General
REED Switches
A REED switch consists of two flattened ferromagnetic reeds which are sealed inside a glass tube
containing a neutral atmosphere. The blades are laid out with a slight overlap at their free ends, so
as to be able to close, and they are spaced a few hundredth of a millimetre apart in the overlap
area (gap).
When the switch is brought into the influence of an appropriate magnetic field, the reeds, or blades,
will assume opposite magnetic polarity, and will attract each other and make contact. When the magnetic
field cancels out, the reeds will separate.
REED switches are manufactured in white room, on automatic sealing machines. In the course of
the sealing process, the inner volume of the switch is filled with a neutral or slightly reducing atmosphere
(Nitrogen + Hydrogen). The switch is thus explosion proof in its very concept, and is also protected from
every form of atmospheric pollution (liquids or gases).
After the manufacture, every switch is inspected unitarily for its electrical characteristics (contact
resistance, breakdown voltage, etc.) and its mechanical characteristics (sensitivity). Each
switch is then graded in relation with the quality of its contact and its sensitivity.
REED Relays
page 1 /14/GB
INT/GEN/Applications/A/25/02/2003
TECHNICAL NOTE : REED SWITCHES
All technical characteristics are subject to change without previous notice.
Caractéristiques sujettes à modifications sans préavis.
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A REED relay consists of one or more switches positioned in the centre of a coil winding
(solenoid). The assembly can be protected by a surrounding magnetic shield, or it can be moulded.
A REED relay is manufactured with the same meticulous care as taken for the protection of the
switches : it is continuously inspected in the course of assembly, impregnation, and moulding.
Overlap
Gap
Leads
Glass to
metal bond
Glass tube
page 2 / 14/GB
INT/GEN/Applications/A/25/02/2003
REED Switches : Technical specifications
1.Nature of the contact
There are two types of REED switches :
-Dry contact switches
-Mercury wetted switches
1.1 Dry REED switches
Dry REED switches are assembled using ferromagnetic materials for the reeds (FERRO-
NICKEL) which are tipped with heat-resisting metal (RHODIUM), to improve their arc-resisting
qualities. These switches can be used in any position.
Like most mechanical contacts, they are subject to bouncing when they close. Depending
on the type of switch the contact resistance is in the order of 50 to 150 milliohms, the switching
capacity varies from 0,4 to 3 Amps., along with a switching voltage
1.2 Mercury wetted REED switches (less and less used)
Mercury wetted REED switches
contain a drop of mercury which irrigates
the contact area by a capillarity effect.
The presence of liquid mercury
makes the vertical positioning of the switch
compulsory (+/- 30°).
The advantage of a mercury wetted
contact is that bouncing is avoided when
the contacts close, that contact resistance
remains very low throughout the useful life
of the switch, and that a lifespan of over
ONE HUNDRED MILLION cycles is
available at nominal current levels.
REED relays can be fitted with a magnetic shield (screen) which protects them from their magnetic
environment, and in a reciprocal manner avoids interference between the field arising from the coil
winding in the relay and neighbouring components.
The applications are large : Interface, telecommunication, modem, automatism, testing machines for
Printed Circuit Boards, testing machines for high voltage relays, low switching, …
REED switches are characterised by :
-The nature of the contact
-Its sensitivity
-The shape of the contact
-Its dimensions
-Switching power
-Its reliabilty
30°
30°
TOP
Mercury
Theard
Dro
p
of mercur
y
Compulsory
vertically
p
osition
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