used for speed, linear position, linear angle and position measurement in automotive, industrial and consumer applications. These devices can be used to sense many physical parameters—ranging from direct measurement of a magnetic field to detection of ocean currents. Hall effect sensors have been successfully used in linear-output applications such as current sensing (motor control protection and disk drives), position sensing (contactless potentiometers and brushless DC motors) and encoded switches (rotary encoders).

Digital-output applications include lens position sensors, proximity sensors, pressure sensors, limit switches, door interlocks and vending machines.


The author is a freelance writer and regular contributor to EFY

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