The Guide to Hall Effect Sensors

When an electric current flows through any material, the electrons within the current naturally move in a straight line, with the electricity creating its own magnetic field as it charges.

If the electrically-charged material is placed between the poles of a permanent magnet, instead of moving in a straight line, the electrons will instead deviate into a curved path as they move through the material. This happens because their own magnetic field is reacting to the contrasting field of the permanent magnet.

As a result of this new curved movement, more electrons are then present at one side of the electrically-charged material. Through this, a potential difference (or voltage) will then appear across the material at right angles to the magnetic field, from both the permanent magnet and the flow of the electric current.

So, how does a Hall effect sensor work?

Using semiconductors (such as silicon), Hall effect sensors work by measuring the changing voltage when the device is placed in a magnetic field. In other words, once a Hall effect sensor detects that it is now in a magnetic field, it is able to sense the position of objects.

Hall effect sensors and magnets

Magnets are intrinsic to Hall effect sensors, which are activated by the presence of an external magnetic field. The device is then able to sense as an object moves either closer or further away, just through the differing strengths of the magnetic field.

As an example, if a Hall effect sensor was placed in a door frame and a magnet on the door, the sensor would be able to detect when the door is open or closed through the presence of the magnetic field. All magnetic fields have two important characteristics. Firstly, what is called a ‘flux density’, which refers to the amount of magnetic flow passing through a unit area, and secondly, all magnets feature two polarities (the North and South poles).

The output signal that comes out from a Hall effect sensor represents the density of a magnetic field around the device. Hall effect sensors have a preset threshold, and when the magnetic flux density exceeds this limit, the device is able to detect the magnetic field by generating an output called the ‘Hall Voltage’.

Hall effect sensors all have a thin piece of semiconductor material inside them, which passes a continuous electrical current through itself to generate a magnetic field. When the device is placed near an external magnet, the magnetic flux exerts a force on the semiconductor material. This force causes a movement of electrons, creating a measurable Hall voltage and activating the Hall effect sensor.

The output Hall voltage from the Hall effect sensor is directly proportional to the strength of the magnetic field passing through the semiconductor material. Often, this output voltage is quite small - equal to only a few microvolts - with many Hall effect devices including built-in DC amplifiers, alongside logic-switching circuits and voltage regulators, which are there to help improve the sensitivity (and therefore effectiveness) of the device.

There are two types of Hall effect sensors: Devices with linear (or analogue) outputs, and those that have digital outputs. Analogue sensors use a continuous voltage output that increases within a strong magnetic field and decreases in a weaker field.

With linear output Hall effect sensors, as the strength of the external magnetic field increases as the device comes into contact with the magnet, the output signal increases in parallel until it reaches the limits imposed by the power supply.

The digital output device, conversely, has a ‘Schmitt trigger’, which is a bistable circuit that steadily increases and decreases the output when the voltage rises and falls to different thresholds.

Thanks to the Schmitt trigger, when the magnetic flux passing through the Hall effect sensor exceeds the device’s preset value, the output from the device switches it from ‘off’ to ‘on’. As the sensor then moves in and out of the magnetic field, the built-in hysteresis in the device eliminates any oscillation of the output signal.

Digital Hall effect sensors

There are two types of digital Hall effect sensors: bipolar and unipolar, which differ depending on the type of magnetic field needed to operate them.

Bipolar digital Hall effect sensors need a positive magnetic field (which comes from the South pole of a magnet) to operate them, and use the negative field (from the North pole) to release them. Unipolar sensors only need a single magnetic South pole to both operate and release them as they move in and out of the magnetic field.

As the output drive capabilities are very small on the majority of digital Hall effect sensors, most of these devices cannot directly switch large electrical loads. Many digital sensors counter this by using an open-collector NPN transistor.

The transistor operates as a switch, shorting out the output terminal to the ground when the magnetic flux density is higher than the Hall effect sensor ‘on’ point. There is a wide range of Hall effect switches available, which are suitable for a variety of different tasks.

There are two ways that Hall effect sensors are used to measure fuel levels. Firstly, in a vertical float system, a permanent magnet is fixed to the surface of a floating object, while the sensor is fitted to the top of the tank, in line with the magnet.

After filling up your car or vehicle, the floating magnet rises to the top of the tank, bringing the magnetic field towards the sensor and increasing the Hall voltage. As the fuel levels lower, the magnet drops, decreasing the Hall voltage in parallel.

As well as measuring fuel, Hall effect sensors are commonly used to time the speed of rotating parts like wheels and shafts, and are particularly useful in various ‘tachometers’, which measure things like speed of the vehicle and RPM.

Hall effect sensors are particularly useful in conditions where things like water, vibrations, or dirt are present, which would impact upon optical and light sensors. This is why they are particularly effective in industrial applications, as magnetic fields cannot be interrupted by external factors.

When a flip cover is closed, the magnet is brought towards the Hall sensor on your smartphone or tablet. Flip covers provide the magnetic field, which generates Hall voltage. Using a Schmitt trigger in the circuit, the mobile device is able to perform particular operations, when the Hall sensor detects this magnetic force, such as turning the screen off.

Through using a Hall effect sensor, battery life is prolonged by disabling the screen when it is covered, and can’t be used.

Hall effect sensors are used over other devices as they are extremely small, easy and cheap to manufacture, and insusceptible to wide temperature changes, which is particularly useful for phones.

While Hall effect sensors are predominantly used to detect objects and spaces, they can also be used for current measurement. As established at the beginning of this guide, the Hall Effect is based on the relationship between magnetic fields and currents, with the devices above utilising magnetic fields to create currents as an output.

By reversing this principle, you can also use a Hall effect sensor for current measurement, by passing the current to get a magnetic field.

Current sensors are an important tool for monitoring the status of equipment, detecting potential variations and ensuring that equipment is safe to use. While pressure switches, optical sensors and zero-speed switches have often been used in the past to monitor equipment, measuring the current input can provide a more accurate insight into equipment performance.

Using both a digital or linear Hall effect sensor to measure current is an effective approach, as these devices are based on the principle that for a given current flow, a proportional magnetic field is produced.

Firstly, the digital Hall effect sensor consists of three key components: the core, the Hall effect device, and signal conditioning circuitry. When the sensor is turned on and exposed to a magnetic field from the core, it produces a potential difference (or voltage) that can be measured and then amplified further into process level signals.

One major benefit of using Hall effect sensors for current measurement is that they are totally isolated from the voltage being monitored, making them a safe way to test equipment without insertion. The measurements are also accurate and repeatable on both AC and DC power, making digital Hall effect sensors an excellent choice for current measurement.

A variant of this is the ratiometric Hall effect sensor, which outputs an analogue voltage that is proportional to the magnetic field intensity. This type of sensor works based on the magnetic polarities, with voltage increasing with the South pole, and decreasing with the North magnetic pole.

A ratiometric Hall effect sensor is an alternative to the digital device discussed above. Instead of switching from off to on outputs, the voltage is measured proportionally to the strength of the magnetic field and magnetic polarity.

As with the digital Hall effect sensors, ratiometric devices can be used to measure the current through a wire. The higher the current, the stronger the magnetic field, and thus a higher output voltage will be measured. Both approaches offer an effective way to measure currents, and test how efficiently electrical equipment is operating.

From current measurement to pressure sensors, fluid flow sensors and object detection, Hall effect sensors and switches have a variety of uses and applications, with these versatile devices an essential part of much electrical equipment.