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
of mercur
Compulsory
vertically
osition
r e l a i s
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