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Hall Effect Sensor

The Hall Effect SensorWe could not end this section on Magnetism (http://www.electronics-tutorials.ws/electromagnetism/magnetism.html) without a mention about magnetic sensors and especially theHall Effect Sensor. Magnetic sensors convert magnetic or magnetically encoded information into electrical signalsfor processing by electronic circuits, and in the Sensors and Transducers (http://www.electronics-tutorials.ws/io/io_1.html) tutorials we looked at inductive proximity sensors and the LDVT as well as solenoid andrelay output actuators.

Magnetic sensors are solid state devices that are becoming more and more popular because they can be used in manydifferent types of application such as sensing position, velocity or directional movement. They are also a popular choice ofsensor for the electronics designer due to their non-contact wear free operation, their low maintenance, robust designand as sealed hall effect devices are immune to vibration, dust and water.

One of the main uses of Magnetic Sensors (http://amazon.com/dp/1580530575/?tag=basicelecttut-20) is in automotivesystems for the sensing of position, distance and speed. For example, the angular position of the crank shaft for the firingangle of the spark plugs, the position of the car seats and seat belts for air-bag control or wheel speed detection for theanti-lock braking system, (ABS).

Magnetic sensors are designed to respond to a wide range of positive and negative magnetic fields in a variety ofdifferent applications and one type of magnet sensor whose output signal is a function of magnetic field density around itis 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 twoimportant characteristics flux density, (B) and polarity (North and South Poles). The output signal from a Hall effect sensoris the function of magnetic field density around the device. When the magnetic flux density around the sensor exceeds a

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certain pre-set threshold, the sensor detects it and generates an output voltage called the Hall Voltage, V . Consider thediagram below.

Hall Effect Sensor Principals

 Hall Effect Sensors consist basically of a thin piece of rectangular p-type semiconductor material such as galliumarsenide (GaAs), indium antimonide (InSb) or indium arsenide (InAs) passing a continuous current through itself. Whenthe device is placed within a magnetic field, the magnetic flux lines exert a force on the semiconductor material whichdeflects the charge carriers, electrons and holes, to either side of the semiconductor slab. This movement of chargecarriers 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 thesemiconductor material by the build-up of these charge carriers. Then the movement of electrons through thesemiconductor material is affected by the presence of an external magnetic field which is at right angles to it and thiseffect 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 whodiscovered it back in the 1870’s with the basic physical principle underlying the Hall effect being Lorentz force. Togenerate a potential difference across the device the magnetic flux lines must be perpendicular, (90 ) to the flow ofcurrent 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. Forexample, 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 nomagnetic field present. They only turn “ON”, (closed circuit condition) when subjected to a magnetic field of sufficientstrength and polarity.

H

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Hall Effect Magnetic SensorThe output voltage, called the Hall voltage, (V ) of the basic Hall Element is directly proportional to the strength of themagnetic field passing through the semiconductor material (output ∝ H). This output voltage can be quite small, only afew microvolts even when subjected to strong magnetic fields so most commercially available Hall effect devices aremanufactured 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 andmagnetic field conditions.

The Hall Effect Sensor

 Hall Effect Sensors are available with either linear or digital outputs. The output signal for linear (analogue) sensors istaken directly from the output of the operational amplifier with the output voltage being directly proportional to themagnetic field passing through the Hall sensor. This output Hall voltage is given as:

Where:

V is the Hall Voltage in volts

R 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

H

H

H

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Linear or analogue sensors give a continuous voltage output that increases with a strong magnetic field and decreaseswith a weak magnetic field. In linear output Hall effect sensors, as the strength of the magnetic field increases the outputsignal from the amplifier will also increase until it begins to saturate by the limits imposed on it by the power supply. Anyadditional increase in the magnetic field will have no effect on the output but drive it more into saturation.

Digital output sensors on the other hand have a Schmitt-trigger with built in hysteresis connected to the op-amp. Whenthe magnetic flux passing through the Hall sensor exceeds a pre-set value the output from the device switches quicklybetween its “OFF” condition to an “ON” condition without any type of contact bounce. This built-in hysteresis eliminatesany oscillation of the output signal as the sensor moves in and out of the magnetic field. Then digital output sensors havejust two states, “ON” and “OFF”.

There are two basic types of digital Hall effect sensor, Bipolar and Unipolar. Bipolarsensors 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 southpole 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 electrical loads as their output drivecapabilities are very small around 10 to 20mA. For large current loads an open-collector(current sinking) NPN Transistor (http://www.electronics-tutorials.ws/transistor/tran_2.html) is added to the output.

This transistor operates in its saturated region as a NPN sink switch which shorts the outputterminal to ground whenever the applied flux density is higher than that of the “ON” pre-setpoint.

The output switching transistor can be either an open emitter transistor, open collectortransistor configuration or both providing a push-pull output type configuration that cansink enough current to directly drive many loads, including relays, motors, LEDs, and lamps.

Hall Effect ApplicationsHall effect sensors are activated by a magnetic field and in many applications the device canbe operated by a single permanent magnet attached to a moving shaft or device. There aremany 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 ensuremaximum sensitivity the magnetic lines of flux must always be perpendicular to the sensingarea of the device and must be of the correct polarity.

