Sheffer LA SERIES - Manual (Page 8)

Principles of Operation for
Magnetic Proximity Switches
Reed Switch Working Principle -
Our reed switch sensors contain
hermetically sealed reed elements
(mechanical contacts) which can be
open or closed in their normal state
depending on the version selected.
When a magnetic field moves within
proximity of the switch, magnetism is
induced into the leads and forces the
contact’s state (open if normally closed
or closed if normally open). Normally
used with pneumatic cylinders that are
built with internal magnetic pistons.
Hall Effect / Magnetoresistive
Working Principle –
Our “Hall effect”
sensors actually use a solid state (no
moving parts) magnetoresistive sensor
instead of a typical Hall effect sensor.
The magnetoresistive sensor responds
to a North or South magnetic pole as
shown to the right by providing a
digital signal to the output control
circuit. The magnetoresistive sensing
technique enables our sensors to out
perform typical Hall effect sensors by
sensing much weaker magnetic fields.
Also, there is no limit to the maximum
strength of the magnetic field a
magnetoresistive sensor will work with
as opposed to a Hall effect sensor.
Sinking (NPN) vs. Sourcing (PNP) -
Our DC Hall effect switches are
available in sinking or sourcing
versions. The basic difference between
these two ways of solid state switching
is as follows:
The SOURCING method connects or
switches one side of the load to the
positive side of the supply. The
negative side is common or connected
directly to the other side of the load as
shown in
Figure 1 . PNP is the acronym
used to describe the transistor that
performs this type of switching in a
solid state sensor.
The SINKING method connects or
switches one side of the load to the
negative side of the supply. The
positive side is common or connected
directly to the other side of the load as
shown in
Figure 2 . NPN is the
acronym used to describe the transistor
that performs this type of switching in
a solid state sensor.
Which Type of Sensor Should I
Use? Reed or Hall Effect?
The type of sensor that is best for a
particular application depends on the
operating parameters and cost related
issues.
Reed Switch Sensors - If initial cost
and versatility are most important,
then reed switch sensors should be
considered. For example, the type 04
reed switch will operate from 5 to 240
volts AC or DC. Generally, one switch
can be stocked to cover a large
majority of common applications.
Please note, reed sensors do not work
well with inrush surge currents and
transients (common to inductive &
capacitive loads; i.e. relays, coils &
long wire runs). If inrush surge
currents and transients must be
accommodated, switch type 24 may
be specified. These parameters should
be given careful consideration when
selecting a proximity device that will
be best suited for an application.
Hall Effect Sensors – In general, if
longevity is a major concern, Hall
effect sensors should be used whenever
they fit within the operating
parameters specified for a given
application. They should receive
special attention when high cycle rates
are required. If Hall effect sensors are
used within their operating range, they
will always out perform and out last
mechanical reed sensors. The initial
added cost associated with a Hall effect
sensor will be outweighed should the
application require high cycle rates.
Special Piston - A special piston must
be utilized in Magnetic Limit Switch
cylinders. In addition to the magnetic
strip and the bronze impregnated
Teflon bearing strip, the piston is
designed with a special endless ring
seal of polyurethane filled with
molybdenum disulphide. As shown in
the illustration, the seal is expanded by
an O-Ring. It seals in both directions.
Low friction and no metal-to-metal
contact afford smooth operation and
extremely long life.
REED SWITCH
HALL EFFECT/ MAGNETORESISTIVE
PISTON CONSTRUCTION
NO METAL-TO-METAL CONTACT
SINKING (NPN) VS. SOURCING (PNP)
NORMALLY OPEN REED
SWITCH OPEN
NORMALLY OPEN REED
SWITCH CLOSED
MAGNET
SENSOR
SUPPLY
MAGNET
OUTPUT
V=0
SENSOR
SUPPLY
OUTPUT
V=0
SINKING SWITCH
SOURCING SWITCH
FIGURE 2
FIGURE 1
SUPPLY
SUPPLY
LOAD
LOAD
SENSORS
Magnetic
1 AC-7