Hall Effect Sensor

In this article we will see the hall effect sensor. Magnetic sensors convert magnetically encoded information into electrical signals for processing by electronic circuits. Magnetic sensors are solid state devices that are becoming increasingly popular because they can be used in many different applications such as position, speed or directional movement. It is also a popular sensor choice for electronic designers due to its contactless, wear-free operation, low maintenance requirements, robust designs and immunity to vibration, dust and water of sealed salon effect devices.

One of the main uses of magnetic sensors is in automotive systems for the detection of position, distance and speed. For example, the angular position of the crankshaft for the ignition angle of the spark plugs, the position of the car seats and seatbelts for airbag control, or wheel speed detection for the anti-lock braking system (ABS), etc.

Magnetic sensors are designed to respond to a wide range of positive and negative magnetic fields in a variety of different applications. A type of magnet sensor that is a function of magnetic field density around the output signal is called the Hall Effect Sensor.

Hall Effect Sensors aredevices activated by an external magnetic field. We know that a magnetic field has two important characteristics: flux density (B) and polarity (North and South Poles). The output signal from the Hall effect sensor is the function of the magnetic field density around the device. When the magnetic flux density around the sensor exceeds a certain preset threshold, the sensor detects it and the Hall Voltage produces an output voltage called VH.

Hall Effect Sensor Principles

Hall Effect Sensor
Hall Effect Sensor Principles

Hall Effect Sensors consist mainly of a thin rectangular piece of p-type semiconductor material such as gallium arsenite (GaAs), indium antimonite (InSb) or indium arsenite (InAs). It passes a constant current through itself. When the device is placed in a magnetic field, magnetic flux lines apply a force on the semiconductor material that deflects load carriers, electrons and holes to both sides of the semiconductor plate. This movement of the load carriers is the result of the magnetic force they experience when passing through semiconductor material.

As these electrons and holes move sideways, a potential difference is produced between the two sides of the semiconductor material by the accumulation of these load carriers. Then the movement of electrons in semiconductor material is influenced by the presence of an external magnetic field that angles itself at a right angle, and this effect is greater in a material with a flat rectangular shape.

The effect of generating a measurable voltage using a magnetic field is called the Hall Effect after Edwin Hall, who discovered in the 1870s that the underlying physical principle of the Hall effect was the Lorentz force. To make a potential difference throughout the device, magnetic flux lines should be perpendicular to the flow of the current (90o) and at the correct polarity, usually at a south pole.

The Hall effect provides information about the type of magnetic pole and the size of the magnetic field. For example, a south pole causes the device to produce a voltage output, while an arctic has no effect. In general, Hall Effect sensors and switches are designed to be "OFF" (open circuit state) when there is no magnetic field. They only switch to an "ON" (closed circuit state) when exposed to a magnetic field with sufficient power and polarity.

Hall Effect Magnetic Sensor

The output voltage of the Basic Hall Element, called Hall voltage (VH), is directly proportional to the power of the magnetic field passing through the semiconductor material (output ∝ H). This output voltage can be quite small, there can be only a few microvolts even when exposed to strong magnetic fields, so most of the Hall effect devices available on the market are produced with built-in DC amplifiers, logic switching circuits and voltage regulators to improve sensor sensitivity, hysteresis and output. Voltage. This also allows the Hall effect sensor to operate in a wider range of power supply and magnetic field conditions.

Hall Effect Sensor

Hall Effect Sensor
Hall Effect Sensor

Hall Effect Sensors are available with linear or digital outputs. The output signal for linear (analog) sensors is directly taken from the output of the operational amplifier, in direct proportion to the magnetic field passing through the output voltage Hall sensor. This output Hall voltage is given as follows:

Hall Effect Sensor

Linear or analog sensors give a continuous voltage output that increases with a strong magnetic field and decreases with a weak magnetic field. In linear-output Hall effect sensors, as the power of the magnetic field increases, the output signal from the amplifier will increase until it begins to saturate with the limits applied to it by the power supply. Any additional increase in the magnetic field will have no effect on the output, but will lead it to greater saturation.

Digital output sensors have a schmitt trigger with built-in hysteresis connected to the op-amp. When the magnetic flux passing through the Hall sensor exceeds a preset value, the output from the device quickly switches from "OFF" to "ON" without any contact jumps. This built-in hysteresis eliminates any oscillation of the output signal as the sensor enters and exits the magnetic field. Then there are only two states of digital output sensors, "ON" and "OFF".

