Re: The Hall Effect

[Luis Marques <marques@rtis.ray.com>]

quk@isham-research.demon.co.uk (Phil Payne) wrote:
> But this is _not_ the right answer - there's no coil in a Hall sensor.
> 
> A Hall sensor is a piece of semiconductor through which a current is
> passed.  Think of something the shape of a football field, with
> electrons flowing between the goals.  If a magnetic field appears,
> perpendicular to the surface, those electrons are going to miss the
> goal and perhaps even leave the field by the side lines.  Which is
> where a Hall sensor has 'sensing' electrodes.

Phil is absolutely right on this one.  A Hall sensor is in its pure form a four
terminal semiconductor device in the presence of a magnetic field (magnet). 
Current flows thru the semiconductor material and a perpendicular magnetic field
tends to deflect the current to one side.  Since current can not flow out of the
sides of the semiconductor (much like water can not go out the sides of a pipe),
an electric potential (voltage) is established perpendicular to both the current
and the magnetic field.  This Hall voltage is just strong enough to overcome the
influence of the magnetic field on the current so that it can flow in a straight
line thru the semiconductor.

If the magnetic field is varied, the Hall voltage will vary also.  And this is
just what happens inside your Bosch distributor or ABS sensors.  Rotating metal
blades pass by the Hall sensor and disturb the magnetic field, resulting in a
pulsating signal whose frequency is directly proportional to the shaft
rotational speed.  BTW, these blades must be made out of some magnetic metal
(iron) or else they will have zero impact on the Hall sensor.

Luis Marques
'87 4kcsq