neil.cairns@virgin.net wrote:
> The 'Hall Effect' is the name given to the principle discovered by a
> chap called, oddly enough, Dr. Hall. It is used in distributors that
> have a chopper-plate inside instead of points, switching on and off the
> magnetic field, by alternately covering and uncovering the Hall chip, (
> a crystal.)
Actually, this is somewhat wrong. I think you are confusing optical
type "chopper" systems (like Crane/Allison) with Hall effect types,
like Lucas/Pertronix.
Optical sensors use a chopper disc, which is spun between an IR
emitter and an IR detector, really just a light beam. The chopper
blades break the light beams at just the right times, and these
breaks are detected by a module which switches the coil
current and fires the spark.
Hall effect devices put spinning magnets in the rotor assembly
in the distributor, and a coil-type detector nearby. (both
usually encased in epoxy so nothing exciting is visible)
The current induced into the coil abruptly changes
polarity at the instant the magnet passes the coil. This
change is detected by a module which fires the spark.
Hall effect units are sturdier than optical types, since
there is no plastic chopper wheel or exposed light units
to become scratched or dirty.
Then main disadvantage I can think of for Hall effect
units is that the effect isn't noticeable except when
the magnets are moving. So you cannot static time a
car with a hall effect sensor. No big deal though, you
can usually eyeball it close enough to get the car
running and set it with a light.
--
Trevor Boicey, Ottawa, Canada.
tboicey@brit.ca, http://www.brit.ca/~tboicey/
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