Hall-Effect Sensors

Based on the principle that if a current is allowed to flow through thin conducting material that is exposed to a magnetic field, another voltage is produced. The switch contains a permanent magnet, a thin semiconductor layer made of gallium arsenate crystal (Hall layer), and a shutter wheel. The Hall layer has a negative and a positive terminal connected to it. Two additional terminals located on either side of the Hall layer are used for the output circuit. The shutter wheel consists of a series of alternating windows and vanes. It creates a magnetic shunt that changes the strength of the magnetic field from the permanent magnet.

The permanent magnet is located directly across from the Hall layer so that its lines of flux will bisect at right angles to the current flow. The permanent magnet is mounted so that a small air gap is between it and the Hall layer.

Hall-Effect Sensors principles of voltage induction

Hall-Effect Sensors principles of voltage induction

A steady current is applied to the crystal of the Hall layer. This produces a signal voltage that is perpendicular to the direction of current flow and magnetic flux. The signal voltage produced is a result of the effect the magnetic field has on the electrons. When the magnetic field bisects the supply current flow, the electrons are deflected toward the Hall layer negative terminal. This results in a weak voltage potential being produced in the Hall switch.

A shutter wheel is attached to a rotational component. As the wheel rotates, the shutters (vanes) will pass in this air gap. When a shutter vane enters the gap, it intercepts the magnetic field and shields the Hall layer from its lines of force. The electrons in the supply current are no longer disrupted and return to a normal state. This results in low voltage potential in the signal circuit of the Hall switch.

The signal voltage leaves the Hall layer as a weak analog signal. To be used by the computer, the signal must be conditioned. It is first amplified because it is too weak to produce a desirable result. The signal is also inverted so that a low input signal is converted into a high output signal. It is then sent through a Schmitt trigger where it is digitized and conditioned into a clean square wave signal. The signal is finally sent to a switching transistor. The computer senses the turning on and off of the switching transistor to determine the frequency of the signals and calculates speed.

The Hall-effect just discussed describes its usage as a switch. It can also be designed as an analog (or linear) sensor that produces an output voltage that is proportional to the applied magnetic field. This makes them useful for determining to position of a component instead of just rotation. For example, this type of sensor can be used to monitor fuel level or to track seat positions in memory seat systems.

A fuel level indication can be accomplished with a Hall-effect sensor by attaching a magnet to the float assembly. As the float moves up and down with the fuel level, the gap between the magnet and the Hall element will change. The gap changes the Hall-effect and thus the output voltage.

As discussed, typical Hall-effect sensors and switches use three wires. However, linear Hall-effect sensors can also be constructed using two wire circuits. This is common on systems that use a DC motor drive. The reference voltage to the sensor is supplied through a pull-up resistor. Typically this reference voltage will be 12 volts. Whenever the motor is operated the reference voltage will be applied. After the motor is turned off, this reference voltage will remain for a short time.

Typical circuit of a Hall-effect switch

Typical circuit of a Hall-effect switch

Internal to the motor assembly is a typical three terminal Hall sensor. The reference voltage is supplied to terminal 1 of the Hall sensor. A pull-up resistor also connects the reference voltage to terminal 3 of the Hall sensor. This becomes the signal circuit. The two pull-up resistors will be of equal value. Terminal 2 of the Hall sensor is connected to the sensor return circuit. A magnet is attached to the motor armature to provide a changing magnetic field once per motor revolution.

When the Hall sensor is off the voltage supplied to the Hall sensor will be near that of the source voltage. Since this is an open circuit condition in the Hall sensor at terminal 3, the voltage drop over the signal circuit pull-up resistor will be 0.

When the motor rotates and the influence of the magnetic field turns on the Hall sensor, the signal terminal 3 is connected to ground within the sensor. This pulls the signal voltage low and results in the formation of a series circuit from the reference supply to terminal 3. Since each of the pull-up resistors are equal the voltage drop will be split between the two. Approximately half the voltage will be dropped across the pull-up resistor in the computer and the other half over the pull-up resistor in the motor assembly. The Hall-effect sensor will remain powered since the reference voltage to terminal 1 is connected between the two resistors and the 6 volts on the circuit is sufficient to operate the sensor.

D8x2.5mm Diametral Magnet Disc N35H

D8x2.5mm Diametral Magnet Disc N35H

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Diametric Disc Magnet NdFeB N35 D6x2.5mm

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D6x2.5mm N35H Diametric Disc Magnets for rotary position sensor

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