Information

Quick-Info Reed-Technology
1. How does a Reed Switch work?
A Reed Switch consists of a pair of ferromagnetic reeds, which overlap
at their free ends (contact area) at a very small distance and are
hermetically sealed in a glass tube. When in the presence of a
magnetic field, the reeds become magnetized to opposite polarity, thus
attracting each other and closing contact.
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2. Different Reed Switch types
Generally speaking the different types are:
› Form A, which is also known as SPST-NO
(
Single Pole Single Throw - Normally Open)
› Form B, which is also known as SPST-NC
(
Single Pole Single Throw - Normally Closed)
› Form C, or change over contact, which is also known as SPDT
(
Single Pole Double Throw)
› Form E is a bi-stable contact. The switching state of this is also called
Latching-Type and remains unchanged (even in the absence of a
magnetic field) until a magnetic field of the opposite polarity is
present
Furthermore, there are a number of special Reed Switches, i.e. for
High Voltage applications or ultra miniature types for implants etc.
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3. Technical Terms
AT stands for Ampere-Turns. It is a standard unit categorizing the
magnetic sensitivity of Reed Switches. The AT value is measured by
centering the Reed Switch in a standard coil, to which an increasing
current is applied. At a certain current (Amps), the Reed Switch closes
by means of the magnetic field generated by the coil. This is the
Operate (or Pull In) value, which is calculated as Amps x number of
coil turns = AT. The
Release (or Drop Out) value can be obtained by
reducing the current passing through the coil until the switch re-
opens.
› High AT value = low sensitivity. Thus, a strong magnetic field is
necessary to operate the Reed Switch (respectively the switching
distance between magnet and switch will be smaller)
› Low AT value = high sensitivity, allowing the switch to operate even
with a weaker magnetic field (respectively the switching distance
between magnet and switch can be increased)
Hysteresis is the difference between the Operate AT and Release AT of
a Reed Switch, i.e. a Reed Switch contact closes when approaching a
magnet to a 10 mm distance. However, the switch will re-open at a
magnet distance of 12 mm.
Some special Reed Switches feature a very low hysteresis
(‘close differential‘ types).
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4. Operation
Generally speaking, four different magnet approaches can be
considered when using permanent magnets.
4.1 One magnet pole faces the Reed Switch providing a maximum of
two operations when moving on the X-axis. Minimal movement of the
magnet over the switch center provides the smallest possible
switching differential.
4.2 Magnet parallel to longitudinal axis of switch.
Approaching the magnet
vertically to the switch on the Y-axis provides
one operation only. Driving the magnet over the full length of the
switch (X-axis) may result in up to three operations. Minimal
movement of the magnet over the switch center provides the smallest
possible switching differential.
4.3 Rotation Switch operation by rotation of magnet. This gives two
operations per complete rotation.
4.4 Operation of switch by shielding. This method requires a
permanently opposite location of the magnet and switch. The switch is
held closed continuously and will only release if the magnet flux is
removed by means of a ferromagnetic shield.
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