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Reed Switches Applications Vol.04 5
Operation
A reed switch encapsulated in a glass tube has two
ferromagnetic reeds which face each other with a
given contact clearance between them, as shown in
diagrams on the right. The glass tube is filled with
nitrogen gas to prevent the activation of the con-
tacts, thus providing improved reliability and ex-
tended life.
Upon receipt of a magnetic field from the outside in
the axial direction of the reed switch, the reeds of
the reed switch are magnetized. The free opposite
ends of the reeds attract each other and come into
contact with each other, to close the circuit. When
the magnetic field is removed, the circuit opens by
means of the resiliency of the reeds.
Contact Material
● Ruthenium (Ru) plated contact
Ruthenium-plated contacts developed by NEC
TOKIN’s unique technique are made of a very
hard material having with a high melting point.
The contacts are resistant to mechanical friction
and heat generation, and they have excellent anti-
sticking performance.
● Rhodium (Rh) plated contact
Power reed switches what are susceptible to rela-
tively large consumption employ Rh-plated contacts,
which results in extended life of the reed switches
(compensating for contact consumption).
Types and Applications
When used in combination with a permanent magnet,
the reed switch finds extensive applications in which
it provides switching and sensing capabilities.
● Construction of Reed Switch
Structure and Principle of
Type
Part No.
Applications
RD-7AA/
General purpose
7B/24B
For general control purposes
NRS-771
(about 10W)
High power
RD-24E
Heavy load (50W)
Direct switching of a 3.4W
Lamp load
RD-8N
lamp (for surge current use)
For general control purposes
Compact
RD-9A/9B
(about 5W)
RD-18A/18B
Ultra compact
/75AA
Light load
NRS-701
Ru
Rh
Au
Items
(Ruthenium)
(Rhodium)
(Gold)
Atomic Weight
101
103
197
Melting Point (K)
2,523
2,233
1,338
Boiling Point (K)
4,173
4,000
2,983
Density (g/cm3)
12.1
12.4
19.3
Specific Heat
0.234
0.238
0.128
(J/gK) 0°C
Hardness (HV)
220
100
25
50.0
54.9
13.5
490×10
6
538×10
6
132×10
6
Tensile Strength
(kgf/mm2) (N/m2)
Crime
Electronic
Electrical
Hot-water
Fan heater VTR
prevention
balances
equipment
supplying
doors
for
apparatus
automobiles
Notebook
Laundry
Rice
Toys
Humidifiers Electronic
computer
machines
cookers
sewing
machines
Cellular
Refrigera-
Printers
Copiers
Vending
Flow
tors
machines
meters
● Operating Principle of Reed Switch
Characteristics of Contact Material
S
SN
S
N
Contact
Glass Tube
Ferromagnetic
Reed