Also to ensure linearity, high field strength magnets are required that produce a largechange in field strength for the required movement. There are several possible paths of motion for detecting a magneticfield, and below are two of the more common sensing configurations using a single magnet: Head-on Detection andSideways Detection.

Head-on DetectionAs its name implies, “head-on detection” requires that the magnetic field is perpendicular to the hall effect sensing deviceand 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, V which in the linear devices represents the strength of the magneticfield, the magnetic flux density, as a function of distance away from the hall effect sensor. The nearer and therefore thestronger the magnetic field, the greater the output voltage and vice versa.

H

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Linear devices can also differentiate between positive and negativemagnetic fields. Non-linear devices can be made to trigger the output “ON”at a pre-set air gap distance away from the magnet for indicating positionaldetection.

Sideways DetectionThe second sensingconfiguration is“sideways detection”.This requires moving

the magnet across the face of the Hall effect element in a sidewaysmotion.

Sideways or slide-by detection is useful for detecting the presence of amagnetic field as it moves across the face of the Hall element within afixed air gap distance for example, counting rotational magnets or thespeed of rotation of motors.

Depending upon the position of the magnetic field as it passes by thezero field centre line of the sensor, a linear output voltage representing both a positive and a negative output can beproduced. 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 insteadof optical and light sensors were the environmental conditions consist of water, vibration, dirt or oil such as in automotiveapplications. 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 isproduced around it. By placing the Hall sensor next to the conductor, electrical currents from a few milliamps intothousands of amperes can be measured from the generated magnetic field without the need of large or expensivetransformers and coils.

As well as detecting the presence or absence of magnets and magnetic fields, Hall effect sensors can also be used todetect ferromagnetic materials such as iron and steel by placing a small permanent “biasing” magnet behind the activearea of the device. The sensor now sits in a permanent and static magnetic field, and any change or disturbance to thismagnetic 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 thetype of device, whether digital or linear. One very simple and easy to construct example is using a Light Emitting Diode(http://www.electronics-tutorials.ws/diode/diode_8.html) as shown below.

Positional Detector

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 This head-on positional detector will be “OFF” when there is no magnetic field present, (0 gauss). When the permanentmagnets south pole (positive gauss) is moved perpendicular towards the active area of the Hall effect sensor the deviceturns “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 forunipolar sensors or exposed to a magnetic north pole (negative gauss) for bipolar sensors. The LED can be replaced witha larger power transistor if the output of the Hall Effect Sensor is required to switch larger current loads.

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27 Responses to “Hall Effect Sensor”Older comments (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-1#comments)

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BABUits realy very usefull

August 21st, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-7869)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=7869#respond)

JC GaydosHow are the hall effect devices utilized in a brushless pwm motor speed controller?

July 19th, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-7427)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=7427#respond)

Wayne Storr (http://www.electronics-tutorials.ws)To detect rotational speed and generate an output proportional to velocity, displacement, or position of arotating shaft.

July 19th, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-7428)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=7428#respond)

PaulThis is exactly what I wish to do using a trigger wheel, a hall effect detector, to then convert to a voltage whichwould have to be variable according to speed to drive a speedometer via an electric motor as I am doing agearbox conversion on an old vehicle could you advise me as to what circuit to use Please

July 20th, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-7439)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=7439#respond)

avadheshCan be use it as a temperature sensor.

Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=7201#respond)

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June 26th, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-7201)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=7201#respond)

Wayne Storr (http://www.electronics-tutorials.ws)Not unless the magnetic field is hot.

June 26th, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-7204)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=7204#respond)

MosesGood information on Hall effect sensing!!

June 5th, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-7028)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=7028#respond)

P D YerunkarGood information on Hall Effect sensor

May 23rd, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-6905)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=6905#respond)

Artur DietrichIs it possible an “two wired” Wheel Speed (ABS) sensor be Hall type?The conector has 2 pins.Best Regards

May 17th, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-6852)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=6852#respond)

Wayne Storr (http://www.electronics-tutorials.ws)Hello Artur, It is possible that your sensor is a Hall Effect Switch, but that means the sensing part (the rotatingwheel or disc) has to be magnetic. I would assume that its some sort of magnetic pulse generator to determine

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the vehicle and individual wheel speed. Basically a magnetic pulse generator consists of a small permanentmagnet with a pick-up coil wrapped around it.

Some form of steel multi-toothed timing disc rotates in front of the sensor affecting its magnetic field as eachtooth passes in front of the sensor producing an AC signal output for the cars electronics. The faster the rotationof the wheel the higher the frequency output. Both hall effect switches and magnetic sensors can be straight 2-wire or a 3-wire with a negative connection through the sensors body to the earth of the cars battery.

May 17th, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-6853)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=6853#respond)

manohar s nadkarniHi there,The existing sensor could be a magneto resistor (which has only two terminalsjust like ordinary resistor), it is a non-linear device.

June 18th, 2015 (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html/comment-page-2#comment-7130)Reply (http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html?replytocom=7130#respond)

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