Trevor Boicey wrote:
>
> 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/
I bow to your greater knowledge, but thats how I understood them!!
Neil
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