Electronics Tutorial 4 - Hall Sensors
The Hall sensor uses the Hall Effect. It is often used to detect the rotary motion of a magnet set into a wheel or axle. Examples of uses are:
Tachometers for diesel engines;
Speed sensors on exercise bikes or rowing machines;
Contactless measurement of direct currents (i.e. without inserting an ammeter);
Feedback for motor control.
The symbol for a Hall sensor is this:
The Hall Effect
It was first discovered in 1879 by an American Physicist, Edwin Herbert Hall (1855 - 1938). He found that when a current-carrying conductor or a semiconductor is placed in a magnetic field, a voltage occurs that is perpendicular to the flow of the current. This is called the Hall Voltage.
The Hall Effect can be observed in conductors, semi-conductors, ionised gases, and plasmas. We will be looking at the effect of magnetic fields in electrons in semi-conductors, because the nature of semi-conductors allow for a relatively high voltage which is easy to measure. It is also easily reproduced in a school or college Physics laboratory. So let's have a look at what happens.
The separation of charges leads to a potential difference, or voltage that is called the Hall voltage (VH). This voltage also causes an electric field, E which is uniform. The idea is shown in the picture below:
You are not expected to use the equation or derive it. However, if you wish to see how the equation is derived and used, please click HERE. The output of a Hall sensor is about 0.1 V. Connected to a circuit, it would give a current of about 1 mA. This can be amplified to give a bigger voltage and current.
Some uses of the Hall Sensor
The picture below shows the Hall Probe used with a data-logger.
The next picture is an animated file of a Hall sensor detecting rotary motion:
Animation by IMeowbot, Wikimedia Commons
You can see the two magnets turning on the shaft. Each time the Hall Sensor detects a magnetic field, the yellow LED lights up.
Anti-lock Braking System (ABS)
These devices are fitted to most cars nowadays. While tyres have a high coefficient of friction on tarmac, it is still possible for the wheels to lock. The car goes into a skid. At best this results in marks on the road and a patch of rubber scraped off the tyres. At worst, the results can be tragic.
A Hall sensor is at the heart of the ABS system. There is a rotating ring with magnets in on each wheel. The Hall sensor gives out a voltage each time the magnets pass. The voltage is detected by a microcontroller or computer that compares the voltage pulses coming from each wheel while braking is happening. The computer has an output that is connected to a valve housing that looks like this:
When one of the wheels turns slower than the others, the situation is detected by the microcontroller and a valve isolates the brake-line from the master cylinder (which is connected to the brake pedal). If the wheel is still turning slower, the brake line is released. Then a pump is automatically turned on to restore the pressure, so that the wheel resumes its braking action. The ABS can be felt operating by an unpleasant juddering. (If you have experienced this while driving, you will know what I mean.) The thing NOT to do is to pump the brake as you would if the car didn't have ABS. Let the system do the controlling for you.
A good driver should rarely have to brake hard, let alone make a crash stop. But sometimes the unexpected happens to the most experienced driver. (It happened to me while I was driving down a hill in snowy conditions and the ABS brought the car under control, before it ended up as an 1200 kg toboggan.)
Finding the Earth's Magnetic Field Using a Hall Sensor
The small Hall sensor in a data-logger probe is not sensitive enough to detect the Earth's magnetic field. However it is possible to have a Hall sensor that is large enough to be sufficiently sensitive. The Earth's magnetic field is in three dimensions. The Hall sensor can pick up the value of a perpendicular magnetic field in one dimension only, so you need to take measurements for the x, y, and z components.
The resultant magnetic field is worked out using:
The directions and angles can also be worked out.
A Hall probe for a data-logger can be used to investigate a more powerful magnetic field in a similar way.
In some mobile telephones, there are Hall sensors that can give you the value of the Earth's magnetic field. The value of the Earth's magnetic field is about 6.5 × 10-5 T, but can be half that value in some places.
Hall Sensors in Electronic Circuits
The Hall sensor in an electronic circuit can be connected to a variety of processing devices. In this case, it's connected to an operational amplifier.
We will consider the behaviour of an operational amplifier in a later tutorial, but it will detect the output from the Hall sensor to give an output voltage.
A student set up a Hall sensor so that the output was connected to the gate of a MOSFET as shown, instead of an operational amplifier.
Explain whether or not this circuit will work.
Hall sensors are often packaged with an amplifier in a chip to give a higher voltage and current, so that they can be used with a MOSFET or a bipolar transistor. Click HERE to see the datasheet of a typical Hall sensor chip.