What is a Hall Effect Sensor.

[Donald Whisnant]

>
>From: Dennis <dke at gramercy.ios.com>
>Date: Tue, 23 Apr 1996 02:10:35 -0400
>Subject: What is a Hall Effect Sensor.
>
>Hi Everyone,
>I'm kind of new here but I have been reading a lot of the posts and I would
>like to ask a question or two. :) A long time ago I read someone say that
>you can pick up the magnet in a speedometer if you have a hall effect
>sensor. I am trying to make something of a wheel possition sensor for class
>to measure the rotating speed of the wheel. I wanted to make it easy to
>install on different vehicles. I would like to know if you can buy a hall
>effect sensor or what is the particular design so that I might build one. 
>
>As I understand it, they are just magnetic coils set up in a daisy fassion
>around a magnet rotating on the end of a circular object. The coils generate
>a positive or negative voltage due to the magnetic field generated, and it
>is the voltage of zero that is measured that tells just when the magnet
>passes the coil. Is this right? 
>
>And does the magnet in a speedometer turn at 1:1 ratio with the drive wheel? 
>
>Thanks in advance,
>
>Dennis Espinoza(Grad Student at California State University Northridge)
>


Dennis...

  Not quite...  A Hall Effect sensor is a semiconductor device that looks
alot like a transistor.  There are two types, current and voltage.  The
current type typically has 4 leads.  Two leads get a constant CURRENT source
input and the other 2 are the magnetic proportional output.  As a magnet is
brought near the current Hall Effect, the output VOLTAGE changes linearly
with the applied magnetic field and the polarity of output voltage indicates
the magnet polarization (north/south).  The voltage type typically has 3
leads, a supply VOLTAGE, a ground reference, and a VOLTAGE output.  The
output voltage is proportional to the applied magnetic field. ...  There
are also two subtypes of these Hall Effects.  One is a "Linear" output, the
other is a "Switch" output.  The "linear" style works as described above and
the "switch" style is the same but with a comparator essentially built in
to turn on or off a digital output when the magnetic field reaches a certain
level (though design varies). ...  The switch types are great for limit
switches and also doing wheel taching and position calculations...

The one thing you must keep in mind with Hall Effects is the characteristics
of a magnetic field...  i.e. the field is an exponentially decaying field which
means that distance does matter -- the farther away, the weaker the field.
Also, angle of incidence matters.  At 90 degrees, no magnetic field will
enter the "sensing surface" of the hall-effect, but as the magnet rotates,
the intesity changes by the cosine of the angle (and consequently the output
varies respectively) ...  This adds some interesting properties to hall-effects,
namely that they can be used to measure angle of inclination or rotation
(though via a cosine rather than linearly) ...  This, however, can have
problems on positioning circuits -- if the magnet or hall-effect moves, then
the "trigger position" changes, though depending on the application, this
usually isn't critical.  Another thing to keep in mind is the hysterises
that is incorporated into the switch version -- namely, they trigger at one
Gauss point (Gauss is the unit of measure for magnetic field) and release at
a different Gauss point.  And these trigger points are typically affected by
temperature changes.  To maintain exact constant trigger points, there must
be some sort of compensation... And, not only are the hall-effects affected
by temperature, but even more so are the magnets (they loose magnetic field
with more heat).  Choosing the proper magnetic material is essential for a 
good design.

Some of the best hall-effects are the current types and one of the leading
makers is F.W. Bell  (you can find them listed in the Thomas Register -- online
on the internet)...  F.W. Bell though ain't cheap -- cheaper units for less
critical projects are available from companies like Allegro (places like
Newark Electronics carry them) ... To give a comparison in price, Allegro
voltage driven switching type are about .50 a piece, and a good F.W. Bell
linear current driven type (the other extreme) is about $28 (or so they
were a couple of years ago)...

The reason for current sourcing verses voltage sourcing is that it is
typically easier to make a constant current source than a constant voltage
source that is temperature stable.  So yes, supply voltage/currents will
affect the output...

Hope this helps in the understanding of Hall Effects... Good luck on the
project...

Donald Whisnant
dewhisna at ix.netcom.com