hall effect circuits

8/7/2019 hall effect circuits

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` Hall Effect

` If an electric current flows through a conductor in a magnetic field, the magnetic field

exerts a transverse force on the moving charge carriers which tends to push them to

one side of the conductor. This is most evident in a thin flat conductor as illustrated. A

buildup of charge at the sides of the conductors will balance this magnetic influence,

producing a measurable voltage between the two sides of the conductor. The

presence of this measurable transverse voltage is called the Hall effect after E. H.

Hall who discovered it in 1879.

` Note that the direction of the current I in the diagram is that of conventional current,

so that the motion of electrons is in the opposite direction. That further confuses all

the "right-hand rule" manipulations





2.HALL VOLTAGE FOR POSITIVE CHARGE CARRIERS

` The transverse voltage (Hall effect) measured in a Hall probe has its origin

in the magnetic force on a moving charge carrier.

` The magnetic force is where is the drift velocity of the charge.

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` The current expressed in terms of the drift velocity

is

` where n is the density of charge carriers.

` And substituting gives

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` CHARGE CARRIERS IN THE HALL EFFECT

` The Hall effect is a conduction phenomenon which is different for different

charge carriers. In most common electrical applications, the conventional

current is used partly because it makes no difference whether you consider

positive or negative charge to be moving. But the Hall voltage has a

different polarity for positive and negative charge carriers, and it has been

used to study the details of conduction in semiconductors and other

materials which show a combination of negative and positive charge

carriers.

` The Hall effect can be used to measure the average drift velocity of the

charge carriers by mechanically moving the Hall probe at different speeds

until the Hall voltage disappears, showing that the charge carriers are now

not moving with respect to the magnetic field. Other types of investigations

of carrier behavior are studied in the quantum Hall effect.



` This is a simple and probably the most reliable motor.` In 1879 Edward Hall placed a thin layer of gold in a strong magnetic field. He

connected a battery to the opposite sides of this film and measured the current

flowing through it. He discovered that a small voltage appeared across this film. This

voltage was proportional to the strength of magnetic field

` multiplied by the current. This effect bears his name.

` For many years the Hall effect was not used in practical applications because thegenerated voltage in the gold film was extremely low. However, in the second half of

the 20th century the mass production of semiconductor chips started. Chips based

on the Hall effect became inexpensive and widely available.




` The Hall effect IC (integrated circuit) is a very small chip which includes

many transistors. It consists of a thin layer of silicon as a Hall generator

(which works better than gold) and several transistor circuits: to amplify the

Hall voltage to a necessary level; to trigger output voltage with its growth;

and to provide stable work regardless of the power supply voltage changes.

The picture below demonstrates the Hall effect IC (between the coin andtwo power transistors):




` The Hall effect IC is a solid state electronic device with no mechanical parts

and therefore it is more reliable than a reed switch. To no surprise it is now

the most widely used sensor in industrial brushless motors. Normally,

however, they include a lot of other components. I tried to design a motor on a Hall effect switch with minimum parts based on the same unified

mechanical design and it worked very well.

` The Hall effect IC is a unipolar switch. It turns on and off when the South

pole of the magnet passes by its branded side. The North pole has no effect

on it, unless it approaches from the back side of the Hall IC. This Hall effect

IC has a built in voltage regulator and may work in the range from 4.5 to24V. The Hall effect IC's included in the kit, however, were tested

extensively; and I found that most of them start working at 3V. This is a

typical Hall effect IC shown from the branded side:

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` The Hall effect switch output current is not sufficient to power this motor, therefore it

also requires a power transistor. You may find information on this component at How

It Works: Reed Switch Motor With A Transistor .

` This is how this motor works:

`

1. When magnet #1 gets close to the Hall IC, the sensor sends a signal to the baseof the power transistor. The transistor opens, and allows a bigger collector current to

flow through the electromagnet. The electromagnet pushes magnet #3 away.

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` When the rotor spins away, magnet #1 stops affecting the Hall IC. Since the signal to

the base of the power transistor has been removed, it is turned off. This disables the

electromagnet.

`

` The rotor continues to spin due to inertia until magnet #2 moves into the working

range of the Hall IC. The Hall IC sends a signal to the base of the transistor. The

transistor opens, and allows a bigger collector current to flow through the

electromagnet. The electromagnet pushes magnet #4 away. This process continues

until the power is disconnected.

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