Hall Effect Sensor
The Hall Effect Device
We could not end this section on
Magnetism without a mention about
magnetic sensors and especially the Hall Effect Sensor. Magnetic sensors convert magnetic or magnetically
encoded information into electrical signals for processing by electronic circuits, and in the
Sensors and Transducers tutorials we looked at
inductive proximity sensors and the LDVT as well as solenoid and relay output actuators.
Magnetic sensors are solid state devices that are becoming more and more popular because they can be used
in many different types of application such as sensing position, velocity or directional movement. They are also a popular
choice of sensor for the electronics designer due to their non-contact wear free operation, their low maintenance, robust
design and as sealed hall effect devices are immune to vibration, dust and water.
One of the main uses of magnetic sensors is in automotive systems for the sensing of position, distance
and speed. For example, the angular position of the crank shaft for the firing angle of the spark plugs, the position of the
car seats and seat belts for air-bag control or wheel speed detection for the anti-lock braking system, (ABS). Magnetic
sensors are designed to respond to a wide range of positive and negative magnetic fields in a variety of different
applications and one type of magnet sensor whose output signal is a function of magnetic field density around it is called
the Hall Effect Sensor.
Hall Effect Sensors are devices which are 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 a Hall effect sensor is the function of magnetic field density around the
device. When the magnetic flux density around the sensor exceeds a certain preset threshold, the sensor detects it and
generates an output voltage called the Hall Voltage, VH. Consider the diagram below.
Hall Effect Sensor
Hall Effect Sensors consist basically of a thin piece of rectangular p-type
semiconductor material such as gallium arsenide (GaAs), indium antimonide (InSb) or indium arsenide (InAs) passing a
continuous current through itself. When the device is placed within a magnetic field, the magnetic flux lines exert a
force on the semiconductor material which deflects the charge carriers, electrons and holes, to either side of the
semiconductor slab. This movement of charge carriers is a result of the magnetic force they experience passing through
the semiconductor material.
As these electrons and holes move side wards a potential difference is produced between the two sides of
the semiconductor material by the build-up of these charge carriers. Then the movement of electrons through the semiconductor
material is affected by the presence of an external magnetic field which is at right angles to it and this effect is greater
in a flat rectangular shaped material.
The effect of generating a measurable voltage by using a magnetic field is called the
Hall Effect after Edwin Hall who discovered it back in the 1870's with the basic physical principle
underlying the Hall effect being Lorentz force. To generate a potential difference across the device the magnetic flux
lines must be perpendicular, (90o) to the flow of current and be of the correct polarity, generally a south pole.
The Hall effect provides information regarding the type of magnetic pole and magnitude of the magnetic
field. For example, a south pole would cause the device to produce a voltage output while a north pole would have no effect.
Generally, Hall Effect sensors and switches are designed to be in the "OFF", (open circuit condition) when there is no
magnetic field present. They only turn "ON", (closed circuit condition) when subjected to a magnetic field of sufficient
strength and polarity.
Hall Effect Magnetic Sensor
The output voltage, called the Hall voltage, (VH) of the basic
Hall Element is directly proportional to the strength of the magnetic field passing through the semiconductor material
(output ∝ H). This output voltage can be quite small, only a few microvolts even
when subjected to strong magnetic fields so most commercially available Hall effect devices are manufactured with built-in
DC amplifiers, logic switching circuits and voltage regulators to improve the sensors sensitivity, hysteresis and output
voltage. This also allows the Hall effect sensor to operate over a wider range of power supplies and magnetic field conditions.
Hall Effect Sensor
Hall Effect Sensors are available with either linear or digital outputs. The output
signal for linear (analogue) sensors is taken directly from the output of the operational amplifier with the output voltage
being directly proportional to the magnetic field passing through the Hall sensor. This output Hall voltage is given as:
- VH is the Hall Voltage in volts
- RH is the Hall Effect co-efficient
- I is the current flow through the sensor in amps
- t is the thickness of the sensor in mm
- B is the Magnetic Flux density in Teslas
Linear or analogue 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 strength of the magnetic field
increases the output signal from the amplifier will also increase until it begins to saturate by the limits imposed on it
by the power supply. Any additional increase in the magnetic field will have no effect on the output but drive it more into
Digital output sensors on the other hand 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 switches
quickly between its "OFF" condition to an "ON" condition without any type of contact bounce. This built-in hysteresis eliminates
any oscillation of the output signal as the sensor moves in and out of the magnetic field. Then digital output sensors have
just two states, "ON" and "OFF".
