Hall Effect Sensors are extensively used in various fields. This post will discuss How they works, their types, applications, advantages and disadvantages.
Introduction to Hall Effect Sensor
Magnetic sensors are solid state devices which generate electrical signals proportional to the magnetic field applied on it. These electrical signals are then further processed by a user specific electronic circuit to give the desired output.
Now days, these Magnetic sensors are capable in responding for a wide range of magnetic fields. One of such magnetic sensor is Hall Effect Sensor whose output (Voltage) is a function of magnetic field density.
An external magnetic field is used to activate these Hall Effect Sensors. When the magnetic flux density in the vicinity of Sensor goes beyond a specific defined threshold, it is detected by the Sensor. On detection, the Sensor generates an output voltage which is also known as Hall Voltage.
Fig. 1 – Hall Effect Sensors
These Hall Effect Sensors are in high demand and have very wide spread applications such as Proximity sensors, Switches, Wheel speed sensors, Positional sensors, etc.
Working Principle of Hall Effect Sensor
Hall Effect Sensor is based on Hall Effect Principle. This principle says that when a conductor or semiconductor with current flowing in one direction is introduced perpendicular to a magnetic field a voltage could be measured at right angles to the current path.
Fig. 2 – Hall Effect Principle – Current Flowing Through a Plate
How Hall Effect Sensor Works
The working of Hall Effect Sensor is described below:
- When an electric current flows through the Sensor, the electrons move through it in a straight line.
- When an external magnetic field acts on the Sensor, the Lorentz force deviates the charge carriers to follow a curved path.
- Due to this, the Negative Charge Electrons will deflect towards one side of the Sensor and the Positive Charge Holes to another.
Fig. 3 – Hall Effect Principle – Deflection of Electrons and Holes
- Due to this accumulation of electrons and holes at different side of the plate, a voltage (potential difference) can be observed between the sides of the plate. The voltage obtained is directly proportional to the electric current and magnetic field strength.
Types of Hall Effect Sensors
The Hall Effect Sensors can be classified into its types on two basis:
- On the basis of Output
- On the basis of Operation
On the Basis of Output
On the basis of Output, the Hall Effect Sensors can be classified into two types: –
- Analog Output Hall Effect Sensors
- Digital Output Hall Effect Sensors
Analog Output Hall Effect Sensors
Fig. 4 – Analog Output Hall Effect Sensor Circuit Diagram
The Analog Output Hall Effect Sensors contains a voltage regulator, a Hall Element and an amplifier. As the name suggests, the output of such type of sensor is analog in nature and proportional to magnetic field strength and the output of Hall Element.
These Sensors have a continuous linear output. Due to this property, they are appropriate to be used as proximity sensors.
Fig. 5 – Output of Analog Output Hall Effect Sensor
Digital Output Hall Effect Sensors
The Digital Output Hall Effect Sensors have only two outputs i.e “ON” and “OFF”. These Sensors have an extra element ‘Schmitt Trigger’ added as compared to Analog Output Hall Effect Sensors.
Fig. 6 – Digital Output Hall Effect Sensor Circuit Diagram
It is the ‘Schmitt Trigger’ that inducts a Hysteresis effect and so two different threshold levels are achieved. Accordingly, the output of the entire circuit will be either Low or High.
Hall Effect Switch is one of such type of sensors. These Digital Output Sensors are widely used as limit switches in CNC Machines, Three Dimensional (3D) printers and positional interlocks in Automated Systems.
Fig. 7 – Output of Digital Output Hall Effect Sensor
On the Basis of Operation
On the basis of Operation, the Hall Effect Sensors can be classified into two types: –
- Bipolar Hall Effect Sensor
- Unipolar Hall Effect Sensor
Bipolar Hall Effect Sensor
As the name suggests, these Sensors require both positive and negative magnetic fields for their operation. Positive magnetic field of magnet’s South Pole is used to activate the sensor and Negative magnetic field of magnet’s North Pole is used to release the sensor.
Fig. 8 – Bipolar Hall Effect Sensor
Unipolar Hall Effect Sensor
As the name suggests, these Sensors require only positive magnetic field of magnet’ south pole to activate as well as release the sensor.
Fig. 9 – Unipolar Hall Effect Sensor
Applications of Hall Effect Sensor
The applications of Hall Effect Sensors have been represented in two categories for ease of understanding.
- Applications of Analog Hall Effect Sensors
- Applications of Digital Hall Effect Sensors
Applications of Analog Hall Effect Sensors
Analog Hall Effect Sensors are utilized for :
- Direct Current sensing in Clamp meters (also known as Tong Testers).
- Wheel speed detection for the anti-lock braking system, (ABS).
- Motor control devices for protection and indications.
- Sensing the availability of Power supply.
- Motion Sensing.
- Sensing the rate of flow.
- Sensing Diaphragm pressure in Diaphragm pressure gauge.
- Sensing Vibration.
- Sensing Ferrous Metal in Ferrous Metal Detectors.
- Voltage Regulation.
Applications of Digital Hall Effect Sensors
Digital Hall Effect Sensors are utilized for :
- Sensing the angular position of the crank shaft for the firing angle of the spark plugs.
- Sensing the position of the car seats and seat belts for air-bag control .
- Wireless Communications.
- Sensing Pressure.
- Sensing Proximity.
- Sensing rate of flow.
- Sensing position of Valves.
- Sensing position of Lens.
Advantages of Hall Effect Sensors
Hall Effect Sensors have the following advantages:
- They can be used for multiple sensor functions like position sensing, speed sensing as well as for sensing the direction of movement too.
- As they are solid state devices, there is absolutely no wear and tear due to absence of moving parts.
- They are almost maintenance free.
- They are robust.
- They are immune to vibration, dust and water.
Disadvantages of Hall Effect Sensors
The Hall Effect Sensors have the following disadvantages: –
- They are not capable to measure current flow at a distance more than 10 cm. The only solution to overcome this issue is to use a very strong magnet that can generate a wide magnetic field.
- Accuracy of the measured value is always a concern as external magnetic fields may affect the values.
- High Temperature affects the conductor resistance. This will in turn affect the charge carrier’s mobility and sensitivity of Hall Effect Sensors.
How large electrical loads can be controlled by Hall Effect Sensors
We already know that a the output drive capability of a Hall Effect Sensor is very low (10 to 20 mA). It is therefore, it can not directly control large electrical loads. However, We can control large electrical loads with Hall Effect Sensors by adding an Open Collector NPN Transistor (current sink) to the output.
The NPN Transistor (current sink) functions in the saturated state as a sink switch. It shorts the output terminal with the ground whenever the flux density is more than the preset ‘ON’ Value.
The output switching transistor may be in different configurations as open emitter transistor, open collector transistor or both. That’s how it provide a pull/push output which enable it to sink enough current to drive large loads directly.