There are two basic types of digital Hall effects sensors, Bipolar and Unipolar. Bipolar sensors require a positive magnetic field (south pole) to operate them and a negative field (north pole) to release them, while unipolar sensors require only a single magnetic south pole to both operate and release them as they enter and out of the magnetic field. field.

Most Hall effect devices cannot directly replace large electrical charges because their output drive capabilities are very small around 10 to 20mA. An open collector (current collapsing) NPN Transistor is added to the output for large current loads.

This transistor operates in its saturated zone as an NPN receiver switch that shorts the output terminal into the ground when the applied flux density is higher than the "ON" preset point.

The output switching transistor can provide an open emitter transistor, open collector transistor configuration or a push-pull output type configuration that can receive enough current to drive many loads directly, including relays, motors, LEDs and lamps.

Hall Effect Applications

Hall effect sensors are activated by a magnetic field, and in many applications the device can be operated by a moving shaft or a single permanent magnet connected to the device. There are many different types of magnet movement, such as detection gestures such as "Head-on", "Sideways", "Push-pull" or "Push-push", etc. Magnetic flux lines should always be perpendicular to the detection area of the device and have the correct polarity to ensure maximum sensitivity, using all kinds of configurations.

In addition, to ensure linearity, high-field strong magnets are required for the necessary movement, producing a large change in field strength. There are several possible ways of moving to detect a magnetic field, and below are two more common detection configurations that use a single magnet: Re-Detection and Side Detection.

Direct Detection

Hall Effect Sensor
Direct Detection

As its name suggests, "head detection" requires the magnetic field to be perpendicular to the hall effect sensing device and approach the sensor directly towards the active face for detection. It's a kind of head-to-head approach.

This head-to-head approach produces an output signal, VH, which represents the strength of the magnetic field, magnetic flux density, as a function of distance from the hall effect sensor in linear devices. The closer the magnetic field and therefore the stronger it is, the greater the output voltage, and vice versa.

Linear devices can also distinguish between positive and negative magnetic fields. Nonlinear devices can be made to trigger an "ON" output at a preset air gap distance away from the magnet to demonstrate positional detection.

Side Detection

The second detection configuration is "side detection". This requires that the magnet be moved sideways along the face of the Hall effect element.

Lateral or floating detection is useful for detecting the presence of a magnetic field when moving within a constant air gap distance along the face of the Hall element, for example, counting rotating magnets or the rotational speed of engines.

Depending on the position of the magnetic field when passing through the sensor's zero-field center line, a linear output voltage can be generated, representing both positive and negative output. This allows directional motion detection, which can be both vertical and horizontal.

Hall Effect Sensors have many different applications, especially as proximity sensors. As with automotive applications, they can be used in place of optical and light sensors in ambient conditions consisting of water, vibration, dirt or oil. Hall effect devices can also be used for current detection.

We know from previous lessons that when a conductor passes through the current, a circular electromagnetic field is produced around it. By placing the Hall sensor next to the conductor, electrical currents can be measured from several milliampers to thousands of ampere from the magnetic field produced without the need for large or expensive transformers and coils.

Hall effect sensors can be used to detect the presence or absence of magnets and magnetic fields, as well as to detect ferromagnetic materials such as iron and steel by placing a small permanent "biased" magnet behind the active area of the device. The sensor is now located within a permanent and static magnetic field, and any changes or distortions in this magnetic field with the addition of an anchored material will be detected with as low sensitivities as possible of mV/G.

Depending on the type of digital or linear device, there are many different ways to connect Hall effect sensors to electrical and electronic circuits. A very simple and easy-to-make example is to use a Light Emitting Diode, as shown below.

Positional Detector

Hall Effect Sensor
Positional Detector

This head-to-head positional detector will be "OFF" when there is no magnetic field (0 gauss). When the permanent magnets are moved vertically towards the south pole (positive gauss), the Active Area of the Hall effect sensor, the device becomes "ON" and lights up the LED. When placed in the "ON" position, the Hall effect sensor remains "ON".

To make the device and therefore the LED "OFF", the magnetic field must be lowered below the release point for unipolar sensors or exposed to the magnetic north pole (negative gauss) for bipolar sensors. If the Hall Effect Sensor needs output to replace larger current loads, the LED can be replaced with a larger power transistor.