There are two basic types of digital Hall effect sensor, 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 move in and out
of the magnetic field.
Most Hall effect devices can not directly switch large loads as their output drive capabilities are very
small around 10 to 20mA. For large current loads an open-collector (current sinking)
NPN Transistor is added to the output.
This transistor operates in its saturated region as a NPN sink switch which shorts the output terminal to ground whenever
the applied flux density is higher than that of the "ON" preset point. This output transistor can be either an open emitter,
open collector or both providing a push-pull output configuration that can sink enough current to directly drive many loads,
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 single permanent magnet attached to a moving shaft or device. There are many different types of magnet movements, such
as "Head-on", "Sideways", "Push-pull" or "Push-push" etc sensing movements. Which every type of configuration is used, to
ensure maximum sensitivity the magnetic lines of flux must always be perpendicular to the sensing area of the device and
must be of the correct polarity.
Also to ensure linearity, high field strength magnets are required that produce a large
change in field strength for the required movement. There are several possible paths of motion for detecting a magnetic field,
and below are two of the more common sensing configurations using a single magnet. Head-on Detection and Sideways Detection.
As its name implies, head-on detection requires that the magnetic field is perpendicular to the sensing
device and that for detection, it approaches the sensor straight on towards the active face. A sort of "head-on" approach.
This head-on approach generates an output signal, VH which in the
linear devices represents the strength of the magnetic field, the magnetic flux density, as a function of distance away from
the sensor. The nearer and therefore the stronger the magnetic field, the greater the output voltage and vice versa.
Linear devices can also differentiate between positive and negative magnetic fields. Non-linear devices
can be made to trigger the output "ON" at a preset air gap distance away from the magnet for indicating positional detection.
The second sensing configuration is sideways detection. This requires moving the magnet across the
face of the Hall element in a sideways motion.
Sideways or slide-by detection is useful for detecting the presence of a magnetic field as it moves across
the face of the Hall element within a fixed air gap distance for example, counting rotational magnets or the speed of rotation.
Depending upon the position of the magnetic field as it passes by the zero field centre line of the
sensor, a linear output voltage representing both a positive and a negative output can be produced. This allows for
directional movement detection which can be vertical as well as horizontal.
There are many different applications for Hall Effect Sensors especially as proximity
sensors. They can be used instead of optical and light sensors were the environmental conditions consist of water, vibration,
dirt or oil such as in automotive applications. Hall effect devices can also be used for current sensing.
We know from the previous tutorials that when a current passes through a conductor, a circular
electromagnetic field is produced around it. By placing the Hall sensor next to the conductor, electrical currents from
a few milliamps into thousands of amperes can be measured from the generated magnetic field without the need of large or
expensive transformers and coils.
As well as detecting the presence or absence of magnets and magnetic fields, Hall effect sensors can also
be used to detect ferromagnetic materials such as iron and steel by placing a small permanent "biasing" magnet behind the
active area of the device. The sensor now sits in a permanent and static magnetic field, and any change or disturbance to
this magnetic field by the introduction of a ferrous material will be detected with sensitivities as low as mV/G possible.
There are many different ways to interface Hall effect sensors to electrical and electronic circuits
depending upon the type of device, whether digital or linear. One very simple and easy to construct example is using a
Light Emitting Diode as shown below.
This head-on positional detector will be "OFF" when there is no magnetic field present, (0 gauss).
When the permanent magnets south pole (positive gauss) is moved perpendicular towards the active area of the Hall effect
sensor the device turns "ON" and lights the LED. Once switched "ON" the Hall effect sensor stays "ON".
To turn the device and therefore the LED "OFF" the magnetic field must be reduced to below the release
point for unipolar sensors or exposed to a magnetic north pole (negative gauss) for bipolar sensors. The LED can be replaced
with a larger power transistor if the output of the Hall effect sensor is required to switch larger current loads.